apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/message/DatabaseMessageSender.java/DatabaseMessageSender/initialize
class DatabaseMessageSender:  @PostConstruct
  private void initialize() {
    cleanExecutorService.submit(() -> {
      while (!cleanStopped.get() && !Thread.currentThread().isInterrupted()) {
        try {
          Long rm = toClean.poll(1, TimeUnit.SECONDS);
          if (rm != null) {
            cleanMessage(rm);
          } else {
            TimeUnit.SECONDS.sleep(5);
          }
        } catch (Throwable ex) {
          Tracer.logError(ex);
        }
      }
    });
  }