Trigger replica recovery restarts by master when primary relocation completes (#23926)

When a primary relocation completes while there are ongoing replica recoveries, the recoveries for these replicas need to be restarted (as a new primary is in charge of replicating changes). Before this commit, the need for a recovery restart was detected by the data nodes that had the replicas, by checking on each cluster state update if the recovery process had completed before the recovery source changed. That code had a race, however, which could lead to a not-fully recovered shard exposing itself as started (see #23904).

This commit takes a different approach: When the primary relocation completes and the master updates the cluster state to move the primary shard from relocating to started, it will reinitialize all initializing replica shards, by giving them a fresh allocation id. Data nodes that have the replica shard will simply detect that the allocation id changed and restart the recovery process (instead of trying to determine the need to restart based on ongoing recoveries).

Note: Removal of the code in IndicesClusterStateService that checks whether the recovery source has changed will not be backported to the 5.x branch. This ensures backward compatibility for the situation where the master node is older and does not have the code changes that have been introduced in this PR.

Closes #23904

core/src/main/java/org/elasticsearch/cluster/routing/RoutingChangesObserver.java

@@ -69,6 +69,12 @@ public interface RoutingChangesObserver {
      */
     void replicaPromoted(ShardRouting replicaShard);
 
+    /**
+     * Called when an initializing replica is reinitialized. This happens when a primary relocation completes, which
+     * reinitializes all currently initializing replicas as their recovery source node changes
+     */
+    void initializedReplicaReinitialized(ShardRouting oldReplica, ShardRouting reinitializedReplica);
+
 
     /**
      * Abstract implementation of {@link RoutingChangesObserver} that does not take any action. Useful for subclasses that only override
@@ -120,6 +126,11 @@ public interface RoutingChangesObserver {
         public void replicaPromoted(ShardRouting replicaShard) {
 
         }
+
+        @Override
+        public void initializedReplicaReinitialized(ShardRouting oldReplica, ShardRouting reinitializedReplica) {
+
+        }
     }
 
     class DelegatingRoutingChangesObserver implements RoutingChangesObserver {
@@ -192,5 +203,12 @@ public interface RoutingChangesObserver {
                 routingChangesObserver.replicaPromoted(replicaShard);
             }
         }
+
+        @Override
+        public void initializedReplicaReinitialized(ShardRouting oldReplica, ShardRouting reinitializedReplica) {
+            for (RoutingChangesObserver routingChangesObserver : routingChangesObservers) {
+                routingChangesObserver.initializedReplicaReinitialized(oldReplica, reinitializedReplica);
+            }
+        }
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/RoutingNodes.java

@@ -451,6 +451,9 @@ public class RoutingNodes implements Iterable<RoutingNode> {
      *
      * Moves the initializing shard to started. If the shard is a relocation target, also removes the relocation source.
      *
+     * If the started shard is a primary relocation target, this also reinitializes currently initializing replicas as their
+     * recovery source changes
+     *
      * @return the started shard
      */
     public ShardRouting startShard(Logger logger, ShardRouting initializingShard, RoutingChangesObserver routingChangesObserver) {
@@ -468,6 +471,30 @@ public class RoutingNodes implements Iterable<RoutingNode> {
                 + initializingShard + " but was: " + relocationSourceShard.getTargetRelocatingShard();
             remove(relocationSourceShard);
             routingChangesObserver.relocationCompleted(relocationSourceShard);
+
+            // if this is a primary shard with ongoing replica recoveries, reinitialize them as their recovery source changed
+            if (startedShard.primary()) {
+                List<ShardRouting> assignedShards = assignedShards(startedShard.shardId());
+                // copy list to prevent ConcurrentModificationException
+                for (ShardRouting routing : new ArrayList<>(assignedShards)) {
+                    if (routing.initializing() && routing.primary() == false) {
+                        if (routing.isRelocationTarget()) {
+                            // find the relocation source
+                            ShardRouting sourceShard = getByAllocationId(routing.shardId(), routing.allocationId().getRelocationId());
+                            // cancel relocation and start relocation to same node again
+                            ShardRouting startedReplica = cancelRelocation(sourceShard);
+                            remove(routing);
+                            routingChangesObserver.shardFailed(routing,
+                                new UnassignedInfo(UnassignedInfo.Reason.REINITIALIZED, "primary changed"));
+                            relocateShard(startedReplica, sourceShard.relocatingNodeId(),
+                                sourceShard.getExpectedShardSize(), routingChangesObserver);
+                        } else {
+                            ShardRouting reinitializedReplica = reinitReplica(routing);
+                            routingChangesObserver.initializedReplicaReinitialized(routing, reinitializedReplica);
+                        }
+                    }
+                }
+            }
         }
         return startedShard;
     }
@@ -730,6 +757,15 @@ public class RoutingNodes implements Iterable<RoutingNode> {
         return reinitializedShard;
     }
 
