<T> void runTasksForExecutor(ClusterStateTaskExecutor<T> executor) {
    final ArrayList<UpdateTask<T>> toExecute = new ArrayList<>();
    final Map<String, ArrayList<T>> processTasksBySource = new HashMap<>();
    synchronized (updateTasksPerExecutor) {
      List<UpdateTask> pending = updateTasksPerExecutor.remove(executor);
      if (pending != null) {
        for (UpdateTask<T> task : pending) {
          if (task.processed.getAndSet(true) == false) {
            logger.trace("will process {}", task.toString(executor));
            toExecute.add(task);
            processTasksBySource
                .computeIfAbsent(task.source, s -> new ArrayList<>())
                .add(task.task);
          } else {
            logger.trace("skipping {}, already processed", task.toString(executor));
          }
        }
      }
    }
    if (toExecute.isEmpty()) {
      return;
    }
    final String tasksSummary =
        processTasksBySource
            .entrySet()
            .stream()
            .map(
                entry -> {
                  String tasks = executor.describeTasks(entry.getValue());
                  return tasks.isEmpty() ? entry.getKey() : entry.getKey() + "[" + tasks + "]";
                })
            .reduce((s1, s2) -> s1 + ", " + s2)
            .orElse("");

    if (!lifecycle.started()) {
      logger.debug("processing [{}]: ignoring, cluster_service not started", tasksSummary);
      return;
    }
    logger.debug("processing [{}]: execute", tasksSummary);
    ClusterState previousClusterState = clusterState;
    if (!previousClusterState.nodes().isLocalNodeElectedMaster() && executor.runOnlyOnMaster()) {
      logger.debug("failing [{}]: local node is no longer master", tasksSummary);
      toExecute.stream().forEach(task -> task.listener.onNoLongerMaster(task.source));
      return;
    }
    ClusterStateTaskExecutor.BatchResult<T> batchResult;
    long startTimeNS = currentTimeInNanos();
    try {
      List<T> inputs =
          toExecute.stream().map(tUpdateTask -> tUpdateTask.task).collect(Collectors.toList());
      batchResult = executor.execute(previousClusterState, inputs);
    } catch (Exception e) {
      TimeValue executionTime =
          TimeValue.timeValueMillis(
              Math.max(0, TimeValue.nsecToMSec(currentTimeInNanos() - startTimeNS)));
      if (logger.isTraceEnabled()) {
        logger.trace(
            (Supplier<?>)
                () ->
                    new ParameterizedMessage(
                        "failed to execute cluster state update in [{}], state:\nversion [{}], source [{}]\n{}{}{}",
                        executionTime,
                        previousClusterState.version(),
                        tasksSummary,
                        previousClusterState.nodes().prettyPrint(),
                        previousClusterState.routingTable().prettyPrint(),
                        previousClusterState.getRoutingNodes().prettyPrint()),
            e);
      }
      warnAboutSlowTaskIfNeeded(executionTime, tasksSummary);
      batchResult =
          ClusterStateTaskExecutor.BatchResult.<T>builder()
              .failures(toExecute.stream().map(updateTask -> updateTask.task)::iterator, e)
              .build(previousClusterState);
    }

    assert batchResult.executionResults != null;
    assert batchResult.executionResults.size() == toExecute.size()
        : String.format(
            Locale.ROOT,
            "expected [%d] task result%s but was [%d]",
            toExecute.size(),
            toExecute.size() == 1 ? "" : "s",
            batchResult.executionResults.size());
    boolean assertsEnabled = false;
    assert (assertsEnabled = true);
    if (assertsEnabled) {
      for (UpdateTask<T> updateTask : toExecute) {
        assert batchResult.executionResults.containsKey(updateTask.task)
            : "missing task result for " + updateTask.toString(executor);
      }
    }

