timoninmaxim commented on code in PR #11391:
URL: https://github.com/apache/ignite/pull/11391#discussion_r1711628585


##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,510 @@
+/*
+ * 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.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+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.apache.ignite.internal.util.typedef.internal.U;
+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, 
SnapshotCheckResponse> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
SnapshotCheckResponse> 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 -> 
onNodeLeft(((DiscoveryEvent)evt).eventNode().id()), EVT_NODE_FAILED, 
EVT_NODE_LEFT);
+    }
+
+    /** Expected to run in a discovery-managed thread. */
+    private void onNodeLeft(UUID nodeId) {
+        Throwable err = new ClusterTopologyCheckedException("Snapshot 
validation stopped. A required node left the cluster " +
+            "[nodeId=" + nodeId + ']');
+
+        Iterator<Map.Entry<String, SnapshotCheckContext>> it = 
contexts.entrySet().iterator();
+
+        while (it.hasNext()) {
+            SnapshotCheckContext ctx = it.next().getValue();
+
+            if (!ctx.req.nodes().contains(nodeId))
+                continue;
+
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+
+            it.remove();

Review Comment:
   Why do you remove context here? Reduce phase is already responsible for 
clean up. 



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,510 @@
+/*
+ * 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.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+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.apache.ignite.internal.util.typedef.internal.U;
+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, 
SnapshotCheckResponse> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
SnapshotCheckResponse> 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 -> 
onNodeLeft(((DiscoveryEvent)evt).eventNode().id()), EVT_NODE_FAILED, 
EVT_NODE_LEFT);
+    }
+
+    /** Expected to run in a discovery-managed thread. */
+    private void onNodeLeft(UUID nodeId) {
+        Throwable err = new ClusterTopologyCheckedException("Snapshot 
validation stopped. A required node left the cluster " +
+            "[nodeId=" + nodeId + ']');
+
+        Iterator<Map.Entry<String, SnapshotCheckContext>> it = 
contexts.entrySet().iterator();
+
+        while (it.hasNext()) {
+            SnapshotCheckContext ctx = it.next().getValue();
+
+            if (!ctx.req.nodes().contains(nodeId))
+                continue;
+
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+
+            it.remove();
+
+            GridFutureAdapter<?> fut = clusterOpFuts.get(ctx.req.reqId);
+
+            if (fut != null)
+                fut.onDone(err);
+        }
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> contexts() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param err The interrupt reason.
+     */
+    void interrupt(Throwable err) {
+        contexts.forEach((snpName, ctx) -> {
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+        });
+
+        contexts.clear();
+
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(err));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, SnapshotCheckResponse> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        contexts.remove(ctx.req.snapshotName());
+
+        if (log.isInfoEnabled())
+            log.info("Finished snapshot validation [req=" + ctx.req + ']');
+
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(reqId);
+
+        if (clusterOpFut == null)
+            return new GridFinishedFuture<>();
+
+        assert results.values().stream().noneMatch(res -> res != null && 
res.metas != null);
+
+        if (ctx.req.allRestoreHandlers) {
+            try {
+                if (!F.isEmpty(errors))
+                    throw F.firstValue(errors);
+
+                Map<ClusterNode, Map<String, SnapshotHandlerResult<?>>> cstRes 
= mapCustomHandlersResults(results, ctx.req.nodes());
+
+                
kctx.cache().context().snapshotMgr().checker().checkCustomHandlersResults(ctx.req.snapshotName(),
 cstRes);
+
+                clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, null));
+            }
+            catch (Throwable err) {
+                clusterOpFut.onDone(err);
+            }
+        }
+        else {
+            Map<ClusterNode, Exception> errors0 = mapErrors(errors);
+
+            if (!F.isEmpty(results)) {
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = mapPartsHashes(results, ctx.req.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+            }
+            else {
+                assert !errors0.isEmpty();
+
+                clusterOpFut.onDone(new 
IgniteSnapshotVerifyException(errors0));
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<SnapshotCheckResponse> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx;
+
+        // The context can be null, if a required node leaves before this 
phase.
