What is federation? When we speak of federation, we are talking about clusters of ESB servers. Users cluster ESB servers in order to eliminate a single point of failure—by using load balancing you can not only spread load across multiple ESB servers, but provide redundancy in case one of the servers goes down.

Federation in SOA is very similar to what the term means in terms of actual government—it is an organization of smaller groups wherein the smaller groups still have some autonomy. In terms of the registry, when we "federate" a service, or deploy it across a cluster of servers, the service may be deployed on several nodes, each of which is running its own registry, but is backed by a shared database. When a service is called by the ServiceInvoker, it may route a request to a server that is down, but then time out and resend the request to a server that is up, like service location.