    ClusterState newClusterState = batchResult.resultingState;
    final ArrayList<UpdateTask<T>> proccessedListeners = new ArrayList<>();
    // fail all tasks that have failed and extract those that are waiting for results
    for (UpdateTask<T> updateTask : toExecute) {
      assert batchResult.executionResults.containsKey(updateTask.task)
          : "missing " + updateTask.toString(executor);
      final ClusterStateTaskExecutor.TaskResult executionResult =
          batchResult.executionResults.get(updateTask.task);
      executionResult.handle(
          () -> proccessedListeners.add(updateTask),
          ex -> {
            logger.debug(
                (Supplier<?>)
                    () ->
                        new ParameterizedMessage(
                            "cluster state update task {} failed", updateTask.toString(executor)),
                ex);
            updateTask.listener.onFailure(updateTask.source, ex);
          });
    }

    if (previousClusterState == newClusterState) {
      for (UpdateTask<T> task : proccessedListeners) {
        if (task.listener instanceof AckedClusterStateTaskListener) {
          // no need to wait for ack if nothing changed, the update can be counted as acknowledged
          ((AckedClusterStateTaskListener) task.listener).onAllNodesAcked(null);
        }
        task.listener.clusterStateProcessed(task.source, previousClusterState, newClusterState);
      }
      TimeValue executionTime =
          TimeValue.timeValueMillis(
              Math.max(0, TimeValue.nsecToMSec(currentTimeInNanos() - startTimeNS)));
      logger.debug(
          "processing [{}]: took [{}] no change in cluster_state", tasksSummary, executionTime);
      warnAboutSlowTaskIfNeeded(executionTime, tasksSummary);
      return;
    }

    try {
      ArrayList<Discovery.AckListener> ackListeners = new ArrayList<>();
      if (newClusterState.nodes().isLocalNodeElectedMaster()) {
        // only the master controls the version numbers
        Builder builder = ClusterState.builder(newClusterState).incrementVersion();
        if (previousClusterState.routingTable() != newClusterState.routingTable()) {
          builder.routingTable(
              RoutingTable.builder(newClusterState.routingTable())
                  .version(newClusterState.routingTable().version() + 1)
                  .build());
        }
        if (previousClusterState.metaData() != newClusterState.metaData()) {
          builder.metaData(
              MetaData.builder(newClusterState.metaData())
                  .version(newClusterState.metaData().version() + 1));
        }
        newClusterState = builder.build();
        for (UpdateTask<T> task : proccessedListeners) {
          if (task.listener instanceof AckedClusterStateTaskListener) {
            final AckedClusterStateTaskListener ackedListener =
                (AckedClusterStateTaskListener) task.listener;
            if (ackedListener.ackTimeout() == null || ackedListener.ackTimeout().millis() == 0) {
              ackedListener.onAckTimeout();
            } else {
              try {
                ackListeners.add(
                    new AckCountDownListener(
                        ackedListener,
                        newClusterState.version(),
                        newClusterState.nodes(),
                        threadPool));
              } catch (EsRejectedExecutionException ex) {
                if (logger.isDebugEnabled()) {
                  logger.debug(
                      "Couldn't schedule timeout thread - node might be shutting down", ex);
                }
                // timeout straightaway, otherwise we could wait forever as the timeout thread has
                // not started
                ackedListener.onAckTimeout();
              }
            }
          }
        }
      }
      final Discovery.AckListener ackListener = new DelegetingAckListener(ackListeners);

      newClusterState.status(ClusterState.ClusterStateStatus.BEING_APPLIED);

      if (logger.isTraceEnabled()) {
        logger.trace(
            "cluster state updated, source [{}]\n{}", tasksSummary, newClusterState.prettyPrint());
      } else if (logger.isDebugEnabled()) {
        logger.debug(
            "cluster state updated, version [{}], source [{}]",
            newClusterState.version(),
            tasksSummary);
      }

      ClusterChangedEvent clusterChangedEvent =
          new ClusterChangedEvent(tasksSummary, newClusterState, previousClusterState);
      // new cluster state, notify all listeners
      final DiscoveryNodes.Delta nodesDelta = clusterChangedEvent.nodesDelta();
      if (nodesDelta.hasChanges() && logger.isInfoEnabled()) {
        String summary = nodesDelta.shortSummary();
        if (summary.length() > 0) {
          logger.info("{}, reason: {}", summary, tasksSummary);
        }
      }

      nodeConnectionsService.connectToAddedNodes(clusterChangedEvent);