+        if (!req.nodes().contains(kctx.localNodeId()) || (ctx = context(null, 
req.requestId())) == null || ctx.locMeta == null)

Review Comment:
   does `ctx.locMeta == null` cover case 
`!req.nodes().contains(kctx.localNodeId())`?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,510 @@
+/*
+ * 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.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+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.apache.ignite.internal.util.typedef.internal.U;
+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, 
SnapshotCheckResponse> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
SnapshotCheckResponse> 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 -> 
onNodeLeft(((DiscoveryEvent)evt).eventNode().id()), EVT_NODE_FAILED, 
EVT_NODE_LEFT);
+    }
+
+    /** Expected to run in a discovery-managed thread. */
+    private void onNodeLeft(UUID nodeId) {
+        Throwable err = new ClusterTopologyCheckedException("Snapshot 
validation stopped. A required node left the cluster " +
+            "[nodeId=" + nodeId + ']');
+
+        Iterator<Map.Entry<String, SnapshotCheckContext>> it = 
contexts.entrySet().iterator();
+
+        while (it.hasNext()) {
+            SnapshotCheckContext ctx = it.next().getValue();
+
+            if (!ctx.req.nodes().contains(nodeId))
+                continue;
+
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+
+            it.remove();
+
+            GridFutureAdapter<?> fut = clusterOpFuts.get(ctx.req.reqId);
+
+            if (fut != null)
+                fut.onDone(err);
+        }
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> contexts() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param err The interrupt reason.
+     */
+    void interrupt(Throwable err) {
+        contexts.forEach((snpName, ctx) -> {
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+        });
+
+        contexts.clear();
+
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(err));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, SnapshotCheckResponse> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        contexts.remove(ctx.req.snapshotName());
+
+        if (log.isInfoEnabled())
+            log.info("Finished snapshot validation [req=" + ctx.req + ']');
+
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(reqId);
+
+        if (clusterOpFut == null)
+            return new GridFinishedFuture<>();
+
+        assert results.values().stream().noneMatch(res -> res != null && 
res.metas != null);
+
+        if (ctx.req.allRestoreHandlers) {
+            try {
+                if (!F.isEmpty(errors))
+                    throw F.firstValue(errors);
+
+                Map<ClusterNode, Map<String, SnapshotHandlerResult<?>>> cstRes 
= mapCustomHandlersResults(results, ctx.req.nodes());
+
+                
kctx.cache().context().snapshotMgr().checker().checkCustomHandlersResults(ctx.req.snapshotName(),
 cstRes);
+
+                clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, null));
+            }
+            catch (Throwable err) {
+                clusterOpFut.onDone(err);
+            }
+        }
+        else {
+            Map<ClusterNode, Exception> errors0 = mapErrors(errors);
+
+            if (!F.isEmpty(results)) {
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = mapPartsHashes(results, ctx.req.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+            }
+            else {
+                assert !errors0.isEmpty();
+
+                clusterOpFut.onDone(new 
IgniteSnapshotVerifyException(errors0));
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<SnapshotCheckResponse> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx;
+
+        // The context can be null, if a required node leaves before this 
phase.
+        if (!req.nodes().contains(kctx.localNodeId()) || (ctx = context(null, 
req.requestId())) == null || ctx.locMeta == null)
+            return new GridFinishedFuture<>();
+
+        IgniteSnapshotManager snpMgr = kctx.cache().context().snapshotMgr();
+
+        if (req.allRestoreHandlers) {
+            ctx.fut = new GridFutureAdapter<>();
+
+            snpMgr.checker().invokeCustomHandlers(ctx.locMeta, 
req.snapshotPath(), req.groups(), true)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        ctx.fut.onDone(err);
+                    else
+                        ctx.fut.onDone(new SnapshotCheckResponse(res));
+                });
+        }
+        else {
+            File snpDir = snpMgr.snapshotLocalDir(req.snapshotName(), 
req.snapshotPath());
+
+            GridFutureAdapter<SnapshotCheckResponse> fut = new 
GridFutureAdapter<>();

Review Comment:
   ctx.fut?



