timoninmaxim commented on code in PR #11391:
URL: https://github.com/apache/ignite/pull/11391#discussion_r1699843834


##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,475 @@
+/*
+ * 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.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;
+
+    /** Operation contexts by name. */
+    private final Map<String, SnapshotCheckContext> contexts = 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) -> {
+            if (contexts.isEmpty())
+                return;
+
+            DiscoveryEvent devt = (DiscoveryEvent)evt;
+
+            Throwable err = new ClusterTopologyCheckedException("Snapshot 
checking stopped. " +
+                "A required node or the initiator node left the cluster: " + 
devt.eventNode() + '.');
+
+            contexts.values().forEach(ctx -> {
+                if (ctx.req.nodes().contains(devt.eventNode().id())) {
+                    ctx.err = err;
+
+                    ctx.fut.onDone(err);
+                }
+            });
+        }, EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> requests() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param th The interrupt reason.
+     */
+    void interrupt(Throwable th) {
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(th));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx != null) {
+            contexts.remove(ctx.req.snapshotName());
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot local validation [req=" + ctx.req 
+ ']');
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                if (ctx.err == null && !F.isEmpty(results)) {
+                    assert results.values().stream().noneMatch(res -> res != 
null && res.metas != null);
+                    assert ctx != null;
+
+                    Map<ClusterNode, Map<PartitionKeyV2, 
PartitionHashRecordV2>> results0 = collectPartsHashes(results,
+                        ctx != null ? ctx.req.nodes() : null);
+
+                    IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                    clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+                }
+                else
+                    finishClusterFutureWithErr(clusterOpFut, ctx.err, errors0);
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        if (req.error() != null)
+            return new GridFinishedFuture<>(req.error());
+
+        SnapshotCheckContext ctx = context(null, req.requestId());
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        assert ctx.req.reqId.equals(req.reqId);
+
+        if (ctx.fut.isDone() && !ctx.fut.isFailed())
+            ctx.fut.reset();
+
+        // Store metas on the initiator node to form the process result 
(SnapshotPartitionsVerifyTaskResult) at the end.
+        if (req.initiatorId().equals(kctx.localNodeId()))
+            ctx.clusterMetas = req.clusterMetas();
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || ctx.locMeta == null)
+            ctx.fut.onDone();
+        else {
+            File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(req.snapshotName(), 
req.snapshotPath());
+
+            
kctx.cache().context().snapshotMgr().checker().checkPartitions(ctx.locMeta, 
snpDir, req.groups(), false, true, false)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        ctx.fut.onDone(err);
+                    else
+                        ctx.fut.onDone(new CheckResultDTO(res));
+                });
+        }
+
+        return ctx.fut;
+    }
+    
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(@Nullable Map<UUID, 
Throwable> errors, @Nullable Set<UUID> requiredNodes) {
+        if (F.isEmpty(errors))
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream()
+            .filter(e -> (requiredNodes == null || 
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 reqId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking context by {@code snpName} or {@code 
reqId}.
+     */
+    private @Nullable SnapshotCheckContext context(@Nullable String snpName, 
UUID reqId) {
+        return snpName == null
+            ? contexts.values().stream().filter(ctx -> 
ctx.req.reqId.equals(reqId)).findFirst().orElse(null)
+            : contexts.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx = 
contexts.computeIfAbsent(req.snapshotName(), snpName -> new 
SnapshotCheckContext(req));
+
+        if (!ctx.req.equals(req)) {
+            return new GridFinishedFuture<>(new 
IllegalStateException("Validation of snapshot '" + req.snapshotName()
+                + "' has already started. Request=" + ctx + '.'));
+        }
+
+        if (!req.nodes.contains(kctx.localNodeId())) {

Review Comment:
   Can we check it before creating the context? If node should be skipped then 
we can just return `new GridFinishedFuture()`.



