Switch to ShardsBatchGatewayAllocator in batch enabled mode for shard…

… explain allocation API

Signed-off-by: Swetha Guptha <gupthasg@amazon.com>

server/src/main/java/org/opensearch/cluster/routing/allocation/AllocationService.java

@@ -724,9 +724,17 @@ private AllocateUnassignedDecision explainUnassignedShardAllocation(ShardRouting
 
     private ExistingShardsAllocator getAllocatorForShard(ShardRouting shardRouting, RoutingAllocation routingAllocation) {
         assert assertInitialized();
-        final String allocatorName = ExistingShardsAllocator.EXISTING_SHARDS_ALLOCATOR_SETTING.get(
-            routingAllocation.metadata().getIndexSafe(shardRouting.index()).getSettings()
-        );
+        String allocatorName;
+        Boolean batchModeEnabled = EXISTING_SHARDS_ALLOCATOR_BATCH_MODE.get(settings);
+        if (batchModeEnabled
+            && routingAllocation.nodes().getMinNodeVersion().onOrAfter(Version.V_2_14_0)
+            && existingShardsAllocators.size() == 2) {
+            allocatorName = ShardsBatchGatewayAllocator.ALLOCATOR_NAME;
+        } else {
+            allocatorName = ExistingShardsAllocator.EXISTING_SHARDS_ALLOCATOR_SETTING.get(
+                routingAllocation.metadata().getIndexSafe(shardRouting.index()).getSettings()
+            );
+        }
         final ExistingShardsAllocator existingShardsAllocator = existingShardsAllocators.get(allocatorName);
         return existingShardsAllocator != null ? existingShardsAllocator : new NotFoundAllocator(allocatorName);
     }