To effectively manage and utilize compute nodes, the Compute service must know their statuses. For example, when a user launches a new VM, the Compute scheduler should send the request to a live node (with enough capacity too, of course). From the Grizzly release and later, the Compute service queries the ServiceGroup API to get the node liveness information.
When a compute worker (running the nova-compute daemon) starts,
it calls the join API to join the compute group, so that every
service that is interested in the information (for example, the scheduler)
can query the group membership or the status of a
particular node. Internally, the ServiceGroup client driver
automatically updates the compute worker status.
The following drivers are implemented: database and ZooKeeper. Further drivers are in review or development, such as memcache.