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,475 @@
+/*
+ * 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.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;
+
+    /** Operation contexts by name. */
+    private final Map<String, SnapshotCheckContext> contexts = 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) -> {
+            if (contexts.isEmpty())
+                return;
+
+            DiscoveryEvent devt = (DiscoveryEvent)evt;
+
+            Throwable err = new ClusterTopologyCheckedException("Snapshot 
checking stopped. " +
+                "A required node or the initiator node left the cluster: " + 
devt.eventNode() + '.');
+
+            contexts.values().forEach(ctx -> {
+                if (ctx.req.nodes().contains(devt.eventNode().id())) {
+                    ctx.err = err;
+
+                    ctx.fut.onDone(err);
+                }
+            });
+        }, EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> requests() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param th The interrupt reason.
+     */
+    void interrupt(Throwable th) {
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(th));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx != null) {
+            contexts.remove(ctx.req.snapshotName());
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot local validation [req=" + ctx.req 
+ ']');
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                if (ctx.err == null && !F.isEmpty(results)) {
+                    assert results.values().stream().noneMatch(res -> res != 
null && res.metas != null);
+                    assert ctx != null;
+
+                    Map<ClusterNode, Map<PartitionKeyV2, 
PartitionHashRecordV2>> results0 = collectPartsHashes(results,
+                        ctx != null ? ctx.req.nodes() : null);
+
+                    IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                    clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+                }
+                else
+                    finishClusterFutureWithErr(clusterOpFut, ctx.err, errors0);
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        if (req.error() != null)
+            return new GridFinishedFuture<>(req.error());
+
+        SnapshotCheckContext ctx = context(null, req.requestId());
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        assert ctx.req.reqId.equals(req.reqId);
+
+        if (ctx.fut.isDone() && !ctx.fut.isFailed())
+            ctx.fut.reset();
+
+        // Store metas on the initiator node to form the process result 
(SnapshotPartitionsVerifyTaskResult) at the end.
+        if (req.initiatorId().equals(kctx.localNodeId()))
+            ctx.clusterMetas = req.clusterMetas();
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || ctx.locMeta == null)
+            ctx.fut.onDone();
+        else {
+            File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(req.snapshotName(), 
req.snapshotPath());
+
+            
kctx.cache().context().snapshotMgr().checker().checkPartitions(ctx.locMeta, 
snpDir, req.groups(), false, true, false)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        ctx.fut.onDone(err);
+                    else
+                        ctx.fut.onDone(new CheckResultDTO(res));
+                });
+        }
+
+        return ctx.fut;
+    }
+    
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(@Nullable Map<UUID, 
Throwable> errors, @Nullable Set<UUID> requiredNodes) {
+        if (F.isEmpty(errors))
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream()
+            .filter(e -> (requiredNodes == null || 
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 reqId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking context by {@code snpName} or {@code 
reqId}.
+     */
+    private @Nullable SnapshotCheckContext context(@Nullable String snpName, 
UUID reqId) {
+        return snpName == null
+            ? contexts.values().stream().filter(ctx -> 
ctx.req.reqId.equals(reqId)).findFirst().orElse(null)
+            : contexts.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx = 
contexts.computeIfAbsent(req.snapshotName(), snpName -> new 
SnapshotCheckContext(req));
+
+        if (!ctx.req.equals(req)) {
+            return new GridFinishedFuture<>(new 
IllegalStateException("Validation of snapshot '" + req.snapshotName()
+                + "' has already started. Request=" + ctx + '.'));
+        }
+
+        if (!req.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation [req=" + req
+                    + "]. Current node is not required.");
+            }
+
+            ctx.fut.onDone();
+        }
+        else {
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas [req=" + ctx.req 
+ ']');
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            Collection<Integer> grpIds = F.isEmpty(req.groups()) ? null : 
F.viewReadOnly(req.groups(), CU::cacheId);
+
+            SnapshotCheckContext ctx0 = ctx;
+
+            snpMgr.checker().checkLocalMetas(
+                snpMgr.snapshotLocalDir(req.snapshotName(), 
req.snapshotPath()),
+                grpIds,
+                kctx.cluster().get().localNode().consistentId()
+            ).whenComplete((locMetas, err) -> {
+                if (err != null)
+                    ctx0.fut.onDone(err);
+                else {
+                    if (!F.isEmpty(locMetas))
+                        ctx0.locMeta = locMetas.get(0);
+
+                    ctx0.fut.onDone(new CheckResultDTO(locMetas));
+                }
+            });
+        }
+
+        return ctx.fut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(snpName(results), reqId);
+
+        Throwable err = ctx == null ? null : ctx.err;
+
+        if (err != null || !F.isEmpty(errors)) {
+            // Context might not be added in the case of concurrent checking 
of the same snapshot.
