timoninmaxim commented on code in PR #11391:
URL: https://github.com/apache/ignite/pull/11391#discussion_r1678752158


##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {

Review Comment:
   The same, can we just throw an exception here instead of setting error in 
the future?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);

Review Comment:
   In SnapshotRestoreProcess, it just throws exceptions on the map phase, and 
cleaning actions are performed on the reduce phase. Could we use the same 
approach here?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {
+            assert locReq.fut() == null;
+
+            locReq.fut(locMetasChkFut);
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas. Request=" + 
locReq + '.');
+
+            Collection<Integer> grpIds = F.isEmpty(locReq.groups()) ? null : 
F.viewReadOnly(locReq.groups(), CU::cacheId);
+
+            // An error can occur when the local future is not initialized yet.
+            Throwable locReqErr = locReq.error();
+
+            if (locReqErr != null)
+                locMetasChkFut.onDone(locReqErr);
+
+            if (!locMetasChkFut.isDone()) {
+                snpMgr.checker().checkLocalMetas(
+                    snpMgr.snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath()),
+                    grpIds,
+                    kctx.cluster().get().localNode().consistentId()
+                ).whenComplete((locMetas, err) -> {
+                    if (err != null)
+                        locMetasChkFut.onDone(err);
+                    else {
+                        if (!F.isEmpty(locMetas))
+                            locReq.meta(locMetas.get(0));
+
+                        locMetasChkFut.onDone(new CheckResultDTO(locMetas));
+                    }
+                });
+            }
+        });
+
+        return locMetasChkFut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckProcessRequest locReq = currentRequest(snpName(results), 
procId);
+
+        assert locReq == null || locReq.opCoordId != null;
+
+        if (locReq == null || stopAndCleanOnError(locReq, errors) || 
!kctx.localNodeId().equals(locReq.opCoordId))
+            return;
+
+        Throwable stopClusterProcErr = null;
+
+        try {
+            assert !F.isEmpty(results) || !F.isEmpty(errors) || locReq.error() 
!= null;
+
+            if (locReq.error() != null)
+                throw locReq.error();

Review Comment:
   I suppose it's already handled in 371L, doesn't it? There is already 
collection of all results from all nodes, then concurrent errors don't affect 
the phase. 



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {
+            assert locReq.fut() == null;
+
+            locReq.fut(locMetasChkFut);
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas. Request=" + 
locReq + '.');
+
+            Collection<Integer> grpIds = F.isEmpty(locReq.groups()) ? null : 
F.viewReadOnly(locReq.groups(), CU::cacheId);
+
+            // An error can occur when the local future is not initialized yet.
+            Throwable locReqErr = locReq.error();
+
+            if (locReqErr != null)
+                locMetasChkFut.onDone(locReqErr);
+
+            if (!locMetasChkFut.isDone()) {
+                snpMgr.checker().checkLocalMetas(
+                    snpMgr.snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath()),
+                    grpIds,
+                    kctx.cluster().get().localNode().consistentId()
+                ).whenComplete((locMetas, err) -> {
+                    if (err != null)
+                        locMetasChkFut.onDone(err);
+                    else {
+                        if (!F.isEmpty(locMetas))
+                            locReq.meta(locMetas.get(0));
+
+                        locMetasChkFut.onDone(new CheckResultDTO(locMetas));
+                    }
+                });
+            }
+        });
+
+        return locMetasChkFut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckProcessRequest locReq = currentRequest(snpName(results), 
procId);
+
+        assert locReq == null || locReq.opCoordId != null;
+
+        if (locReq == null || stopAndCleanOnError(locReq, errors) || 
!kctx.localNodeId().equals(locReq.opCoordId))
+            return;
+
+        Throwable stopClusterProcErr = null;
+
+        try {
+            assert !F.isEmpty(results) || !F.isEmpty(errors) || locReq.error() 
!= null;
+
+            if (locReq.error() != null)
+                throw locReq.error();
+
+            locReq.metas = new HashMap<>();
+
+            Map<ClusterNode, Exception> errs = new HashMap<>();
+
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be non-baseline (not required).
+                if (locReq.nodes.contains(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsHashes == null;
+                    assert kctx.cluster().get().node(nodeId) != null;
+
+                    locReq.metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);
+                }
+            });
+
+            errors.forEach((nodeId, nodeErr) -> {

Review Comment:
   If `errors` are not empty this code is never executed. It is handled within 
`stopAndCleanOnError`



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);

Review Comment:
   you still have in the same class `locReq` and `locRq`. 



