Update Routing table builder

Author: ankikuma

server/src/main/java/org/elasticsearch/cluster/metadata/MetadataAutoshardIndexService.java

@@ -146,18 +146,23 @@ public ClusterState applyAutoshardIndexRequest(
         final String indexName = request.index();
         final IndexMetadata sourceMetadata = currentState.metadata().index(indexName);
         int sourceNumShards = sourceMetadata.getNumberOfShards();
+        int targetNumShards = sourceNumShards * 2;
 
         Settings.Builder settingsBuilder = Settings.builder().put(sourceMetadata.getSettings());
         settingsBuilder.remove(IndexMetadata.SETTING_NUMBER_OF_SHARDS);
-        settingsBuilder.put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, sourceNumShards * 2);
+        settingsBuilder.put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, targetNumShards);
 
         final Map<Index, Settings> updates = Maps.newHashMapWithExpectedSize(1);
         updates.put(sourceMetadata.getIndex(), settingsBuilder.build());
         final Metadata newMetadata = currentState.metadata().withIndexSettingsUpdates(updates);
 
+        /*
         var routingTableBuilder = RoutingTable.builder(allocationService.getShardRoutingRoleStrategy(), currentState.routingTable())
             .remove(indexName)
             .addAsNew(newMetadata.index(indexName));
+        */
+
+        var routingTableBuilder = RoutingTable.builder().updateNumberOfShards(targetNumShards, indexName);
 
         ClusterState updated = ClusterState.builder(currentState)
             .incrementVersion()

server/src/main/java/org/elasticsearch/cluster/routing/RoutingTable.java

@@ -449,7 +449,8 @@ private static void addShard(
                 .addShard(shardRoutingEntry);
         }
 
-        public Builder updateNumberOfShards(final int numberOfShards, final String index) {
+        /* Update the number of shards for an existing index in serverless */
+        public Builder updateNumberOfShards(final int newShardCount, final String index) {
             if (indicesRouting == null) {
                 throw new IllegalStateException("once build is called the builder cannot be reused");
             }
@@ -458,15 +459,33 @@ public Builder updateNumberOfShards(final int numberOfShards, final String index
                 // ignore index missing failure, its closed...
                 return this;
             }
+            // Replica count (should be 0 for serverless)
             int currentNumberOfReplicas = indexRoutingTable.shard(0).size() - 1; // remove the required primary
-            int numberOfOldShards = indexRoutingTable.size();
+            int oldShardCount = indexRoutingTable.size();
+            assert (newShardCount % oldShardCount == 0) : "New shard count must be multiple of old shard count";
             IndexRoutingTable.Builder builder = new IndexRoutingTable.Builder(shardRoutingRoleStrategy, indexRoutingTable.getIndex());
+            builder.ensureShardArray(newShardCount);
+
             // re-add existing shards
-            builder.ensureShardArray(numberOfOldShards);
-            for (int i = 0; i < numberOfOldShards; i++) {
+            for (int i = 0; i < oldShardCount; i++) {
                 builder.addIndexShard(new IndexShardRoutingTable.Builder(indexRoutingTable.shard(i)));
             }
-            // See addReplica and addIndexShard to create the new shards
+
+            int numNewShards = newShardCount - oldShardCount;
+            // Add new shards
+            for (int i = 0; i < numNewShards; i++) {
+                ShardId shardId = new ShardId(indexRoutingTable.getIndex(), oldShardCount + i);
+                ShardRouting shardRouting = ShardRouting.newUnassigned(
+                    shardId,
+                    true,
+                    RecoverySource.EmptyStoreRecoverySource.INSTANCE,
+                    new UnassignedInfo(UnassignedInfo.Reason.SHARD_ADDED, null), // A new Reason needed in UnassignedInfo
+                    ShardRouting.Role.INDEX_ONLY); // A new role API similar to newReplicaRole() needed in shardRoutingRoleStrategy ?
+                IndexShardRoutingTable.Builder indexShardRoutingBuilder = IndexShardRoutingTable.builder(shardId);
+                indexShardRoutingBuilder.addShard(shardRouting);
+                builder.addIndexShard(indexShardRoutingBuilder);
+            }
+            // No need to add replicas because no replicas on serverless.
             indicesRouting.put(index, builder.build());
             return this;
         }

server/src/main/java/org/elasticsearch/cluster/routing/UnassignedInfo.java

@@ -175,7 +175,11 @@ public enum Reason {
         /**
          * Replica is unpromotable and the primary failed.
          */
-        UNPROMOTABLE_REPLICA
+        UNPROMOTABLE_REPLICA,
+        /**
+         * New shard added as part of index resize (auto sharding)
+         */
+        SHARD_ADDED
     }
 
     /**