+            if (ctx != null) {
+                contexts.remove(ctx.req.snapshotName());
+
+                if (log.isInfoEnabled())
+                    log.info("Finished snapshot local validation [req=" + 
ctx.req + ']');
+            }
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                finishClusterFutureWithErr(clusterOpFut, err, errors0);
+            }
+
+            return;
+        }
+
+        if (ctx == null || 
!ctx.req.operationalNodeId().equals(kctx.localNodeId()))
+            return;
+
+        Map<ClusterNode, List<SnapshotMetadata>> metas = new HashMap<>();
+
+        try {
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be non-baseline (not required).
+                if (ctx.req.nodes().contains(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsHashes == null;
+                    assert kctx.cluster().get().node(nodeId) != null;
+
+                    metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);
+                }
+            });
+
+            SnapshotMetadataVerificationTaskResult metasRes = new 
SnapshotMetadataVerificationTaskResult(
+                metas,
+                SnapshotChecker.reduceMetasResults(ctx.req.snapshotName(), 
ctx.req.snapshotPath(), metas, null,
+                    kctx.cluster().get().localNode().consistentId())
+            );
+
+            if (!F.isEmpty(metasRes.exceptions()))
+                throw new IgniteSnapshotVerifyException(metasRes.exceptions());
+        }
+        catch (Throwable th) {
+            err = th;
+        }
+
+        phase2PartsHashes.start(reqId, new 
SnapshotCheckProcessRequest(ctx.req, err, metas));
+
+        if (log.isDebugEnabled())
+            log.debug("Started partitions validation as part of the snapshot 
checking [req=" + ctx.req + ']');
+    }
+
+    /** Finds current snapshot name from the metas. */
+    private @Nullable String snpName(@Nullable Map<UUID, CheckResultDTO> 
results) {
+        if (F.isEmpty(results))
+            return null;
+
+        for (CheckResultDTO nodeRes : results.values()) {
+            if (nodeRes == null || F.isEmpty(nodeRes.metas))
+                continue;
+
+            assert nodeRes.metas.get(0) != null : "Empty snapshot metadata in 
the results";
+            assert !F.isEmpty(nodeRes.metas.get(0).snapshotName()) : "Empty 
snapshot name in a snapshot metadata.";
+
+            return nodeRes.metas.get(0).snapshotName();
+        }
+
+        return null;
+    }
+
+    /** Starts the snapshot full validation. */
+    public IgniteInternalFuture<SnapshotPartitionsVerifyTaskResult> start(
+        String snpName,
+        @Nullable String snpPath,
+        @Nullable Collection<String> grpNames,
+        boolean inclCstHndlrs
+    ) {
+        assert !F.isEmpty(snpName);
+
+        UUID reqId = UUID.randomUUID();
+
+        List<UUID> requiredNodes = new 
ArrayList<>(F.viewReadOnly(kctx.discovery().discoCache().aliveBaselineNodes(), 
F.node2id()));

Review Comment:
   Why not Set?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,475 @@
+/*
+ * 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.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;
+
+    /** Operation contexts by name. */
+    private final Map<String, SnapshotCheckContext> contexts = 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) -> {
+            if (contexts.isEmpty())
+                return;
+
+            DiscoveryEvent devt = (DiscoveryEvent)evt;
+
+            Throwable err = new ClusterTopologyCheckedException("Snapshot 
checking stopped. " +
+                "A required node or the initiator node left the cluster: " + 
devt.eventNode() + '.');
+
+            contexts.values().forEach(ctx -> {
+                if (ctx.req.nodes().contains(devt.eventNode().id())) {
+                    ctx.err = err;
+
+                    ctx.fut.onDone(err);
+                }
+            });
+        }, EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> requests() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param th The interrupt reason.
