void shutdown() {
    shuttingDown = true;
    LOG.info("Shutting down compaction txn heartbeater service.");
    for (CompactionHeartbeater heartbeater : tasks.values()) {
      try {
        heartbeater.stop();
      } catch (InterruptedException e) {
        LOG.warn("Shutdownhook thread was interrupted during shutting down the CompactionHeartbeatService.");
      }
    }
    tasks.clear();
    clientPool.close();
    LOG.info("Compaction txn heartbeater service is successfully stopped.");
  }
Shuts down the service, by closing its underlying resources. Be aware that after shutdown this service is no
longer usable, there is no way to re-initialize it.
ql/src/java/org/apache/hadoop/hive/ql/txn/compactor/CompactionHeartbeatService.java