Enable write load constraint decider via feature flag (#135246)

server/src/internalClusterTest/java/org/elasticsearch/index/shard/IndexShardIT.java

@@ -305,6 +305,9 @@ public class IndexShardIT extends ESSingleNodeTestCase {
     }
 
     public void testNodeWriteLoadsArePresent() {
+        // Disable write load decider to begin with
+        setWriteLoadDeciderEnablement(WriteLoadConstraintSettings.WriteLoadDeciderStatus.DISABLED);
+
         InternalClusterInfoService clusterInfoService = (InternalClusterInfoService) getInstanceFromNode(ClusterInfoService.class);
         ClusterInfoServiceUtils.refresh(clusterInfoService);
         Map<String, NodeUsageStatsForThreadPools> nodeThreadPoolStats = clusterInfoService.getClusterInfo()
@@ -315,15 +318,10 @@ public class IndexShardIT extends ESSingleNodeTestCase {
         assertTrue(nodeThreadPoolStats.isEmpty());
 
         // Enable collection for node write loads.
-        updateClusterSettings(
-            Settings.builder()
-                .put(
-                    WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey(),
-                    randomBoolean()
-                        ? WriteLoadConstraintSettings.WriteLoadDeciderStatus.ENABLED
-                        : WriteLoadConstraintSettings.WriteLoadDeciderStatus.LOW_THRESHOLD_ONLY
-                )
-                .build()
+        setWriteLoadDeciderEnablement(
+            randomBoolean()
+                ? WriteLoadConstraintSettings.WriteLoadDeciderStatus.ENABLED
+                : WriteLoadConstraintSettings.WriteLoadDeciderStatus.LOW_THRESHOLD_ONLY
         );
         try {
             // Force a ClusterInfo refresh to run collection of the node thread pool usage stats.
@@ -346,9 +344,7 @@ public class IndexShardIT extends ESSingleNodeTestCase {
                 assertThat(writeThreadPoolStats.maxThreadPoolQueueLatencyMillis(), greaterThanOrEqualTo(0L));
             }
         } finally {
-            updateClusterSettings(
-                Settings.builder().putNull(WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey()).build()
-            );
+            clearWriteLoadDeciderEnablementSetting();
         }
     }
 
@@ -365,27 +361,25 @@ public class IndexShardIT extends ESSingleNodeTestCase {
 
         final InternalClusterInfoService clusterInfoService = (InternalClusterInfoService) getInstanceFromNode(ClusterInfoService.class);
 
-        // Not collecting stats yet because allocation write load stats collection is disabled by default.
-        {
-            ClusterInfoServiceUtils.refresh(clusterInfoService);
-            final Map<ShardId, Double> shardWriteLoads = clusterInfoService.getClusterInfo().getShardWriteLoads();
-            assertNotNull(shardWriteLoads);
-            assertTrue(shardWriteLoads.isEmpty());
-        }
-
-        // Turn on collection of write-load stats.
-        updateClusterSettings(
-            Settings.builder()
-                .put(
-                    WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey(),
-                    randomBoolean()
-                        ? WriteLoadConstraintSettings.WriteLoadDeciderStatus.ENABLED
-                        : WriteLoadConstraintSettings.WriteLoadDeciderStatus.LOW_THRESHOLD_ONLY
-                )
-                .build()
-        );
+        // Explicitly disable write load decider
+        setWriteLoadDeciderEnablement(WriteLoadConstraintSettings.WriteLoadDeciderStatus.DISABLED);
 