+     */
+    void interrupt(Throwable th) {
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(th));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx != null) {
+            contexts.remove(ctx.req.snapshotName());
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot local validation [req=" + ctx.req 
+ ']');
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                if (ctx.err == null && !F.isEmpty(results)) {
+                    assert results.values().stream().noneMatch(res -> res != 
null && res.metas != null);
+                    assert ctx != null;
+
+                    Map<ClusterNode, Map<PartitionKeyV2, 
PartitionHashRecordV2>> results0 = collectPartsHashes(results,
+                        ctx != null ? ctx.req.nodes() : null);
+
+                    IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                    clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+                }
+                else
+                    finishClusterFutureWithErr(clusterOpFut, ctx.err, errors0);
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        if (req.error() != null)
+            return new GridFinishedFuture<>(req.error());
+
+        SnapshotCheckContext ctx = context(null, req.requestId());
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        assert ctx.req.reqId.equals(req.reqId);
+
+        if (ctx.fut.isDone() && !ctx.fut.isFailed())
+            ctx.fut.reset();
+
+        // Store metas on the initiator node to form the process result 
(SnapshotPartitionsVerifyTaskResult) at the end.
+        if (req.initiatorId().equals(kctx.localNodeId()))
+            ctx.clusterMetas = req.clusterMetas();
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || ctx.locMeta == null)
+            ctx.fut.onDone();
+        else {
+            File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(req.snapshotName(), 
req.snapshotPath());
+
+            
kctx.cache().context().snapshotMgr().checker().checkPartitions(ctx.locMeta, 
snpDir, req.groups(), false, true, false)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        ctx.fut.onDone(err);
+                    else
+                        ctx.fut.onDone(new CheckResultDTO(res));
+                });
+        }
+
+        return ctx.fut;
+    }
+    
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(@Nullable Map<UUID, 
Throwable> errors, @Nullable Set<UUID> requiredNodes) {
+        if (F.isEmpty(errors))
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream()
+            .filter(e -> (requiredNodes == null || 
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 reqId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking context by {@code snpName} or {@code 
reqId}.
+     */
+    private @Nullable SnapshotCheckContext context(@Nullable String snpName, 
UUID reqId) {
+        return snpName == null
+            ? contexts.values().stream().filter(ctx -> 
ctx.req.reqId.equals(reqId)).findFirst().orElse(null)
+            : contexts.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx = 
contexts.computeIfAbsent(req.snapshotName(), snpName -> new 
SnapshotCheckContext(req));
+
+        if (!ctx.req.equals(req)) {
+            return new GridFinishedFuture<>(new 
IllegalStateException("Validation of snapshot '" + req.snapshotName()
+                + "' has already started. Request=" + ctx + '.'));
+        }
+
+        if (!req.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation [req=" + req
+                    + "]. Current node is not required.");
+            }
+
+            ctx.fut.onDone();
+        }
+        else {
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas [req=" + ctx.req 
+ ']');
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            Collection<Integer> grpIds = F.isEmpty(req.groups()) ? null : 
F.viewReadOnly(req.groups(), CU::cacheId);
+
+            SnapshotCheckContext ctx0 = ctx;
+
+            snpMgr.checker().checkLocalMetas(
+                snpMgr.snapshotLocalDir(req.snapshotName(), 
req.snapshotPath()),
+                grpIds,
+                kctx.cluster().get().localNode().consistentId()
+            ).whenComplete((locMetas, err) -> {
+                if (err != null)
+                    ctx0.fut.onDone(err);
+                else {
+                    if (!F.isEmpty(locMetas))
+                        ctx0.locMeta = locMetas.get(0);
+
+                    ctx0.fut.onDone(new CheckResultDTO(locMetas));
+                }
+            });
+        }
+
+        return ctx.fut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(snpName(results), reqId);
+
+        Throwable err = ctx == null ? null : ctx.err;
+
+        if (err != null || !F.isEmpty(errors)) {
+            // Context might not be added in the case of concurrent checking 
of the same snapshot.
+            if (ctx != null) {
+                contexts.remove(ctx.req.snapshotName());
+
+                if (log.isInfoEnabled())
+                    log.info("Finished snapshot local validation [req=" + 
ctx.req + ']');
+            }
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                finishClusterFutureWithErr(clusterOpFut, err, errors0);
+            }
+
+            return;
+        }
+
+        if (ctx == null || 
!ctx.req.operationalNodeId().equals(kctx.localNodeId()))
+            return;
+
+        Map<ClusterNode, List<SnapshotMetadata>> metas = new HashMap<>();
+
+        try {
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be non-baseline (not required).
