WriteLoadConstraintDecider: Don't require shard write load to return canRemain=NOT_PREFERRED (#137416)

Author: nicktindall

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

@@ -129,13 +129,6 @@ public Decision canRemain(IndexMetadata indexMetadata, ShardRouting shardRouting
             return Decision.single(Decision.Type.YES, NAME, "canRemain() is not enabled");
         }
 
-        // Check whether the shard being relocated has any write load estimate. If it does not, then this decider has no opinion.
-        var allShardWriteLoads = allocation.clusterInfo().getShardWriteLoads();
-        var shardWriteLoad = allShardWriteLoads.get(shardRouting.shardId());
-        if (shardWriteLoad == null || shardWriteLoad == 0) {
-            return Decision.single(Decision.Type.YES, NAME, "Shard has no estimated write load. Decider takes no action.");
-        }
-
         var allNodeUsageStats = allocation.clusterInfo().getNodeUsageStatsForThreadPools();
         var nodeUsageStatsForThreadPools = allNodeUsageStats.get(node.nodeId());
         if (nodeUsageStatsForThreadPools == null) {

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

@@ -234,8 +234,8 @@ public void testWriteLoadDeciderCanRemain() {
             ).type()
         );
         assertEquals(
-            "A shard without write load should remain on a node with queuing above the threshold",
-            Decision.Type.YES,
+            "A shard with no write load can still return NOT_PREFERRED",
+            Decision.Type.NOT_PREFERRED,
             writeLoadDecider.canRemain(
                 testHarness.clusterState.metadata().getProject().index(indexName),
                 testHarness.shardRoutingNoWriteLoad,