public Set<String> getHosts(String serviceName, String componentName) {
    Set<String> hosts = m_topology.get(serviceName + "/" + componentName);
    if (null == hosts) {
      hosts = new HashSet<>();
    }

    return hosts;
  }
Gets any hosts where the matching service and component are installed.

@param serviceName
         the service name
@param componentName
         the component name
@return the set of hosts where the component is installed, or an empty set.
ambari-server-spi/src/main/java/org/apache/ambari/spi/ClusterInformation.java