+                if (ctx.req.nodes().contains(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsHashes == null;
+                    assert kctx.cluster().get().node(nodeId) != null;
+
+                    metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);
+                }
+            });
+
+            SnapshotMetadataVerificationTaskResult metasRes = new 
SnapshotMetadataVerificationTaskResult(
+                metas,
+                SnapshotChecker.reduceMetasResults(ctx.req.snapshotName(), 
ctx.req.snapshotPath(), metas, null,
+                    kctx.cluster().get().localNode().consistentId())
+            );
+
+            if (!F.isEmpty(metasRes.exceptions()))
+                throw new IgniteSnapshotVerifyException(metasRes.exceptions());
+        }
+        catch (Throwable th) {
+            err = th;
+        }
+
+        phase2PartsHashes.start(reqId, new 
SnapshotCheckProcessRequest(ctx.req, err, metas));
+
+        if (log.isDebugEnabled())
+            log.debug("Started partitions validation as part of the snapshot 
checking [req=" + ctx.req + ']');
+    }
+
+    /** Finds current snapshot name from the metas. */
+    private @Nullable String snpName(@Nullable Map<UUID, CheckResultDTO> 
results) {
+        if (F.isEmpty(results))
+            return null;
+
+        for (CheckResultDTO nodeRes : results.values()) {
+            if (nodeRes == null || F.isEmpty(nodeRes.metas))
+                continue;
+
+            assert nodeRes.metas.get(0) != null : "Empty snapshot metadata in 
the results";
+            assert !F.isEmpty(nodeRes.metas.get(0).snapshotName()) : "Empty 
snapshot name in a snapshot metadata.";
+
+            return nodeRes.metas.get(0).snapshotName();
+        }
+
+        return null;
+    }
+
+    /** Starts the snapshot full validation. */
+    public IgniteInternalFuture<SnapshotPartitionsVerifyTaskResult> start(
+        String snpName,
+        @Nullable String snpPath,
+        @Nullable Collection<String> grpNames,
+        boolean inclCstHndlrs
+    ) {
+        assert !F.isEmpty(snpName);
+
+        UUID reqId = UUID.randomUUID();
+
+        List<UUID> requiredNodes = new 
ArrayList<>(F.viewReadOnly(kctx.discovery().discoCache().aliveBaselineNodes(), 
F.node2id()));
+
+        // Initiator is also required.
+        requiredNodes.add(kctx.localNodeId());
+
+        SnapshotCheckProcessRequest req = new SnapshotCheckProcessRequest(
+            reqId,
+            kctx.localNodeId(),
+            requiredNodes.get((int)(Math.random() * requiredNodes.size())),
+            requiredNodes,
+            snpName,
+            snpPath,
+            grpNames,
+            0,
+            inclCstHndlrs,
+            null
+        );
+
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
new GridFutureAdapter<>();
+
+        clusterOpFut.listen(fut -> {
+            clusterOpFuts.remove(reqId);
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot checking process [req=" + req + 
']');
+        });
+
+        clusterOpFuts.put(reqId, clusterOpFut);
+
+        phase1CheckMetas.start(req.requestId(), req);
+
+        return clusterOpFut;
+    }
+
+    /** Properly sets errror to the cluster operation future. */
+    static boolean finishClusterFutureWithErr(
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut,
+        Throwable propogatedError,
+        Map<ClusterNode, Exception> nodeErrors
+    ) {
+        assert propogatedError != null || !F.isEmpty(nodeErrors);
+
+        if (propogatedError == null)
+            return clusterOpFut.onDone(new 
IgniteSnapshotVerifyException(nodeErrors));
+        else if (propogatedError instanceof IgniteSnapshotVerifyException)
+            return clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(null,
+                new 
IdleVerifyResultV2(((IgniteSnapshotVerifyException)propogatedError).exceptions())));
+        else
+            return clusterOpFut.onDone(propogatedError);
+    }
+
+    /** Converts failure to an exception if it is not. */
+    private static Exception asException(Throwable th) {
+        return th instanceof Exception ? (Exception)th : new 
IgniteException(th);
+    }
+
+    /** Operation context. */
+    private static final class SnapshotCheckContext {
+        /** Request. */
+        private final SnapshotCheckProcessRequest req;
+
+        /** Working future. */
+        private final GridFutureAdapter<CheckResultDTO> fut = new 