+    private ShardRouting reinitReplica(ShardRouting shard) {
+        assert shard.primary() == false : "shard must be a replica: " + shard;
+        assert shard.initializing() : "can only reinitialize an initializing replica: " + shard;
+        assert shard.isRelocationTarget() == false : "replication target cannot be reinitialized: " + shard;
+        ShardRouting reinitializedShard = shard.reinitializeReplicaShard();
+        updateAssigned(shard, reinitializedShard);
+        return reinitializedShard;
+    }
+
     private void updateAssigned(ShardRouting oldShard, ShardRouting newShard) {
         assert oldShard.shardId().equals(newShard.shardId()) :
             "can only update " + oldShard + " by shard with same shard id but was " + newShard;

core/src/main/java/org/elasticsearch/cluster/routing/ShardRouting.java

@@ -393,6 +393,17 @@ public final class ShardRouting implements Writeable, ToXContent {
             allocationId, UNAVAILABLE_EXPECTED_SHARD_SIZE);
     }
 
+    /**
+     * Reinitializes a replica shard, giving it a fresh allocation id
+     */
+    public ShardRouting reinitializeReplicaShard() {
+        assert state == ShardRoutingState.INITIALIZING : this;
+        assert primary == false : this;
+        assert isRelocationTarget() == false : this;
+        return new ShardRouting(shardId, currentNodeId, null, primary, ShardRoutingState.INITIALIZING,
+            recoverySource, unassignedInfo, AllocationId.newInitializing(), expectedShardSize);
+    }
+
     /**
      * Set the shards state to <code>STARTED</code>. The shards state must be
      * <code>INITIALIZING</code> or <code>RELOCATING</code>. Any relocation will be

core/src/main/java/org/elasticsearch/cluster/routing/allocation/AllocationService.java

@@ -41,7 +41,9 @@ import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.gateway.GatewayAllocator;
 
+import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.Iterator;
 import java.util.List;
 import java.util.function.Function;
@@ -88,6 +90,9 @@ public class AllocationService extends AbstractComponent {
         routingNodes.unassigned().shuffle();
         RoutingAllocation allocation = new RoutingAllocation(allocationDeciders, routingNodes, clusterState,
             clusterInfoService.getClusterInfo(), currentNanoTime(), false);
+        // as starting a primary relocation target can reinitialize replica shards, start replicas first
+        startedShards = new ArrayList<>(startedShards);
+        Collections.sort(startedShards, Comparator.comparing(ShardRouting::primary));
         applyStartedShards(allocation, startedShards);
         gatewayAllocator.applyStartedShards(allocation, startedShards);
         reroute(allocation);

core/src/main/java/org/elasticsearch/cluster/routing/allocation/RoutingNodesChangedObserver.java

@@ -96,6 +96,17 @@ public class RoutingNodesChangedObserver implements RoutingChangesObserver {
         setChanged();
     }
 
