code: private static void enqueueCallbackTask(final ShufflerParams shufflerParams, final String url,
        final String taskName) {
      RetryHelper.runWithRetries(callable(new Runnable() {
        @Override
        public void run() {
          String hostname = ModulesServiceFactory.getModulesService().getVersionHostname(
              shufflerParams.getCallbackModule(), shufflerParams.getCallbackVersion());
          Queue queue = QueueFactory.getQueue(shufflerParams.getCallbackQueue());
          String separater = shufflerParams.getCallbackPath().contains("?") ? "&" : "?";
          try {
            queue.add(TaskOptions.Builder.withUrl(shufflerParams.getCallbackPath() + separater + url)
                .method(TaskOptions.Method.GET).header("Host", hostname).taskName(taskName));
          } catch (TaskAlreadyExistsException e) {
            // harmless dup.
          }
        }
      }), RETRY_PARAMS, EXCEPTION_HANDLER);
    }
docs: Notifies the caller that the job has completed.