void reduce(int slot) {
   AtomicReferenceArray shardResponses = responsesByItemAndShard.get(slot);
   PercolateResponse reducedResponse =
       TransportPercolateAction.reduce(
           (PercolateRequest) percolateRequests.get(slot), shardResponses, percolatorService);
   reducedResponses.set(slot, reducedResponse);
   assert expectedOperations.get() >= 1
       : "slot[" + slot + "] expected options should be >= 1 but is " + expectedOperations.get();
   if (expectedOperations.decrementAndGet() == 0) {
     finish();
   }
 }
    ASyncAction(
        MultiPercolateRequest multiPercolateRequest,
        List<Object> percolateRequests,
        ActionListener<MultiPercolateResponse> finalListener,
        ClusterState clusterState) {
      this.finalListener = finalListener;
      this.multiPercolateRequest = multiPercolateRequest;
      this.percolateRequests = percolateRequests;
      responsesByItemAndShard = new AtomicReferenceArray<>(percolateRequests.size());
      expectedOperationsPerItem = new AtomicReferenceArray<>(percolateRequests.size());
      reducedResponses = new AtomicArray<>(percolateRequests.size());

      // Resolving concrete indices and routing and grouping the requests by shard
      requestsByShard = new HashMap<>();
      // Keep track what slots belong to what shard, in case a request to a shard fails on all
      // copies
      shardToSlots = new HashMap<>();
      int expectedResults = 0;
      for (int slot = 0; slot < percolateRequests.size(); slot++) {
        Object element = percolateRequests.get(slot);
        assert element != null;
        if (element instanceof PercolateRequest) {
          PercolateRequest percolateRequest = (PercolateRequest) element;
          String[] concreteIndices;
          try {
            concreteIndices =
                indexNameExpressionResolver.concreteIndices(clusterState, percolateRequest);
          } catch (IndexNotFoundException e) {
            reducedResponses.set(slot, e);
            responsesByItemAndShard.set(slot, new AtomicReferenceArray(0));
            expectedOperationsPerItem.set(slot, new AtomicInteger(0));
            continue;
          }
          Map<String, Set<String>> routing =
              indexNameExpressionResolver.resolveSearchRouting(
                  clusterState, percolateRequest.routing(), percolateRequest.indices());
          // TODO: I only need shardIds, ShardIterator(ShardRouting) is only needed in
          // TransportShardMultiPercolateAction
          GroupShardsIterator shards =
              clusterService
                  .operationRouting()
                  .searchShards(
                      clusterState, concreteIndices, routing, percolateRequest.preference());
          if (shards.size() == 0) {
            reducedResponses.set(slot, new UnavailableShardsException(null, "No shards available"));
            responsesByItemAndShard.set(slot, new AtomicReferenceArray(0));
            expectedOperationsPerItem.set(slot, new AtomicInteger(0));
            continue;
          }

          // The shard id is used as index in the atomic ref array, so we need to find out how many
          // shards there are regardless of routing:
          int numShards =
              clusterService
                  .operationRouting()
                  .searchShardsCount(clusterState, concreteIndices, null);
          responsesByItemAndShard.set(slot, new AtomicReferenceArray(numShards));
          expectedOperationsPerItem.set(slot, new AtomicInteger(shards.size()));
          for (ShardIterator shard : shards) {
            ShardId shardId = shard.shardId();
            TransportShardMultiPercolateAction.Request requests = requestsByShard.get(shardId);
            if (requests == null) {
              requestsByShard.put(
                  shardId,
                  requests =
                      new TransportShardMultiPercolateAction.Request(
                          multiPercolateRequest,
                          shardId.getIndex(),
                          shardId.getId(),
                          percolateRequest.preference()));
            }
            logger.trace("Adding shard[{}] percolate request for item[{}]", shardId, slot);
            requests.add(
                new TransportShardMultiPercolateAction.Request.Item(
                    slot, new PercolateShardRequest(shardId, percolateRequest)));

            IntArrayList items = shardToSlots.get(shardId);
            if (items == null) {
              shardToSlots.put(shardId, items = new IntArrayList());
            }
            items.add(slot);
          }
          expectedResults++;
        } else if (element instanceof Throwable || element instanceof MultiGetResponse.Failure) {
          logger.trace("item[{}] won't be executed, reason: {}", slot, element);
          reducedResponses.set(slot, element);
          responsesByItemAndShard.set(slot, new AtomicReferenceArray(0));
          expectedOperationsPerItem.set(slot, new AtomicInteger(0));
        }
      }
      expectedOperations = new AtomicInteger(expectedResults);
    }