Configuration nodes also provide a discovery service that the clients can use to locate the service providers (i.e. other nodes providing a particular service). It is used as a directory service. e.g. the control nodes are designed to scale horizontally: the vrouter agent can use the discovery service to determine which control-nodes to connect to (as that list may change as the cluster grows).
For example, when the vRouter agent in a compute node wants to connect to a control node (to be more precise: to an active-active pair of Control VM’s) it uses service discovery to discover the IP address of the control node. The clients use local configuration, DHCP or DNS to locate the service discovery server.
Service providers e.g. DNS, IF-MAP, etc. register with the discovery service as they come online, and anything requiring the use of the services subscribes. Usefully for debugging node issues you can take a look at the discovery service, by connecting to the server at port 5998 e.g. http://x.y.z.230:5998/services and see what services are registered:
And who are using them:
So you can see, for example, that the the vRouters are subscribed to the XMPP service, and the dns-server.
As more services/nodes are brought online they are registered with the discovery service and will be used. Likewise, if a service stops responding to heartbeats, the discovery will remove that service from the active list, and the clients will use the alternatives.