      // if we are the master, publish the new state to all nodes
      // we publish here before we send a notification to all the listeners, since if it fails
      // we don't want to notify
      if (newClusterState.nodes().isLocalNodeElectedMaster()) {
        logger.debug("publishing cluster state version [{}]", newClusterState.version());
        try {
          clusterStatePublisher.accept(clusterChangedEvent, ackListener);
        } catch (Discovery.FailedToCommitClusterStateException t) {
          final long version = newClusterState.version();
          logger.warn(
              (Supplier<?>)
                  () ->
                      new ParameterizedMessage(
                          "failing [{}]: failed to commit cluster state version [{}]",
                          tasksSummary,
                          version),
              t);
          proccessedListeners.forEach(task -> task.listener.onFailure(task.source, t));
          return;
        }
      }

      // update the current cluster state
      clusterState = newClusterState;
      logger.debug("set local cluster state to version {}", newClusterState.version());
      try {
        // nothing to do until we actually recover from the gateway or any other block indicates we
        // need to disable persistency
        if (clusterChangedEvent.state().blocks().disableStatePersistence() == false
            && clusterChangedEvent.metaDataChanged()) {
          final Settings incomingSettings = clusterChangedEvent.state().metaData().settings();
          clusterSettings.applySettings(incomingSettings);
        }
      } catch (Exception ex) {
        logger.warn("failed to apply cluster settings", ex);
      }
      for (ClusterStateListener listener : preAppliedListeners) {
        try {
          listener.clusterChanged(clusterChangedEvent);
        } catch (Exception ex) {
          logger.warn("failed to notify ClusterStateListener", ex);
        }
      }

      nodeConnectionsService.disconnectFromRemovedNodes(clusterChangedEvent);

      newClusterState.status(ClusterState.ClusterStateStatus.APPLIED);

      for (ClusterStateListener listener : postAppliedListeners) {
        try {
          listener.clusterChanged(clusterChangedEvent);
        } catch (Exception ex) {
          logger.warn("failed to notify ClusterStateListener", ex);
        }
      }

      // manual ack only from the master at the end of the publish
      if (newClusterState.nodes().isLocalNodeElectedMaster()) {
        try {
          ackListener.onNodeAck(newClusterState.nodes().getLocalNode(), null);
        } catch (Exception e) {
          final DiscoveryNode localNode = newClusterState.nodes().getLocalNode();
          logger.debug(
              (Supplier<?>)
                  () ->
                      new ParameterizedMessage(
                          "error while processing ack for master node [{}]", localNode),
              e);
        }
      }

      for (UpdateTask<T> task : proccessedListeners) {
        task.listener.clusterStateProcessed(task.source, previousClusterState, newClusterState);
      }

      try {
        executor.clusterStatePublished(clusterChangedEvent);
      } catch (Exception e) {
        logger.error(
            (Supplier<?>)
                () ->
                    new ParameterizedMessage(
                        "exception thrown while notifying executor of new cluster state publication [{}]",
                        tasksSummary),
            e);
      }

      TimeValue executionTime =
          TimeValue.timeValueMillis(
              Math.max(0, TimeValue.nsecToMSec(currentTimeInNanos() - startTimeNS)));
      logger.debug(
          "processing [{}]: took [{}] done applying updated cluster_state (version: {}, uuid: {})",
          tasksSummary,
          executionTime,
          newClusterState.version(),
          newClusterState.stateUUID());
      warnAboutSlowTaskIfNeeded(executionTime, tasksSummary);
    } catch (Exception e) {
      TimeValue executionTime =
          TimeValue.timeValueMillis(
              Math.max(0, TimeValue.nsecToMSec(currentTimeInNanos() - startTimeNS)));
      final long version = newClusterState.version();
      final String stateUUID = newClusterState.stateUUID();
      final String prettyPrint = newClusterState.prettyPrint();
      logger.warn(
          (Supplier<?>)
              () ->
                  new ParameterizedMessage(
                      "failed to apply updated cluster state in [{}]:\nversion [{}], uuid [{}], source [{}]\n{}",
                      executionTime,
                      version,
                      stateUUID,
                      tasksSummary,
                      prettyPrint),
          e);
      // TODO: do we want to call updateTask.onFailure here?
    }
  }
 @Override
 public void onNodeAck(DiscoveryNode node, @Nullable Exception e) {
   for (Discovery.AckListener listener : listeners) {
     listener.onNodeAck(node, e);
   }
 }