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/snapshot/SnapshotCheckProcess.java:
##########
@@ -0,0 +1,510 @@
+/*
+ * 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.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+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.apache.ignite.internal.util.typedef.internal.U;
+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, 
SnapshotCheckResponse> phase1CheckMetas;
+
+    /** Partition hashes second phase subprocess.  */
+    private final DistributedProcess<SnapshotCheckProcessRequest, 
SnapshotCheckResponse> 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 -> 
onNodeLeft(((DiscoveryEvent)evt).eventNode().id()), EVT_NODE_FAILED, 
EVT_NODE_LEFT);
+    }
+
+    /** Expected to run in a discovery-managed thread. */
+    private void onNodeLeft(UUID nodeId) {
+        Throwable err = new ClusterTopologyCheckedException("Snapshot 
validation stopped. A required node left the cluster " +
+            "[nodeId=" + nodeId + ']');
+
+        Iterator<Map.Entry<String, SnapshotCheckContext>> it = 
contexts.entrySet().iterator();
+
+        while (it.hasNext()) {
+            SnapshotCheckContext ctx = it.next().getValue();
+
+            if (!ctx.req.nodes().contains(nodeId))
+                continue;
+
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+
+            it.remove();
+
+            GridFutureAdapter<?> fut = clusterOpFuts.get(ctx.req.reqId);
+
+            if (fut != null)
+                fut.onDone(err);
+        }
+    }
+
+    /** */
+    Map<String, SnapshotCheckContext> contexts() {
+        return Collections.unmodifiableMap(contexts);
+    }
+
+    /**
+     * Stops all the processes with the passed exception.
+     *
+     * @param err The interrupt reason.
+     */
+    void interrupt(Throwable err) {
+        contexts.forEach((snpName, ctx) -> {
+            if (ctx.fut != null)
+                ctx.fut.onDone(err);
+        });
+
+        contexts.clear();
+
+        clusterOpFuts.forEach((reqId, fut) -> fut.onDone(err));
+    }
+
+    /** Phase 2 and process finish. */
+    private IgniteInternalFuture<?> reduceValidatePartsAndFinish(
+        UUID reqId,
+        Map<UUID, SnapshotCheckResponse> results,
+        Map<UUID, Throwable> errors
+    ) {
+        SnapshotCheckContext ctx = context(null, reqId);
+
+        if (ctx == null)
+            return new GridFinishedFuture<>();
+
+        contexts.remove(ctx.req.snapshotName());
+
+        if (log.isInfoEnabled())
+            log.info("Finished snapshot validation [req=" + ctx.req + ']');
+
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(reqId);
+
+        if (clusterOpFut == null)
+            return new GridFinishedFuture<>();
+
+        assert results.values().stream().noneMatch(res -> res != null && 
res.metas != null);
+
+        if (ctx.req.allRestoreHandlers) {
+            try {
+                if (!F.isEmpty(errors))
+                    throw F.firstValue(errors);
+
+                Map<ClusterNode, Map<String, SnapshotHandlerResult<?>>> cstRes 
= mapCustomHandlersResults(results, ctx.req.nodes());
+
+                
kctx.cache().context().snapshotMgr().checker().checkCustomHandlersResults(ctx.req.snapshotName(),
 cstRes);
+
+                clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, null));
+            }
+            catch (Throwable err) {
+                clusterOpFut.onDone(err);
+            }
+        }
+        else {
+            Map<ClusterNode, Exception> errors0 = mapErrors(errors);
+
+            if (!F.isEmpty(results)) {
+                Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
results0 = mapPartsHashes(results, ctx.req.nodes());
+
+                IdleVerifyResultV2 chkRes = 
SnapshotChecker.reduceHashesResults(results0, errors0);
+
+                clusterOpFut.onDone(new 
SnapshotPartitionsVerifyTaskResult(ctx.clusterMetas, chkRes));
+            }
+            else {
+                assert !errors0.isEmpty();
+
+                clusterOpFut.onDone(new 
IgniteSnapshotVerifyException(errors0));
+            }
+        }
+
+        return new GridFinishedFuture<>();
+    }
+
+    /** Phase 2 beginning.  */
+    private IgniteInternalFuture<SnapshotCheckResponse> 
validateParts(SnapshotCheckProcessRequest req) {
+        SnapshotCheckContext ctx;
+
+        // The context can be null, if a required node leaves before this 
phase.
