Short-circuit rebalancing when disabled (#40966)

Today if `cluster.routing.rebalance.enable: none` then rebalancing is disabled,
but we still execute `balanceByWeights()` and perform some rather expensive
calculations before discovering that we cannot rebalance any shards. In a large
cluster this can make cluster state updates occur rather slowly. With this
change we check earlier whether rebalancing is globally disabled and, if so,
avoid the rebalancing process entirely.

Relates #40942 which was reverted because of egregiously faulty tests.

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

@@ -86,16 +86,21 @@ public class EnableAllocationDecider extends AllocationDecider {
         clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING, this::setEnableRebalance);
     }
 
-    public void setEnableRebalance(Rebalance enableRebalance) {
+    private void setEnableRebalance(Rebalance enableRebalance) {
         this.enableRebalance = enableRebalance;
     }
 
-    public void setEnableAllocation(Allocation enableAllocation) {
+    private void setEnableAllocation(Allocation enableAllocation) {
         this.enableAllocation = enableAllocation;
     }
 
     @Override
     public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
+        return canAllocate(shardRouting, allocation);
+    }
+
+    @Override
+    public Decision canAllocate(ShardRouting shardRouting, RoutingAllocation allocation) {
         if (allocation.ignoreDisable()) {
             return allocation.decision(Decision.YES, NAME,
                 "explicitly ignoring any disabling of allocation due to manual allocation commands via the reroute API");
@@ -136,10 +141,29 @@ public class EnableAllocationDecider extends AllocationDecider {
         }
     }
 
+    @Override
+    public Decision canRebalance(RoutingAllocation allocation) {
+        if (allocation.ignoreDisable()) {
+            return allocation.decision(Decision.YES, NAME, "allocation is explicitly ignoring any disabling of rebalancing");
+        }
+
+        if (enableRebalance == Rebalance.NONE) {
+            for (IndexMetaData indexMetaData : allocation.metaData()) {
+                if (INDEX_ROUTING_REBALANCE_ENABLE_SETTING.exists(indexMetaData.getSettings())
+                    && INDEX_ROUTING_REBALANCE_ENABLE_SETTING.get(indexMetaData.getSettings()) != Rebalance.NONE) {
+                    return allocation.decision(Decision.YES, NAME, "rebalancing is permitted on one or more indices");
+                }
+            }
+            return allocation.decision(Decision.NO, NAME, "no rebalancing is allowed due to %s", setting(enableRebalance, false));
+        }
+
+        return allocation.decision(Decision.YES, NAME, "rebalancing is not globally disabled");
+    }
+
     @Override
     public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
         if (allocation.ignoreDisable()) {
-            return allocation.decision(Decision.YES, NAME, "allocation is explicitly ignoring any disabling of relocation");
+            return allocation.decision(Decision.YES, NAME, "allocation is explicitly ignoring any disabling of rebalancing");
         }
 
         Settings indexSettings = allocation.metaData().getIndexSafe(shardRouting.index()).getSettings();

server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/EnableAllocationShortCircuitTests.java

