A service (such as HDFS, MapReduce, HBase) runs in a cluster. It has roles, which are the actual entities (NameNode, DataNodes, etc.) that perform the service's functions.
HDFS services and health checksIn CDH4, HDFS services may not present any health checks. This will happen if the service has more than one nameservice configured. In those cases, the health information will be available by fetching information about the nameservices instead.
The health summary is still available, and reflects a service-wide summary.