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {
+            assert locReq.fut() == null;
+
+            locReq.fut(locMetasChkFut);
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas. Request=" + 
locReq + '.');
+
+            Collection<Integer> grpIds = F.isEmpty(locReq.groups()) ? null : 
F.viewReadOnly(locReq.groups(), CU::cacheId);
+
+            // An error can occur when the local future is not initialized yet.
+            Throwable locReqErr = locReq.error();
+
+            if (locReqErr != null)
+                locMetasChkFut.onDone(locReqErr);
+
+            if (!locMetasChkFut.isDone()) {

Review Comment:
   Looks like it can be replaced with `if (locReq.error() == null`. In case of 
existing error, the reduce phase should clean.



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {

Review Comment:
   Let's add javadocs for `extReq`. I suppose it means `external`. I'm not 
sure, why it isn't just a `req`?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {
+            assert locReq.fut() == null;
+
+            locReq.fut(locMetasChkFut);
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas. Request=" + 
locReq + '.');
+
+            Collection<Integer> grpIds = F.isEmpty(locReq.groups()) ? null : 
F.viewReadOnly(locReq.groups(), CU::cacheId);
+
+            // An error can occur when the local future is not initialized yet.
+            Throwable locReqErr = locReq.error();
+
+            if (locReqErr != null)
+                locMetasChkFut.onDone(locReqErr);
+
+            if (!locMetasChkFut.isDone()) {

Review Comment:
   Actually, do we really need this check at all? The future inside 
`checkLocalMetas` isn't interruptible, then in case of error happened a moment 
later after the check, it unconditionally checks all metas. I think we can skip 
this `if` at all in favor of simplicity.



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {
+            assert locReq.fut() == null;
+
+            locReq.fut(locMetasChkFut);
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas. Request=" + 
locReq + '.');
+
+            Collection<Integer> grpIds = F.isEmpty(locReq.groups()) ? null : 
F.viewReadOnly(locReq.groups(), CU::cacheId);
+
+            // An error can occur when the local future is not initialized yet.
+            Throwable locReqErr = locReq.error();
+
+            if (locReqErr != null)
+                locMetasChkFut.onDone(locReqErr);
+
+            if (!locMetasChkFut.isDone()) {
+                snpMgr.checker().checkLocalMetas(
+                    snpMgr.snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath()),
+                    grpIds,
+                    kctx.cluster().get().localNode().consistentId()
+                ).whenComplete((locMetas, err) -> {
+                    if (err != null)
+                        locMetasChkFut.onDone(err);
+                    else {
+                        if (!F.isEmpty(locMetas))
+                            locReq.meta(locMetas.get(0));
+
+                        locMetasChkFut.onDone(new CheckResultDTO(locMetas));
+                    }
+                });
+            }
+        });
+
+        return locMetasChkFut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckProcessRequest locReq = currentRequest(snpName(results), 
procId);
+
+        assert locReq == null || locReq.opCoordId != null;
+
+        if (locReq == null || stopAndCleanOnError(locReq, errors) || 
!kctx.localNodeId().equals(locReq.opCoordId))
+            return;
+
+        Throwable stopClusterProcErr = null;
+
+        try {
+            assert !F.isEmpty(results) || !F.isEmpty(errors) || locReq.error() 
!= null;
+
+            if (locReq.error() != null)
+                throw locReq.error();
+
+            locReq.metas = new HashMap<>();
+
+            Map<ClusterNode, Exception> errs = new HashMap<>();
+
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be non-baseline (not required).
+                if (locReq.nodes.contains(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsHashes == null;
+                    assert kctx.cluster().get().node(nodeId) != null;
+
+                    locReq.metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);

Review Comment:
   `locReq.metas` are volatile and it's not a concurrent collection. Then I 
suppose it protects the reference only. Then common practice is to fully 
prepare collection first and after that set the reference.



