private static HashedWheelTimer getTimer() {
    HashedWheelTimer timer = TIMER;
    if (timer != null) {
      return timer;
    }
    synchronized (RegionReplicationFlushRequester.class) {
      timer = TIMER;
      if (timer != null) {
        return timer;
      }
      timer = new HashedWheelTimer(
        new ThreadFactoryBuilder().setNameFormat("RegionReplicationFlushRequester-Timer-pool-%d")
          .setDaemon(true).setUncaughtExceptionHandler(Threads.LOGGING_EXCEPTION_HANDLER).build(),
        500, TimeUnit.MILLISECONDS);
      TIMER = timer;
    }
    return timer;
  }
The minimum interval between two flush requests
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/regionreplication/RegionReplicationFlushRequester.java