public Set<String> invalidateHosts(AlertDefinition definition) {
    return invalidateHosts(definition.getClusterId(),
        definition.getSource().getType(), definition.getName(),
        definition.getServiceName(), definition.getComponentName());
  }
Invalidate the hashes of any host that would be affected by the specified
definition. If the definition is an {@link SourceType#AGGREGATE}, this will
return an empty set since aggregates do not affect hosts.

@param definition
         the definition to use to find the hosts to invlidate (not
         {@code null}).
@return the hosts that were invalidated, or an empty set (never
        {@code null}).
ambari-server/src/main/java/org/apache/ambari/server/state/alert/AlertDefinitionHash.java