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,549 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.cache.persistence.snapshot;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.IgniteLogger;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.events.DiscoveryEvent;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.IgniteInternalFuture;
+import org.apache.ignite.internal.cluster.ClusterTopologyCheckedException;
+import org.apache.ignite.internal.management.cache.IdleVerifyResultV2;
+import org.apache.ignite.internal.management.cache.PartitionKeyV2;
+import 
org.apache.ignite.internal.processors.cache.verify.PartitionHashRecordV2;
+import org.apache.ignite.internal.util.distributed.DistributedProcess;
+import org.apache.ignite.internal.util.future.GridFinishedFuture;
+import org.apache.ignite.internal.util.future.GridFutureAdapter;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.CU;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.events.EventType.EVT_NODE_FAILED;
+import static org.apache.ignite.events.EventType.EVT_NODE_LEFT;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_CHECK_METAS;
+import static 
org.apache.ignite.internal.util.distributed.DistributedProcess.DistributedProcessType.SNAPSHOT_VALIDATE_PARTS;
+
+/** Distributed process of snapshot checking (with the partition hashes). */
+public class SnapshotCheckProcess {
+    /** */
+    private final IgniteLogger log;
+
+    /** */
+    private final GridKernalContext kctx;
+
+    /** Snapshot check requests per snapshot on every node. */
+    private final Map<String, SnapshotCheckProcessRequest> requests = new 
ConcurrentHashMap<>();
+
+    /** Cluster-wide operation futures per snapshot called from current node. 
*/
+    private final Map<UUID, 
GridFutureAdapter<SnapshotPartitionsVerifyTaskResult>> clusterOpFuts = new 
ConcurrentHashMap<>();
+
+    /** Check metas first phase subprocess. */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
CheckResultDTO> phase2PartsHashes;
+
+    /** */
+    public SnapshotCheckProcess(GridKernalContext kctx) {
+        this.kctx = kctx;
+
+        log = kctx.log(getClass());
+
+        phase1CheckMetas = new DistributedProcess<>(kctx, 
SNAPSHOT_CHECK_METAS, this::prepareAndCheckMetas,
+            this::reducePreparationAndMetasCheck);
+
+        phase2PartsHashes = new DistributedProcess<>(kctx, 
SNAPSHOT_VALIDATE_PARTS, this::validateParts,
+            this::reduceValidatePartsAndFinish);
+
+        kctx.event().addLocalEventListener((evt) -> 
nodeLeft(((DiscoveryEvent)evt).eventNode()), EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckProcessRequest> requests() {
+        return Collections.unmodifiableMap(requests);
+    }
+
+    /**
+     * Stops the process with the passed exception.
+     *
+     * @param th The interrupt reason.
+     * @param rqFilter If not {@code null}, used to filter which 
requests/process to stop. If {@code null}, stops all the validations.
+     */
+    void interrupt(Throwable th, @Nullable 
Function<SnapshotCheckProcessRequest, Boolean> rqFilter) {
+        requests.values().forEach(rq -> {
+            if (rqFilter == null || rqFilter.apply(rq)) {
+                rq.error(th);
+
+                clean(rq.requestId(), th, null, null);
+            }
+        });
+    }
+
+    /** Stops the related validation if the node is a mandatory one. */
+    private void nodeLeft(ClusterNode node) {
+        if (node.isClient() || requests.isEmpty())
+            return;
+
+        interrupt(
+            new ClusterTopologyCheckedException("Snapshot checking stopped. A 
node left the cluster: " + node + '.'),
+            rq -> rq.nodes().contains(node.id())
+        );
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        clean(procId, null, results, errors);
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckProcessRequest locReq;
+
+        if (stopAndCleanOnError(req, null) || (locReq = 
requests.get(req.snapshotName())) == null)
+            return new GridFinishedFuture<>();
+
+        assert locReq.equals(req);
+
+        // Store metas to collect cluster operation result later.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(req.requestId());
+
+        if (clusterOpFut != null)
+            locReq.metas = req.metas;
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || locReq.meta() == null)
+            return new GridFinishedFuture<>();
+
+        GridFutureAdapter<CheckResultDTO> locPartsChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locPartsChkFut, () -> {
+            locReq.fut(locPartsChkFut);
+
+            // An error can occure when the local future is still null.
+            Throwable lorReqErr = locReq.error();
+
+            if (lorReqErr != null)
+                locPartsChkFut.onDone(lorReqErr);
+
+            if (!locPartsChkFut.isDone()) {
+                File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath());
+
+                