+    @Override
+    public void initializedReplicaReinitialized(ShardRouting oldReplica, ShardRouting reinitializedReplica) {
+        assert oldReplica.initializing() && oldReplica.primary() == false :
+            "expected initializing replica shard " + oldReplica;
+        assert reinitializedReplica.initializing() && reinitializedReplica.primary() == false :
+            "expected reinitialized replica shard " + reinitializedReplica;
+        assert oldReplica.allocationId().getId().equals(reinitializedReplica.allocationId().getId()) == false :
+            "expected allocation id to change for reinitialized replica shard (old: " + oldReplica + " new: " + reinitializedReplica + ")";
+        setChanged();
+    }
+
     /**
      * Marks the allocation as changed.
      */

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -416,6 +416,9 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
                 // active primaries.
                 throw new IndexShardRelocatedException(shardId(), "Shard is marked as relocated, cannot safely move to state " + newRouting.state());
             }
+            assert newRouting.active() == false || state == IndexShardState.STARTED || state == IndexShardState.RELOCATED ||
+                state == IndexShardState.CLOSED :
+                "routing is active, but local shard state isn't. routing: " + newRouting + ", local state: " + state;
             this.shardRouting = newRouting;
             persistMetadata(newRouting, currentRouting);
         }
@@ -498,6 +501,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
      * @return the previous shard state
      */
     private IndexShardState changeState(IndexShardState newState, String reason) {
+        assert Thread.holdsLock(mutex);
         logger.debug("state: [{}]->[{}], reason [{}]", state, newState, reason);
         IndexShardState previousState = state;
         state = newState;

core/src/main/java/org/elasticsearch/indices/cluster/IndicesClusterStateService.java

@@ -403,20 +403,6 @@ public class IndicesClusterStateService extends AbstractLifecycleComponent imple
                     // state may result in a new shard being initialized while having the same allocation id as the currently started shard.
                     logger.debug("{} removing shard (not active, current {}, new {})", shardId, currentRoutingEntry, newShardRouting);
                     indexService.removeShard(shardId.id(), "removing shard (stale copy)");
-                } else {
-                    // remove shards where recovery source has changed. This re-initializes shards later in createOrUpdateShards
-                    if (newShardRouting.recoverySource() != null && newShardRouting.recoverySource().getType() == Type.PEER) {
-                        RecoveryState recoveryState = shard.recoveryState();
-                        final DiscoveryNode sourceNode = findSourceNodeForPeerRecovery(logger, routingTable, nodes, newShardRouting);
-                        if (recoveryState.getSourceNode().equals(sourceNode) == false) {
-                            if (recoveryTargetService.cancelRecoveriesForShard(shardId, "recovery source node changed")) {
-                                // getting here means that the shard was still recovering
-                                logger.debug("{} removing shard (recovery source changed), current [{}], global [{}], shard [{}])",
-                                    shardId, recoveryState.getSourceNode(), sourceNode, newShardRouting);
-                                indexService.removeShard(shardId.id(), "removing shard (recovery source node changed)");
-                            }
-                        }
-                    }
                 }
             }
         }

core/src/main/java/org/elasticsearch/indices/recovery/PeerRecoveryTargetService.java

@@ -126,17 +126,6 @@ public class PeerRecoveryTargetService extends AbstractComponent implements Inde
         }
     }
 
-    /**
-     * Cancel all ongoing recoveries for the given shard.
-     *
-     * @param reason  reason for cancellation
-     * @param shardId shard ID for which to cancel recoveries
-     * @return {@code true} if a recovery was cancelled
-     */
-    public boolean cancelRecoveriesForShard(ShardId shardId, String reason) {
-        return onGoingRecoveries.cancelRecoveriesForShard(shardId, reason);
-    }
-
     public void startRecovery(final IndexShard indexShard, final DiscoveryNode sourceNode, final RecoveryListener listener) {
         // create a new recovery status, and process...
         final long recoveryId = onGoingRecoveries.startRecovery(indexShard, sourceNode, listener, recoverySettings.activityTimeout());

core/src/test/java/org/elasticsearch/cluster/routing/allocation/StartedShardsRoutingTests.java

