Most remote services provided by the JBoss application server, including JNDI, EJB, RMI and JBoss Remoting, require the client to obtain (e.g., to look up and download) a stub (or proxy) object. The stub object is generated by the server and it implements the business interface of the service. The client then makes local method calls against the stub object. The call is automatically routed across the network and invoked against service objects managed in the server. In a clustering environment, the server-generated stub object is also an interceptor that understand how to route calls to nodes in the cluster. The stub object figures out how to find the appropriate server node, marshal call parameters, un-marshall call results, return the results to the caller client.

The stub interceptors have updated knowledge about the cluster. For instance, they know the IP addresses of all available server nodes, the algorithm to distribute load across nodes (see next section), and how to failover the request if the target node not available. With every service request, the server node updates the stub interceptor with the latest changes in the cluster. For instance, if a node drops out of the cluster, each of the client stub interceptor is updated with the new configuration the next time it connects to any active node in the cluster. All the manipulations on the service stub are transparent to the client application. The client-side interceptor clustering architecture is illustrated in Figure 1.2, “The client-side interceptor (proxy) architecture for clustering”.