GridFutureAdapter<>();
+
+        /** Collected cluster metas. */
+        @Nullable private Map<ClusterNode, List<SnapshotMetadata>> 
clusterMetas;
+        
+        /** Snapshot local metadata. */
+        @Nullable private SnapshotMetadata locMeta;

Review Comment:
   Can we calculate it from `clusterMetas?`?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,475 @@
+/*
+ * 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.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;
+
+    /** Operation contexts by name. */
+    private final Map<String, SnapshotCheckContext> contexts = 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) -> {
+            if (contexts.isEmpty())
+                return;
+
+            DiscoveryEvent devt = (DiscoveryEvent)evt;
+
+            Throwable err = new ClusterTopologyCheckedException("Snapshot 
checking stopped. " +
+                "A required node or the initiator node left the cluster: " + 
devt.eventNode() + '.');
+
+            contexts.values().forEach(ctx -> {
+                if (ctx.req.nodes().contains(devt.eventNode().id())) {
+                    ctx.err = err;
+
+                    ctx.fut.onDone(err);
+                }
+            });
+        }, EVT_NODE_FAILED, EVT_NODE_LEFT);
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> requests() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param th The interrupt reason.
+     */
+    void interrupt(Throwable th) {
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(th));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx != null) {
+            contexts.remove(ctx.req.snapshotName());
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot local validation [req=" + ctx.req 
+ ']');
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                if (ctx.err == null && !F.isEmpty(results)) {
+                    assert results.values().stream().noneMatch(res -> res != 
null && res.metas != null);
+                    assert ctx != null;
+
+                    Map<ClusterNode, Map<PartitionKeyV2, 
PartitionHashRecordV2>> results0 = collectPartsHashes(results,
+                        ctx != null ? ctx.req.nodes() : null);
+
+                    IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                    clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+                }
+                else
+                    finishClusterFutureWithErr(clusterOpFut, ctx.err, errors0);
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<CheckResultDTO> 
validateParts(SnapshotCheckProcessRequest req) {
+        if (req.error() != null)
+            return new GridFinishedFuture<>(req.error());
+
+        SnapshotCheckContext ctx = context(null, req.requestId());
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        assert ctx.req.reqId.equals(req.reqId);
+
+        if (ctx.fut.isDone() && !ctx.fut.isFailed())
+            ctx.fut.reset();
+
+        // Store metas on the initiator node to form the process result 
(SnapshotPartitionsVerifyTaskResult) at the end.
+        if (req.initiatorId().equals(kctx.localNodeId()))
+            ctx.clusterMetas = req.clusterMetas();
+
+        // Local meta might be null if current node started after the snapshot 
creation or placement.
+        if (!req.nodes.contains(kctx.localNodeId()) || ctx.locMeta == null)
+            ctx.fut.onDone();
+        else {
+            File snpDir = 
kctx.cache().context().snapshotMgr().snapshotLocalDir(req.snapshotName(), 
req.snapshotPath());
+
+            
kctx.cache().context().snapshotMgr().checker().checkPartitions(ctx.locMeta, 
snpDir, req.groups(), false, true, false)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        ctx.fut.onDone(err);
+                    else
+                        ctx.fut.onDone(new CheckResultDTO(res));
+                });
+        }
+
+        return ctx.fut;
+    }
+    
+    /** */
+    private Map<ClusterNode, Exception> collectErrors(@Nullable Map<UUID, 
Throwable> errors, @Nullable Set<UUID> requiredNodes) {
+        if (F.isEmpty(errors))
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream()
+            .filter(e -> (requiredNodes == null || 
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 reqId  If {@code snpName} is {@code null}, is used to find the 
operation request.
+     * @return Current snapshot checking context by {@code snpName} or {@code 
reqId}.