@@ -33,11 +33,14 @@ import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.ShardRoutingState;
 import org.elasticsearch.cluster.routing.TestShardRouting;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.shard.ShardId;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.List;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.not;
@@ -84,4 +87,69 @@ public class StartedShardsRoutingTests extends ESAllocationTestCase {
         assertThat(shardRouting.currentNodeId(), equalTo("node2"));
         assertThat(shardRouting.relocatingNodeId(), nullValue());
     }
+
+    public void testRelocatingPrimariesWithInitializingReplicas() {
+        AllocationService allocation = createAllocationService();
+
+        logger.info("--> building initial cluster state");
+        AllocationId primaryId = AllocationId.newRelocation(AllocationId.newInitializing());
+        AllocationId replicaId = AllocationId.newInitializing();
+        boolean relocatingReplica = randomBoolean();
+        if (relocatingReplica) {
+            replicaId = AllocationId.newRelocation(replicaId);
+        }
+
+        final IndexMetaData indexMetaData = IndexMetaData.builder("test")
+            .settings(settings(Version.CURRENT))
+            .numberOfShards(1).numberOfReplicas(1)
+            .putInSyncAllocationIds(0,
+                relocatingReplica ? Sets.newHashSet(primaryId.getId(), replicaId.getId()) : Sets.newHashSet(primaryId.getId()))
+            .build();
+        final Index index = indexMetaData.getIndex();
+        ClusterState.Builder stateBuilder = ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING.getDefault(Settings.EMPTY))
+            .nodes(DiscoveryNodes.builder().add(newNode("node1")).add(newNode("node2")).add(newNode("node3")).add(newNode("node4")))
+            .metaData(MetaData.builder().put(indexMetaData, false));
+
+        final ShardRouting relocatingPrimary = TestShardRouting.newShardRouting(
+            new ShardId(index, 0), "node1", "node2", true, ShardRoutingState.RELOCATING, primaryId);
+        final ShardRouting replica = TestShardRouting.newShardRouting(
+            new ShardId(index, 0), "node3", relocatingReplica ? "node4" : null, false,
+            relocatingReplica ? ShardRoutingState.RELOCATING : ShardRoutingState.INITIALIZING, replicaId);
+
+        stateBuilder.routingTable(RoutingTable.builder().add(IndexRoutingTable.builder(index)
+            .addIndexShard(new IndexShardRoutingTable.Builder(relocatingPrimary.shardId())
+                .addShard(relocatingPrimary)
+                .addShard(replica)
+                .build()))
+            .build());
+
+
+        ClusterState state = stateBuilder.build();
+
+        logger.info("--> test starting of relocating primary shard with initializing / relocating replica");
+        ClusterState newState = allocation.applyStartedShards(state, Arrays.asList(relocatingPrimary.getTargetRelocatingShard()));
+        assertNotEquals(newState, state);
+        assertTrue(newState.routingTable().index("test").allPrimaryShardsActive());
+        ShardRouting startedReplica = newState.routingTable().index("test").shard(0).replicaShards().get(0);
+        if (relocatingReplica) {
+            assertTrue(startedReplica.relocating());
+            assertEquals(replica.currentNodeId(), startedReplica.currentNodeId());
+            assertEquals(replica.relocatingNodeId(), startedReplica.relocatingNodeId());
+            assertEquals(replica.allocationId().getId(), startedReplica.allocationId().getId());
+            assertNotEquals(replica.allocationId().getRelocationId(), startedReplica.allocationId().getRelocationId());
+        } else {
+            assertTrue(startedReplica.initializing());
+            assertEquals(replica.currentNodeId(), startedReplica.currentNodeId());
+            assertNotEquals(replica.allocationId().getId(), startedReplica.allocationId().getId());
+        }
+
+        logger.info("--> test starting of relocating primary shard together with initializing / relocating replica");
+        List<ShardRouting> startedShards = new ArrayList<>();
+        startedShards.add(relocatingPrimary.getTargetRelocatingShard());
+        startedShards.add(relocatingReplica ? replica.getTargetRelocatingShard() : replica);
+        Collections.shuffle(startedShards, random());
+        newState = allocation.applyStartedShards(state, startedShards);
+        assertNotEquals(newState, state);
+        assertTrue(newState.routingTable().index("test").shard(0).allShardsStarted());
+    }
 }

