public RequestStatusResponse start(final String cluster, final String hostname)
      throws SystemException,
      NoSuchParentResourceException,
      UnsupportedPropertyException {

    return ((HostComponentResourceProvider) getResourceProvider(Resource.Type.HostComponent)).
        start(cluster, hostname);
  }
If the rack info provided in the request differs from the rack info of the host
update the rack info of the host with the value from the host request

@param cluster The cluster to check user privileges against. User is required
               to have {@link RoleAuthorization#HOST_ADD_DELETE_HOSTS} rights on the cluster.
@param host The host of which rack information is to be updated
@param hostRequest
@return true is host was updated otherwise false
@throws AmbariException
@throws AuthorizationException
ambari-server/src/main/java/org/apache/ambari/server/controller/internal/HostResourceProvider.java