+     */
+    private @Nullable SnapshotCheckContext context(@Nullable String snpName, 
UUID reqId) {
+        return snpName == null
+            ? contexts.values().stream().filter(ctx -> 
ctx.req.reqId.equals(reqId)).findFirst().orElse(null)
+            : contexts.get(snpName);
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<CheckResultDTO> 
prepareAndCheckMetas(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx = 
contexts.computeIfAbsent(req.snapshotName(), snpName -> new 
SnapshotCheckContext(req));
+
+        if (!ctx.req.equals(req)) {
+            return new GridFinishedFuture<>(new 
IllegalStateException("Validation of snapshot '" + req.snapshotName()
+                + "' has already started. Request=" + ctx + '.'));
+        }
+
+        if (!req.nodes.contains(kctx.localNodeId())) {
+            if (log.isDebugEnabled()) {
+                log.debug("Skipping snapshot local metadatas collecting for 
snapshot validation [req=" + req
+                    + "]. Current node is not required.");
+            }
+
+            ctx.fut.onDone();
+        }
+        else {
+            if (log.isDebugEnabled())
+                log.debug("Checking local snapshot metadatas [req=" + ctx.req 
+ ']');
+
+            IgniteSnapshotManager snpMgr = 
kctx.cache().context().snapshotMgr();
+
+            Collection<Integer> grpIds = F.isEmpty(req.groups()) ? null : 
F.viewReadOnly(req.groups(), CU::cacheId);
+
+            SnapshotCheckContext ctx0 = ctx;
+
+            snpMgr.checker().checkLocalMetas(
+                snpMgr.snapshotLocalDir(req.snapshotName(), 
req.snapshotPath()),
+                grpIds,
+                kctx.cluster().get().localNode().consistentId()
+            ).whenComplete((locMetas, err) -> {
+                if (err != null)
+                    ctx0.fut.onDone(err);
+                else {
+                    if (!F.isEmpty(locMetas))
+                        ctx0.locMeta = locMetas.get(0);
+
+                    ctx0.fut.onDone(new CheckResultDTO(locMetas));
+                }
+            });
+        }
+
+        return ctx.fut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID reqId,
+        Map<UUID, CheckResultDTO> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(snpName(results), reqId);
+
+        Throwable err = ctx == null ? null : ctx.err;
+
+        if (err != null || !F.isEmpty(errors)) {
+            // Context might not be added in the case of concurrent checking 
of the same snapshot.
+            if (ctx != null) {
+                contexts.remove(ctx.req.snapshotName());
+
+                if (log.isInfoEnabled())
+                    log.info("Finished snapshot local validation [req=" + 
ctx.req + ']');
+            }
+
+            GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut 
= clusterOpFuts.get(reqId);
+
+            if (clusterOpFut != null) {
+                Map<ClusterNode, Exception> errors0 = collectErrors(errors, 
ctx != null ? ctx.req.nodes() : null);
+
+                finishClusterFutureWithErr(clusterOpFut, err, errors0);
+            }
+
+            return;
+        }
+
+        if (ctx == null || 
!ctx.req.operationalNodeId().equals(kctx.localNodeId()))
+            return;
+
+        Map<ClusterNode, List<SnapshotMetadata>> metas = new HashMap<>();
+
+        try {
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be non-baseline (not required).
