@Subscribe
  @AllowConcurrentEvents
  public void onAmbariEvent(HostsRemovedEvent event) {
    LOG.debug("Received event {}", event);

    // remove any current alerts for the removed hosts
    for (String hostName : event.getHostNames()) {
      m_alertsDao.removeCurrentByHost(hostName);
    }
  }
Handles the {@link HostsRemovedEvent} by performing the following actions:
<ul>
<li>Removes all {@link AlertCurrentEntity} for the removed hosts</li>
</ul>
ambari-server/src/main/java/org/apache/ambari/server/events/listeners/alerts/AlertHostListener.java