@@ -0,0 +1,233 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.elasticsearch.cluster.routing.allocation.decider;
+
+import org.elasticsearch.Version;
+import org.elasticsearch.cluster.ClusterModule;
+import org.elasticsearch.cluster.ClusterName;
+import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.ESAllocationTestCase;
+import org.elasticsearch.cluster.EmptyClusterInfoService;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.cluster.node.DiscoveryNodes;
+import org.elasticsearch.cluster.routing.RoutingNode;
+import org.elasticsearch.cluster.routing.RoutingTable;
+import org.elasticsearch.cluster.routing.ShardRouting;
+import org.elasticsearch.cluster.routing.ShardRoutingState;
+import org.elasticsearch.cluster.routing.allocation.AllocationService;
+import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
+import org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator;
+import org.elasticsearch.common.settings.ClusterSettings;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.plugins.ClusterPlugin;
+import org.elasticsearch.test.gateway.TestGatewayAllocator;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+import static org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider.CLUSTER_ROUTING_ALLOCATION_ENABLE_SETTING;
+import static org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider.CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING;
+import static org.elasticsearch.cluster.routing.allocation.decider.EnableAllocationDecider.INDEX_ROUTING_REBALANCE_ENABLE_SETTING;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.greaterThan;
+
+public class EnableAllocationShortCircuitTests extends ESAllocationTestCase {
+
+    private static ClusterState createClusterStateWithAllShardsAssigned() {
+        AllocationService allocationService = createAllocationService(Settings.EMPTY);
+
+        final int numberOfNodes = randomIntBetween(1, 5);
+        final DiscoveryNodes.Builder discoveryNodesBuilder = DiscoveryNodes.builder();
+        for (int i = 0; i < numberOfNodes; i++) {
+            discoveryNodesBuilder.add(newNode("node" + i));
+        }
+
+        final MetaData.Builder metadataBuilder = MetaData.builder();
+        final RoutingTable.Builder routingTableBuilder = RoutingTable.builder();
+        for (int i = randomIntBetween(1, 10); i >= 0; i--) {
+            final IndexMetaData indexMetaData = IndexMetaData.builder("test" + i).settings(settings(Version.CURRENT))
+                .numberOfShards(1).numberOfReplicas(randomIntBetween(0, numberOfNodes - 1)).build();
+            metadataBuilder.put(indexMetaData, true);
+            routingTableBuilder.addAsNew(indexMetaData);
+        }
+
+        ClusterState clusterState = ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING.get(Settings.EMPTY))
+            .nodes(discoveryNodesBuilder).metaData(metadataBuilder).routingTable(routingTableBuilder.build()).build();
+
+        while (clusterState.getRoutingNodes().hasUnassignedShards()
+               || clusterState.getRoutingNodes().shardsWithState(ShardRoutingState.INITIALIZING).isEmpty() == false) {
+            clusterState = allocationService.applyStartedShards(clusterState,
+                clusterState.getRoutingNodes().shardsWithState(ShardRoutingState.INITIALIZING));
+            clusterState = allocationService.reroute(clusterState, "reroute");
+        }
+
+        return clusterState;
+    }
+
+    public void testRebalancingAttemptedIfPermitted() {
+        ClusterState clusterState = createClusterStateWithAllShardsAssigned();
+
+        final RebalanceShortCircuitPlugin plugin = new RebalanceShortCircuitPlugin();
+        AllocationService allocationService = createAllocationService(Settings.builder()
+                .put(CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING.getKey(),
+                    randomFrom(EnableAllocationDecider.Rebalance.ALL,
+                        EnableAllocationDecider.Rebalance.PRIMARIES,
+                        EnableAllocationDecider.Rebalance.REPLICAS).name()),
+            plugin);
+        allocationService.reroute(clusterState, "reroute").routingTable();
+        assertThat(plugin.rebalanceAttempts, greaterThan(0));
+    }
+
+    public void testRebalancingSkippedIfDisabled() {
+        ClusterState clusterState = createClusterStateWithAllShardsAssigned();
+
+        final RebalanceShortCircuitPlugin plugin = new RebalanceShortCircuitPlugin();
+        AllocationService allocationService = createAllocationService(Settings.builder()
+                .put(CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING.getKey(), EnableAllocationDecider.Allocation.NONE.name()),
+            plugin);
+        allocationService.reroute(clusterState, "reroute").routingTable();
+        assertThat(plugin.rebalanceAttempts, equalTo(0));
+    }
+
+    public void testRebalancingSkippedIfDisabledIncludingOnSpecificIndices() {
+        ClusterState clusterState = createClusterStateWithAllShardsAssigned();
+        final IndexMetaData indexMetaData = randomFrom(clusterState.metaData().indices().values().toArray(IndexMetaData.class));
+        clusterState = ClusterState.builder(clusterState).metaData(MetaData.builder(clusterState.metaData())
+            .put(IndexMetaData.builder(indexMetaData).settings(Settings.builder().put(indexMetaData.getSettings())
+                .put(INDEX_ROUTING_REBALANCE_ENABLE_SETTING.getKey(), EnableAllocationDecider.Rebalance.NONE.name()))).build()).build();
+
+        final RebalanceShortCircuitPlugin plugin = new RebalanceShortCircuitPlugin();
+        AllocationService allocationService = createAllocationService(Settings.builder()