+                if (ctx.req.nodes().contains(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsHashes == null;
+                    assert kctx.cluster().get().node(nodeId) != null;
+
+                    metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);
+                }
+            });
+
+            SnapshotMetadataVerificationTaskResult metasRes = new 
SnapshotMetadataVerificationTaskResult(
+                metas,
+                SnapshotChecker.reduceMetasResults(ctx.req.snapshotName(), 
ctx.req.snapshotPath(), metas, null,
+                    kctx.cluster().get().localNode().consistentId())
+            );
+
+            if (!F.isEmpty(metasRes.exceptions()))
+                throw new IgniteSnapshotVerifyException(metasRes.exceptions());
+        }
+        catch (Throwable th) {
+            err = th;
+        }
+
+        phase2PartsHashes.start(reqId, new 
SnapshotCheckProcessRequest(ctx.req, err, metas));
+
+        if (log.isDebugEnabled())
+            log.debug("Started partitions validation as part of the snapshot 
checking [req=" + ctx.req + ']');
+    }
+
+    /** Finds current snapshot name from the metas. */
+    private @Nullable String snpName(@Nullable Map<UUID, CheckResultDTO> 
results) {
+        if (F.isEmpty(results))
+            return null;
+
+        for (CheckResultDTO nodeRes : results.values()) {
+            if (nodeRes == null || F.isEmpty(nodeRes.metas))
+                continue;
+
+            assert nodeRes.metas.get(0) != null : "Empty snapshot metadata in 
the results";
+            assert !F.isEmpty(nodeRes.metas.get(0).snapshotName()) : "Empty 
snapshot name in a snapshot metadata.";
+
+            return nodeRes.metas.get(0).snapshotName();
+        }
+
+        return null;
+    }
+
+    /** Starts the snapshot full validation. */
+    public IgniteInternalFuture<SnapshotPartitionsVerifyTaskResult> start(
+        String snpName,
+        @Nullable String snpPath,
+        @Nullable Collection<String> grpNames,
+        boolean inclCstHndlrs
+    ) {
+        assert !F.isEmpty(snpName);
+
+        UUID reqId = UUID.randomUUID();
+
+        List<UUID> requiredNodes = new 
ArrayList<>(F.viewReadOnly(kctx.discovery().discoCache().aliveBaselineNodes(), 
F.node2id()));
+
+        // Initiator is also required.
+        requiredNodes.add(kctx.localNodeId());
+
+        SnapshotCheckProcessRequest req = new SnapshotCheckProcessRequest(
+            reqId,
+            kctx.localNodeId(),
+            requiredNodes.get((int)(Math.random() * requiredNodes.size())),
+            requiredNodes,
+            snpName,
+            snpPath,
+            grpNames,
+            0,
+            inclCstHndlrs,
+            null
+        );
+
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
new GridFutureAdapter<>();
+
+        clusterOpFut.listen(fut -> {
+            clusterOpFuts.remove(reqId);
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot checking process [req=" + req + 
']');
+        });
+
+        clusterOpFuts.put(reqId, clusterOpFut);
+
+        phase1CheckMetas.start(req.requestId(), req);
+
+        return clusterOpFut;
+    }
+
+    /** Properly sets errror to the cluster operation future. */
+    static boolean finishClusterFutureWithErr(
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut,
+        Throwable propogatedError,
+        Map<ClusterNode, Exception> nodeErrors
+    ) {
+        assert propogatedError != null || !F.isEmpty(nodeErrors);
+
+        if (propogatedError == null)
+            return clusterOpFut.onDone(new 
IgniteSnapshotVerifyException(nodeErrors));
+        else if (propogatedError instanceof IgniteSnapshotVerifyException)
+            return clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(null,
+                new 
IdleVerifyResultV2(((IgniteSnapshotVerifyException)propogatedError).exceptions())));
+        else
+            return clusterOpFut.onDone(propogatedError);
+    }
+
+    /** Converts failure to an exception if it is not. */
+    private static Exception asException(Throwable th) {
+        return th instanceof Exception ? (Exception)th : new 
IgniteException(th);
+    }
+
+    /** Operation context. */
+    private static final class SnapshotCheckContext {
+        /** Request. */
+        private final SnapshotCheckProcessRequest req;
+
+        /** Working future. */
+        private final GridFutureAdapter<CheckResultDTO> fut = new 
GridFutureAdapter<>();
+
+        /** Collected cluster metas. */
+        @Nullable private Map<ClusterNode, List<SnapshotMetadata>> 
clusterMetas;
+        
+        /** Snapshot local metadata. */
+        @Nullable private SnapshotMetadata locMeta;
+
+        /** An error occured. */
+        @Nullable private volatile Throwable err;
+
+        /** Creates operation context. */
+        private SnapshotCheckContext(SnapshotCheckProcessRequest req) {
+            this.req = req;
+        }
+
+        /** */
+        boolean isNodeRequired(UUID nodeId) {

Review Comment:
   prefix `is` isn't used for internal classes.



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