core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -284,7 +284,7 @@ public class IndexShardTests extends IndexShardTestCase {
                 true, ShardRoutingState.INITIALIZING, AllocationId.newRelocation(AllocationId.newInitializing())));
         } else if (randomBoolean()) {
             // simulate promotion
-            indexShard = newShard(shardId, false);
+            indexShard = newStartedShard(false);
             ShardRouting replicaRouting = indexShard.routingEntry();
             indexShard.updatePrimaryTerm(indexShard.getPrimaryTerm() + 1);
             ShardRouting primaryRouting = TestShardRouting.newShardRouting(replicaRouting.shardId(), replicaRouting.currentNodeId(), null,

core/src/test/java/org/elasticsearch/indices/cluster/ClusterStateChanges.java

@@ -41,9 +41,12 @@ import org.elasticsearch.action.support.master.MasterNodeRequest;
 import org.elasticsearch.action.support.master.TransportMasterNodeAction;
 import org.elasticsearch.action.support.master.TransportMasterNodeActionUtils;
 import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.ClusterStateTaskExecutor;
+import org.elasticsearch.cluster.ClusterStateTaskExecutor.ClusterTasksResult;
 import org.elasticsearch.cluster.ClusterStateUpdateTask;
 import org.elasticsearch.cluster.EmptyClusterInfoService;
 import org.elasticsearch.cluster.action.shard.ShardStateAction;
+import org.elasticsearch.cluster.action.shard.ShardStateAction.ShardEntry;
 import org.elasticsearch.cluster.metadata.AliasValidator;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
@@ -217,23 +220,29 @@ public class ClusterStateChanges extends AbstractComponent {
     }
 
     public ClusterState applyFailedShards(ClusterState clusterState, List<FailedShard> failedShards) {
-        List<ShardStateAction.ShardEntry> entries = failedShards.stream().map(failedShard ->
-            new ShardStateAction.ShardEntry(failedShard.getRoutingEntry().shardId(), failedShard.getRoutingEntry().allocationId().getId(),
+        List<ShardEntry> entries = failedShards.stream().map(failedShard ->
+            new ShardEntry(failedShard.getRoutingEntry().shardId(), failedShard.getRoutingEntry().allocationId().getId(),
                 0L, failedShard.getMessage(), failedShard.getFailure()))
             .collect(Collectors.toList());
-        try {
-            return shardFailedClusterStateTaskExecutor.execute(clusterState, entries).resultingState;
-        } catch (Exception e) {
-            throw ExceptionsHelper.convertToRuntime(e);
-        }
+        return runTasks(shardFailedClusterStateTaskExecutor, clusterState, entries);
     }
 
     public ClusterState applyStartedShards(ClusterState clusterState, List<ShardRouting> startedShards) {
-        List<ShardStateAction.ShardEntry> entries = startedShards.stream().map(startedShard ->
-            new ShardStateAction.ShardEntry(startedShard.shardId(), startedShard.allocationId().getId(), 0L, "shard started", null))
+        List<ShardEntry> entries = startedShards.stream().map(startedShard ->
+            new ShardEntry(startedShard.shardId(), startedShard.allocationId().getId(), 0L, "shard started", null))
             .collect(Collectors.toList());
+        return runTasks(shardStartedClusterStateTaskExecutor, clusterState, entries);
+    }
+
+    private <T> ClusterState runTasks(ClusterStateTaskExecutor<T> executor, ClusterState clusterState, List<T> entries) {
         try {
-            return shardStartedClusterStateTaskExecutor.execute(clusterState, entries).resultingState;
+            ClusterTasksResult<T> result = executor.execute(clusterState, entries);
+            for (ClusterStateTaskExecutor.TaskResult taskResult : result.executionResults.values()) {
+                if (taskResult.isSuccess() == false) {
+                    throw taskResult.getFailure();
+                }
+            }
+            return result.resultingState;
         } catch (Exception e) {
             throw ExceptionsHelper.convertToRuntime(e);
         }