Future<?> submit(final Runnable task) {
      if (!isShutdown()) {
        synchronized (this) {
          if (!isShutdown()) {
            return this.executor.submit(task);
          }
        }
      }

      LOGGER.warn("{} has not been launched, ignore submit task", this.name);
      return null;
    }
Downstream Data Flow Executor: execute poll task
iotdb-client/session/src/main/java/org/apache/iotdb/session/subscription/consumer/base/SubscriptionExecutorServiceManager.java