Simulate moves using cluster_concurrent_rebalance=1

BalancedShardAllocator is prone to perform unnecessary moves when
cluster_concurrent_rebalance is set to high values (>2).
(See elastic#87279)
Above allocator is used in DesiredBalanceComputer. Since we do not move actual
shard data during calculation it is possible to artificially set above setting
to 1 to avoid unnecessary moves in desired balance.

Author: idegtiarenko

server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ConcurrentRebalanceAllocationDecider.java

@@ -62,10 +62,17 @@ public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation alloca
 
     @Override
     public Decision canRebalance(RoutingAllocation allocation) {
+        int relocatingShards = allocation.routingNodes().getRelocatingShardCount();
+        if (allocation.isSimulating() && relocatingShards >= 1) {
+            // BalancedShardAllocator is prone to perform unnecessary moves when cluster_concurrent_rebalance is set to high values (>2).
+            // (See https://github.com/elastic/elasticsearch/issues/87279)
+            // Above allocator is used in DesiredBalanceComputer. Since we do not move actual shard data during calculation
+            // it is possible to artificially set above setting to 1 to avoid unnecessary moves in desired balance.
+            return allocation.decision(Decision.THROTTLE, NAME, "allocation should move one shard at the time when simulating");
+        }
         if (clusterConcurrentRebalance == -1) {
             return allocation.decision(Decision.YES, NAME, "unlimited concurrent rebalances are allowed");
         }
-        int relocatingShards = allocation.routingNodes().getRelocatingShardCount();
         if (relocatingShards >= clusterConcurrentRebalance) {
             return allocation.decision(
                 Decision.THROTTLE,

server/src/test/java/org/elasticsearch/cluster/routing/allocation/IndexBalanceTests.java

@@ -12,16 +12,27 @@
 import org.apache.logging.log4j.Logger;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.ESAllocationTestCase;
 import org.elasticsearch.cluster.TestShardRoutingRoleStrategies;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.metadata.Metadata;
 import org.elasticsearch.cluster.node.DiscoveryNodes;
+import org.elasticsearch.cluster.routing.AllocationId;
+import org.elasticsearch.cluster.routing.IndexRoutingTable;
 import org.elasticsearch.cluster.routing.RoutingNodes;
 import org.elasticsearch.cluster.routing.RoutingTable;
+import org.elasticsearch.cluster.routing.ShardRoutingState;
+import org.elasticsearch.cluster.routing.TestShardRouting;
+import org.elasticsearch.cluster.routing.allocation.allocator.DesiredBalanceShardsAllocator;
 import org.elasticsearch.cluster.routing.allocation.decider.ClusterRebalanceAllocationDecider;
+import org.elasticsearch.common.UUIDs;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.shard.ShardId;
+
+import java.util.Set;
+import java.util.function.IntFunction;
 