kctx.cache().context().snapshotMgr().checker().checkPartitions(locReq.meta(), 
snpDir, locReq.groups(), false, true, false)
+                    .whenComplete((res, err) -> {
+                        if (err != null)
+                            locPartsChkFut.onDone(err);
+                        else
+                            locPartsChkFut.onDone(new CheckResultDTO(res));
+                    });
+            }
+        });
+
+        return locPartsChkFut;
+    }
+
+    /**
+     * If required, stops and clean related validation if errors occured
+     *
+     * @return {@code True} if the validation was stopped and cleaned. {@code 
False} otherwise.
+     */
+    private boolean stopAndCleanOnError(SnapshotCheckProcessRequest req, 
@Nullable Map<UUID, Throwable> occurredErrors) {
+        assert req != null;
+
+        assert F.isEmpty(occurredErrors) || 
req.nodes().containsAll(occurredErrors.keySet());
+
+        if (F.isEmpty(occurredErrors) && req.error() == null)
+            return false;
+
+        clean(req.requestId(), req.error(), null, occurredErrors);
+
+        return true;
+    }
+
+    /** Cleans certain snapshot validation. */
+    private void clean(
+        UUID rqId,
+        @Nullable Throwable opErr,
+        @Nullable Map<UUID, CheckResultDTO> results,
+        @Nullable Map<UUID, Throwable> errors
+    ) {
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.remove(rqId);
+
+        stopFutureOnAnyFailure(clusterOpFut, () -> {
+            SnapshotCheckProcessRequest locRq = currentRequest(null, rqId);
+
+            Throwable err = opErr;
+
+            if (locRq != null)
+                err = stopAndCleanLocRequest(locRq, err);
+
+            if (clusterOpFut == null || clusterOpFut.isDone())
+                return;
+
+            boolean finished;
+
+            // Nodes' results and errors reducing is available on a required 
node where the process request must be registered.
+            assert (F.isEmpty(errors) && F.isEmpty(results)) || locRq != null;
+
+            Map<ClusterNode, Exception> errors0 = locRq == null || 
F.isEmpty(errors)
+                ? Collections.emptyMap()
+                : collectErrors(errors, locRq.nodes());
+
+            if (err == null && !F.isEmpty(results)) {
+                assert results.values().stream().noneMatch(res -> res != null 
&& res.metas != null);
+
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = collectPartsHashes(results, locRq.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                finished = clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(locRq.metas, chkRes));
+            }
+            else
+                finished = finishClusterFutureWithErr(clusterOpFut, err, 
errors0);
+
+            if (finished && log.isInfoEnabled())
+                log.info("Snapshot validation process finished, req: " + locRq 
+ '.');
+        });
+    }
+
+    /** */
+    private Throwable stopAndCleanLocRequest(SnapshotCheckProcessRequest 
locRq, @Nullable Throwable err) {
+        if (err == null && locRq.error() != null)
+            err = locRq.error();
+
+        GridFutureAdapter<?> locWorkingFut = locRq.fut();
+
+        boolean finished = false;
+
+        // Try to stop local working future ASAP.
+        if (locWorkingFut != null)
+            finished = err == null ? locWorkingFut.onDone() : 
locWorkingFut.onDone(err);
+
+        requests.remove(locRq.snapshotName());
+
+        if (finished && log.isInfoEnabled())
+            log.info("Finished snapshot local validation, req: " + locRq + 
'.');
+
+        return err;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(Map<UUID, Throwable> 
errors, Set<UUID> requiredNodes) {
+        if (errors.isEmpty())
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream().filter(e -> 
requiredNodes.contains(e.getKey()) && e.getValue() != null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> asException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
collectPartsHashes(
+        @Nullable Map<UUID, CheckResultDTO> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        return results.entrySet().stream()
+            .filter(e -> requiredNodes.contains(e.getKey()) && e.getValue() != 
null)
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()), e -> e.getValue().partsHashes));
+    }
+
+    /**
+     * @param snpName Snapshot name of the validation process. If {@code 
null}, ignored.
+     * @param procId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking request by {@code snpName} or {@code 
procId}.
+     */
+    private @Nullable SnapshotCheckProcessRequest currentRequest(@Nullable 
String snpName, UUID procId) {
+        return snpName == null
+            ? requests.values().stream().filter(rq -> 
rq.requestId().equals(procId)).findFirst().orElse(null)
+            : requests.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest extReq) {
+        SnapshotCheckProcessRequest locReq = 
