int getAvailableCount() {
      if (!isShutdown()) {
        synchronized (this) {
          if (!isShutdown()) {
            // TODO: temporarily disable multiple poll
            return 0;
            // return Math.max(
            //    ((ThreadPoolExecutor) this.executor).getCorePoolSize()
            //        - ((ThreadPoolExecutor) this.executor).getActiveCount(),
            //    0);
          }
        }
      }

      LOGGER.warn("{} has not been launched, return zero", this.name);
      return 0;
    }
Downstream Data Flow Executor: execute poll task
iotdb-client/session/src/main/java/org/apache/iotdb/session/subscription/consumer/base/SubscriptionExecutorServiceManager.java