         try {
+            // Stats should not be collected when the decider is disabled
+            {
+                ClusterInfoServiceUtils.refresh(clusterInfoService);
+                final Map<ShardId, Double> shardWriteLoads = clusterInfoService.getClusterInfo().getShardWriteLoads();
+                assertNotNull(shardWriteLoads);
+                assertTrue(shardWriteLoads.isEmpty());
+            }
+
+            // Turn on collection of write-load stats.
+            setWriteLoadDeciderEnablement(
+                randomBoolean()
+                    ? WriteLoadConstraintSettings.WriteLoadDeciderStatus.ENABLED
+                    : WriteLoadConstraintSettings.WriteLoadDeciderStatus.LOW_THRESHOLD_ONLY
+            );
+
             // Force a ClusterInfo refresh to run collection of the write-load stats.
             ClusterInfoServiceUtils.refresh(clusterInfoService);
             final Map<ShardId, Double> shardWriteLoads = clusterInfoService.getClusterInfo().getShardWriteLoads();
@@ -404,12 +398,14 @@ public class IndexShardIT extends ESSingleNodeTestCase {
                 assertThat(maximumLoadRecorded, greaterThan(0.0));
             }
         } finally {
-            updateClusterSettings(
-                Settings.builder().putNull(WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey()).build()
-            );
+            clearWriteLoadDeciderEnablementSetting();
         }
     }
 
+    private void clearWriteLoadDeciderEnablementSetting() {
+        updateClusterSettings(Settings.builder().putNull(WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey()).build());
+    }
+
     public void testMaxHeapPerNodeIsPresent() {
         InternalClusterInfoService clusterInfoService = (InternalClusterInfoService) getInstanceFromNode(ClusterInfoService.class);
         ClusterInfoServiceUtils.refresh(clusterInfoService);
@@ -764,6 +760,12 @@ public class IndexShardIT extends ESSingleNodeTestCase {
         }
     }
 
+    private void setWriteLoadDeciderEnablement(WriteLoadConstraintSettings.WriteLoadDeciderStatus status) {
+        updateClusterSettings(
+            Settings.builder().put(WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey(), status).build()
+        );
+    }
+
     public static final IndexShard recoverShard(IndexShard newShard) throws IOException {
         DiscoveryNode localNode = DiscoveryNodeUtils.builder("foo").roles(emptySet()).build();
         newShard.markAsRecovering("store", new RecoveryState(newShard.routingEntry(), localNode, null));

server/src/main/java/org/elasticsearch/cluster/routing/allocation/WriteLoadConstraintSettings.java

@@ -15,6 +15,7 @@ import org.elasticsearch.common.Priority;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.unit.RatioValue;
+import org.elasticsearch.common.util.FeatureFlag;
 import org.elasticsearch.core.TimeValue;
 
 /**
@@ -23,6 +24,7 @@ import org.elasticsearch.core.TimeValue;
 public class WriteLoadConstraintSettings {
 
     private static final String SETTING_PREFIX = "cluster.routing.allocation.write_load_decider.";
+    private static final FeatureFlag WRITE_LOAD_DECIDER_FEATURE_FLAG = new FeatureFlag("write_load_decider");
 
     public enum WriteLoadDeciderStatus {
         /**
@@ -59,7 +61,7 @@ public class WriteLoadConstraintSettings {
     public static final Setting<WriteLoadDeciderStatus> WRITE_LOAD_DECIDER_ENABLED_SETTING = Setting.enumSetting(
         WriteLoadDeciderStatus.class,
         SETTING_PREFIX + "enabled",
-        WriteLoadDeciderStatus.DISABLED,
+        WRITE_LOAD_DECIDER_FEATURE_FLAG.isEnabled() ? WriteLoadDeciderStatus.ENABLED : WriteLoadDeciderStatus.DISABLED,
         Setting.Property.Dynamic,
         Setting.Property.NodeScope
     );

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

@@ -43,9 +43,14 @@ public class WriteLoadConstraintDeciderTests extends ESAllocationTestCase {
         String indexName = "test-index";
         var testHarness = createClusterStateAndRoutingAllocation(indexName);
 
-        // The write load decider is disabled by default.
-
-        var writeLoadDecider = createWriteLoadConstraintDecider(Settings.builder().build());
+        var writeLoadDecider = createWriteLoadConstraintDecider(
+            Settings.builder()
+                .put(
+                    WriteLoadConstraintSettings.WRITE_LOAD_DECIDER_ENABLED_SETTING.getKey(),
+                    WriteLoadConstraintSettings.WriteLoadDeciderStatus.DISABLED
+                )
+                .build()
+        );
 
         assertEquals(
             Decision.Type.YES,