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

    Collection<String> hosts = event.getHosts();
    long clusterId = event.getClusterId();

    // no hosts, nothing to do
    if (null == hosts || hosts.isEmpty()) {
      return;
    }

    AlertDefinitionHash hash = m_alertDefinitionHash.get();
    hash.enqueueAgentCommands(clusterId, hosts);
  }
Handles {@link AlertHashInvalidationEvent} by performing the following
tasks:
<ul>
<li>Enqueuing {@link AlertDefinitionCommand}</li>
</ul>

@param event
         the event being handled.
ambari-server/src/main/java/org/apache/ambari/server/events/listeners/alerts/AlertHashInvalidationListener.java