 import static org.elasticsearch.cluster.routing.ShardRoutingState.INITIALIZING;
 import static org.elasticsearch.cluster.routing.ShardRoutingState.STARTED;
@@ -440,4 +451,76 @@ public void testBalanceAllNodesStartedAddIndex() {
         assertThat(routingNodes.node("node2").shardsWithState("test1", STARTED).count(), equalTo(2L));
         assertThat(routingNodes.node("node3").shardsWithState("test1", STARTED).count(), equalTo(2L));
     }
+
+    /**
+     * {@see https://github.com/elastic/elasticsearch/issues/87279}
+     */
+    public void testRebalance() {
+        final var settings = Settings.builder()
+            .put("cluster.routing.allocation.cluster_concurrent_rebalance", randomIntBetween(3, 9))
+            .build();
+        final var allocationService = createAllocationService(settings);
+        assumeTrue("", allocationService.shardsAllocator instanceof DesiredBalanceShardsAllocator);
+
+        final var discoveryNodesBuilder = DiscoveryNodes.builder();
+        for (int nodeIndex = 0; nodeIndex < 3; nodeIndex++) {
+            discoveryNodesBuilder.add(newNode("node-" + nodeIndex));
+        }
+
+        final var metadataBuilder = Metadata.builder();
+        final var routingTableBuilder = RoutingTable.builder();
+
+        addIndex(metadataBuilder, routingTableBuilder, "index-0", 4, shardId -> "node-" + (shardId / 2));
+        addIndex(metadataBuilder, routingTableBuilder, "index-1", 4, shardId -> "node-" + (shardId / 2));
+        addIndex(metadataBuilder, routingTableBuilder, "index-2", 1, shardId -> "node-2");
+
+        final var clusterState = ClusterState.builder(ClusterName.DEFAULT)
+            .nodes(discoveryNodesBuilder)
+            .metadata(metadataBuilder)
+            .routingTable(routingTableBuilder)
+            .build();
+
+        final var reroutedState = applyStartedShardsUntilNoChange(clusterState, allocationService);
+        // node-0 and node-1 has 4 shards each. node-2 has only [index-2][0]. It should not be moved to achieve even balance.
+        assertThat(reroutedState.getRoutingTable().index("index-2").shard(0).primaryShard().currentNodeId(), equalTo("node-2"));
+    }
+
+    private static void addIndex(
+        Metadata.Builder metadataBuilder,
+        RoutingTable.Builder routingTableBuilder,
+        String indexName,
+        int numberOfShards,
+        IntFunction<String> assignmentFunction
+    ) {
+        final var inSyncIds = randomList(numberOfShards, numberOfShards, () -> UUIDs.randomBase64UUID(random()));
+        final var indexMetadataBuilder = IndexMetadata.builder(indexName)
+            .settings(
+                Settings.builder()
+                    .put("index.number_of_shards", numberOfShards)
+                    .put("index.number_of_replicas", 0)
+                    .put("index.version.created", Version.CURRENT)
+                    .build()
+            );
+        for (int shardId = 0; shardId < numberOfShards; shardId++) {
+            indexMetadataBuilder.putInSyncAllocationIds(shardId, Set.of(inSyncIds.get(shardId)));
+        }
+        metadataBuilder.put(indexMetadataBuilder);
+        final var indexId = metadataBuilder.get(indexName).getIndex();
+        final var indexRoutingTableBuilder = IndexRoutingTable.builder(indexId);
+
+        for (int shardId = 0; shardId < numberOfShards; shardId++) {
+            indexRoutingTableBuilder.addShard(
+                TestShardRouting.newShardRouting(
+                    new ShardId(indexId, shardId),
+                    assignmentFunction.apply(shardId),
+                    null,
+                    true,
+                    ShardRoutingState.STARTED,
+                    AllocationId.newInitializing(inSyncIds.get(shardId))
+                )
+            );
+        }
+
+        routingTableBuilder.add(indexRoutingTableBuilder);
+    }
 }

test/framework/src/main/java/org/elasticsearch/cluster/ESAllocationTestCase.java

@@ -337,7 +337,8 @@ protected static class MockAllocationService extends AllocationService {
 
         private volatile long nanoTimeOverride = -1L;
 
-        private final GatewayAllocator gatewayAllocator;
+        public final GatewayAllocator gatewayAllocator;
+        public final ShardsAllocator shardsAllocator;
 
         public MockAllocationService(
             AllocationDeciders allocationDeciders,
@@ -355,6 +356,7 @@ public MockAllocationService(
                 TestShardRoutingRoleStrategies.DEFAULT_ROLE_ONLY
             );
             this.gatewayAllocator = gatewayAllocator;
+            this.shardsAllocator = shardsAllocator;
         }
 
         public void setNanoTimeOverride(long nanoTime) {
@@ -365,10 +367,6 @@ public void setNanoTimeOverride(long nanoTime) {
         protected long currentNanoTime() {
             return nanoTimeOverride == -1L ? super.currentNanoTime() : nanoTimeOverride;
         }
-
-        public GatewayAllocator getGatewayAllocator() {
-            return gatewayAllocator;
-        }
     }
 
     /**