+        if (!req.nodes().contains(kctx.localNodeId()) || (ctx = context(null, 
req.requestId())) == null || ctx.locMeta == null)
+            return new GridFinishedFuture<>();
+
+        IgniteSnapshotManager snpMgr = kctx.cache().context().snapshotMgr();
+
+        if (req.allRestoreHandlers) {
+            ctx.fut = new GridFutureAdapter<>();
+
+            snpMgr.checker().invokeCustomHandlers(ctx.locMeta, 
req.snapshotPath(), req.groups(), true)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        ctx.fut.onDone(err);
+                    else
+                        ctx.fut.onDone(new SnapshotCheckResponse(res));
+                });
+        }
+        else {
+            File snpDir = snpMgr.snapshotLocalDir(req.snapshotName(), 
req.snapshotPath());
+
+            GridFutureAdapter<SnapshotCheckResponse> fut = new 
GridFutureAdapter<>();
+
+            snpMgr.checker().checkPartitions(ctx.locMeta, snpDir, 
req.groups(), false, true, false)
+                .whenComplete((res, err) -> {
+                    if (err != null)
+                        fut.onDone(err);
+                    else
+                        fut.onDone(new SnapshotCheckResponse(res));
+                });
+        }
+
+        return ctx.fut;
+    }
+
+    /** */
+    private Map<ClusterNode, Exception> mapErrors(@Nullable Map<UUID, 
Throwable> errors) {
+        if (F.isEmpty(errors))
+            return Collections.emptyMap();
+
+        return errors.entrySet().stream()
+            .collect(Collectors.toMap(e -> 
kctx.cluster().get().node(e.getKey()),
+                e -> e.getValue() instanceof Exception ? 
(Exception)e.getValue() : new IgniteException(e.getValue())));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<PartitionKeyV2, PartitionHashRecordV2>> 
mapPartsHashes(
+        @Nullable Map<UUID, SnapshotCheckResponse> results,
+        Collection<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        // A not required node can leave the cluster and its result can be 
null.
+        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()));
+    }
+
+    /** */
+    private Map<ClusterNode, Map<String, SnapshotHandlerResult<?>>> 
mapCustomHandlersResults(
+        Map<UUID, SnapshotCheckResponse> results,
+        Set<UUID> requiredNodes
+    ) {
+        if (F.isEmpty(results))
+            return Collections.emptyMap();
+
+        // A not required node can leave the cluster and its result can be 
null.
+        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().customHandlersResults()));
+    }
+
+    /**
+     * @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) {
+        SnapshotCheckContext ctx = snpName == null
+            ? contexts.values().stream().filter(ctx0 -> 
ctx0.req.reqId.equals(reqId)).findFirst().orElse(null)
+            : contexts.get(snpName);
+
+        assert ctx == null || ctx.req.reqId.equals(reqId);
+
+        return ctx;
+    }
+
+    /** Phase 1 beginning: prepare, collect and check local metas. */
+    private IgniteInternalFuture<SnapshotCheckResponse> 
prepareAndCheckMetas(SnapshotCheckProcessRequest req) {
+        if (!req.nodes().contains(kctx.localNodeId()))
+            return new GridFinishedFuture<>();
+
+        SnapshotCheckContext ctx = 
contexts.computeIfAbsent(req.snapshotName(), snpName -> new 
SnapshotCheckContext(req));
+
+        if (!ctx.req.requestId().equals(req.requestId())) {
+            return new GridFinishedFuture<>(new 
IllegalStateException("Validation of snapshot '" + req.snapshotName()
+                + "' has already started. Request=" + ctx + '.'));
+        }
+
+        // Excludes non-baseline initiator.
+        if (!baseline(kctx.localNodeId()))
+            return new GridFinishedFuture<>();
+
+        IgniteSnapshotManager snpMgr = kctx.cache().context().snapshotMgr();
+
+        Collection<Integer> grpIds = F.isEmpty(req.groups()) ? null : 
F.viewReadOnly(req.groups(), CU::cacheId);
+
+        ctx.fut = new GridFutureAdapter<>();
+
+        snpMgr.checker().checkLocalMetas(
+            snpMgr.snapshotLocalDir(req.snapshotName(), req.snapshotPath()),
+            grpIds,
+            kctx.cluster().get().localNode().consistentId()
+        ).whenComplete((locMetas, err) -> {
+            if (err != null)
+                ctx.fut.onDone(err);
+            else
+                ctx.fut.onDone(new SnapshotCheckResponse(locMetas));
+        });
+
+        return ctx.fut;
+    }
+
+    /** Phase 1 end. */
+    private void reducePreparationAndMetasCheck(
+        UUID reqId,
+        Map<UUID, SnapshotCheckResponse> results,
+        Map<UUID, Throwable> errors
+    ) {
+        String snpName = snpName(results);
+
+        SnapshotCheckContext ctx = context(snpName, reqId);
+
+        // The context is not stored in the case of concurrent check of the 
same snapshot but the operation future is registered.