+                .put(CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING.getKey(), EnableAllocationDecider.Rebalance.NONE.name()),
+            plugin);
+        allocationService.reroute(clusterState, "reroute").routingTable();
+        assertThat(plugin.rebalanceAttempts, equalTo(0));
+    }
+
+    public void testRebalancingAttemptedIfDisabledButOverridenOnSpecificIndices() {
+        ClusterState clusterState = createClusterStateWithAllShardsAssigned();
+        final IndexMetaData indexMetaData = randomFrom(clusterState.metaData().indices().values().toArray(IndexMetaData.class));
+        clusterState = ClusterState.builder(clusterState).metaData(MetaData.builder(clusterState.metaData())
+            .put(IndexMetaData.builder(indexMetaData).settings(Settings.builder().put(indexMetaData.getSettings())
+                .put(INDEX_ROUTING_REBALANCE_ENABLE_SETTING.getKey(),
+                    randomFrom(EnableAllocationDecider.Rebalance.ALL,
+                        EnableAllocationDecider.Rebalance.PRIMARIES,
+                        EnableAllocationDecider.Rebalance.REPLICAS).name()))).build()).build();
+
+        final RebalanceShortCircuitPlugin plugin = new RebalanceShortCircuitPlugin();
+        AllocationService allocationService = createAllocationService(Settings.builder()
+                .put(CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING.getKey(), EnableAllocationDecider.Rebalance.NONE.name()),
+            plugin);
+        allocationService.reroute(clusterState, "reroute").routingTable();
+        assertThat(plugin.rebalanceAttempts, greaterThan(0));
+    }
+
+    public void testAllocationSkippedIfDisabled() {
+        final AllocateShortCircuitPlugin plugin = new AllocateShortCircuitPlugin();
+        AllocationService allocationService = createAllocationService(Settings.builder()
+                .put(CLUSTER_ROUTING_ALLOCATION_ENABLE_SETTING.getKey(), EnableAllocationDecider.Allocation.NONE.name()),
+            plugin);
+
+        MetaData metaData = MetaData.builder()
+            .put(IndexMetaData.builder("test").settings(settings(Version.CURRENT)).numberOfShards(1).numberOfReplicas(0))
+            .build();
+
+        RoutingTable routingTable = RoutingTable.builder()
+            .addAsNew(metaData.index("test"))
+            .build();
+
+        ClusterState clusterState = ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING.getDefault(Settings.EMPTY))
+            .metaData(metaData).routingTable(routingTable).nodes(DiscoveryNodes.builder().add(newNode("node1"))).build();
+
+        allocationService.reroute(clusterState, "reroute").routingTable();
+        assertThat(plugin.canAllocateAttempts, equalTo(0));
+    }
+
+    private static AllocationService createAllocationService(Settings.Builder settings, ClusterPlugin plugin) {
+        final ClusterSettings emptyClusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
+        List<AllocationDecider> deciders = new ArrayList<>(ClusterModule.createAllocationDeciders(settings.build(), emptyClusterSettings,
+                Collections.singletonList(plugin)));
+        return new MockAllocationService(
+            new AllocationDeciders(deciders),
+            new TestGatewayAllocator(), new BalancedShardsAllocator(Settings.EMPTY), EmptyClusterInfoService.INSTANCE);
+    }
+
+    private static class RebalanceShortCircuitPlugin implements ClusterPlugin {
+        int rebalanceAttempts;
+
+        @Override
+        public Collection<AllocationDecider> createAllocationDeciders(Settings settings, ClusterSettings clusterSettings) {
+            return Collections.singletonList(new RebalanceShortCircuitAllocationDecider());
+        }
+
+        private class RebalanceShortCircuitAllocationDecider extends AllocationDecider {
+
+            @Override
+            public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
+                rebalanceAttempts++;
+                return super.canRebalance(shardRouting, allocation);
+            }
+
+            @Override
+            public Decision canRebalance(RoutingAllocation allocation) {
+                rebalanceAttempts++;
+                return super.canRebalance(allocation);
+            }
+        }
+    }
+
+    private static class AllocateShortCircuitPlugin implements ClusterPlugin {
+        int canAllocateAttempts;
+
+        @Override
+        public Collection<AllocationDecider> createAllocationDeciders(Settings settings, ClusterSettings clusterSettings) {
+            return Collections.singletonList(new AllocateShortCircuitAllocationDecider());
+        }
+
+        private class AllocateShortCircuitAllocationDecider extends AllocationDecider {
+
+            @Override
+            public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
+                canAllocateAttempts++;
+                return super.canAllocate(shardRouting, node, allocation);
+            }
+
+            @Override
+            public Decision canAllocate(ShardRouting shardRouting, RoutingAllocation allocation) {
+                canAllocateAttempts++;
+                return super.canAllocate(shardRouting, allocation);
+            }
+
+            @Override
+            public Decision canAllocate(IndexMetaData indexMetaData, RoutingNode node, RoutingAllocation allocation) {
+                canAllocateAttempts++;
+                return super.canAllocate(indexMetaData, node, allocation);
+            }
+
+            @Override
+            public Decision canAllocate(RoutingNode node, RoutingAllocation allocation) {
+                canAllocateAttempts++;
+                return super.canAllocate(node, allocation);
+            }
+        }
+    }
+}