requests.computeIfAbsent(extReq.snapshotName(), snpName -> extReq);
+
+        if (!locReq.equals(extReq)) {
+            Throwable err = new IllegalStateException("Validation of snapshot 
'" + extReq.snapshotName()
+                + "' has already started. Request=" + locReq + '.');
+
+            clean(extReq.requestId(), err, null, null);
+
+            return new GridFinishedFuture<>(err);
+        }
+
+        if (!extReq.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation, request=" + extReq
+                    + ". Current node is not required.");
+            }
+
+            return new GridFinishedFuture<>();
+        }
+
+        GridFutureAdapter<CheckResultDTO> locMetasChkFut = new 
GridFutureAdapter<>();
+
+        stopFutureOnAnyFailure(locMetasChkFut, () -> {
+            assert locReq.fut() == null;
+
+            locReq.fut(locMetasChkFut);
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas. Request=" + 
locReq + '.');
+
+            Collection<Integer> grpIds = F.isEmpty(locReq.groups()) ? null : 
F.viewReadOnly(locReq.groups(), CU::cacheId);
+
+            // An error can occur when the local future is not initialized yet.
+            Throwable locReqErr = locReq.error();
+
+            if (locReqErr != null)
+                locMetasChkFut.onDone(locReqErr);
+
+            if (!locMetasChkFut.isDone()) {
+                snpMgr.checker().checkLocalMetas(
+                    snpMgr.snapshotLocalDir(locReq.snapshotName(), 
locReq.snapshotPath()),
+                    grpIds,
+                    kctx.cluster().get().localNode().consistentId()
+                ).whenComplete((locMetas, err) -> {
+                    if (err != null)
+                        locMetasChkFut.onDone(err);
+                    else {
+                        if (!F.isEmpty(locMetas))
+                            locReq.meta(locMetas.get(0));
+
+                        locMetasChkFut.onDone(new CheckResultDTO(locMetas));
+                    }
+                });
+            }
+        });
+
+        return locMetasChkFut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID procId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckProcessRequest locReq = currentRequest(snpName(results), 
procId);
+
+        assert locReq == null || locReq.opCoordId != null;
+
+        if (locReq == null || stopAndCleanOnError(locReq, errors) || 
!kctx.localNodeId().equals(locReq.opCoordId))
+            return;
+
+        Throwable stopClusterProcErr = null;
+
+        try {
+            assert !F.isEmpty(results) || !F.isEmpty(errors) || locReq.error() 
!= null;
+
+            if (locReq.error() != null)
+                throw locReq.error();
+
+            locReq.metas = new HashMap<>();
+
+            Map<ClusterNode, Exception> errs = new HashMap<>();
+
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be non-baseline (not required).
+                if (locReq.nodes.contains(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsHashes == null;
+                    assert kctx.cluster().get().node(nodeId) != null;
+
+                    locReq.metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);
+                }
+            });
+
+            errors.forEach((nodeId, nodeErr) -> {
+                assert locReq.nodes.contains(nodeId);
+                assert kctx.cluster().get().node(nodeId) != null;
+                assert nodeErr != null;
+
+                errs.put(kctx.cluster().get().node(nodeId), 
asException(nodeErr));
+            });
+
+            SnapshotMetadataVerificationTaskResult metasRes = new 
SnapshotMetadataVerificationTaskResult(
+                locReq.metas,
+                SnapshotChecker.reduceMetasResults(locReq.snapshotName(), 
locReq.snapshotPath(), locReq.metas, errs,
+                    kctx.cluster().get().localNode().consistentId())
+            );
+
+            if (!F.isEmpty(metasRes.exceptions()))
+                stopClusterProcErr = new 
IgniteSnapshotVerifyException(metasRes.exceptions());
+        }
+        catch (Throwable th) {
+            stopClusterProcErr = th;
+        }
+
+        if (stopClusterProcErr != null)
+            locReq.error(stopClusterProcErr);
+
+        phase2PartsHashes.start(procId, locReq);
+
+        if (log.isDebugEnabled())
+            log.debug("Started partitions validation as part of the snapshot 
checking. Request=" + locReq + '.');
+    }
+
+    /** Finds current snapshot name from the metas. */
+    private @Nullable String snpName(@Nullable Map<UUID, CheckResultDTO> 
results) {
+        if (F.isEmpty(results))
+            return null;
+
+        // Ensure the same snapshot name or empty.
+        assert F.flatCollections(results.values().stream().filter(r -> r != 
null && r.metas != null).map(res -> res.metas)

Review Comment:
   It looks complicated. I suppose it can be check during map phase. Or, maybe, 
while constructing `CheckResultDTO`?



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