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
clusterOpFuts.get(reqId);
+
+        try {
+            if (!F.isEmpty(errors))
+                throw new IgniteSnapshotVerifyException(mapErrors(errors));
+
+            if (ctx == null)
+                return;
+
+            Map<ClusterNode, List<SnapshotMetadata>> metas = new HashMap<>();
+
+            results.forEach((nodeId, nodeRes) -> {
+                // A node might be not required. It gives null result. But a 
required node might have invalid empty result
+                // which must be validated.
+                if (ctx.req.nodes().contains(nodeId) && baseline(nodeId)) {
+                    assert nodeRes != null && nodeRes.partsResults == null;
+
+                    metas.put(kctx.cluster().get().node(nodeId), 
nodeRes.metas);
+                }
+            });
+
+            Map<ClusterNode, Exception> metasCheck = 
SnapshotChecker.reduceMetasResults(ctx.req.snapshotName(), 
ctx.req.snapshotPath(),
+                metas, null, kctx.cluster().get().localNode().consistentId());
+
+            if (!F.isEmpty(metasCheck))
+                throw new IgniteSnapshotVerifyException(metasCheck);
+
+            List<SnapshotMetadata> locMetas = 
metas.get(kctx.cluster().get().localNode());
+
+            ctx.locMeta = F.isEmpty(locMetas) ? null : locMetas.get(0);
+
+            if (clusterOpFut != null)
+                ctx.clusterMetas = metas;
+
+            if (U.isLocalNodeCoordinator(kctx.discovery()))
+                phase2PartsHashes.start(reqId, ctx.req);
+        }
+        catch (Exception e) {
+            if (ctx != null) {
+                contexts.remove(ctx.req.snapshotName());
+
+                if (log.isInfoEnabled())
+                    log.info("Finished snapshot validation [req=" + ctx.req + 
']');
+            }
+
+            if (clusterOpFut != null)
+                clusterOpFut.onDone(e);
+        }
+    }
+
+    /** Finds current snapshot name from the metas. */
+    private @Nullable String snpName(@Nullable Map<UUID, 
SnapshotCheckResponse> results) {
+        if (F.isEmpty(results))
+            return null;
+
+        for (SnapshotCheckResponse 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 validation process.
+     *
+     * @param snpName Snapshot name.
+     * @param snpPath Snapshot directory path.
+     * @param grpNames List of cache group names.
+     * @param allRestoreHandlers If {@code true}, all the registered {@link 
IgniteSnapshotManager#handlers()} of type
+     *                    {@link SnapshotHandlerType#RESTORE} are invoked. 
Otherwise, only snapshot metadatas and partition
+     *                    hashes are validated.
+     */
+    public IgniteInternalFuture<SnapshotPartitionsVerifyTaskResult> start(
+        String snpName,
+        @Nullable String snpPath,
+        @Nullable Collection<String> grpNames,
+        boolean allRestoreHandlers
+    ) {
+        assert !F.isEmpty(snpName);
+
+        UUID reqId = UUID.randomUUID();
+
+        Set<UUID> requiredNodes = new 
HashSet<>(F.viewReadOnly(kctx.discovery().discoCache().aliveBaselineNodes(), 
F.node2id()));
+
+        // Initiator is also a required node. It collects the final oparation 
result.
+        requiredNodes.add(kctx.localNodeId());
+
+        SnapshotCheckProcessRequest req = new SnapshotCheckProcessRequest(
+            reqId,
+            requiredNodes,
+            snpName,
+            snpPath,
+            grpNames,
+            0,
+            allRestoreHandlers
+        );
+
+        GridFutureAdapter<SnapshotPartitionsVerifyTaskResult> clusterOpFut = 
new GridFutureAdapter<>();
+
+        clusterOpFut.listen(fut -> {
+            clusterOpFuts.remove(reqId);
+
+            if (log.isInfoEnabled())
+                log.info("Finished snapshot validation process [req=" + req + 
']');
+        });
+
+        clusterOpFuts.put(reqId, clusterOpFut);
+
+        phase1CheckMetas.start(req.requestId(), req);
+
+        return clusterOpFut;
+    }
+
+    /** @return {@code True} if the provided node id is id of a baseline node. 
*/
+    private boolean baseline(UUID nodeId) {
+        return CU.baselineNode(kctx.cluster().get().node(nodeId), 
kctx.state().clusterState());
+    }
+
+    /** Operation context. */
+    private static final class SnapshotCheckContext {

Review Comment:
   Let's use same java modifiers for classes `SnapshotCheckContext` and 
`SnapshotCheckResponse` and for their fields and functions.



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