Whenever you deploy a web application to a WebLogic cluster, you need something sitting before the cluster (i.e., between the client and the cluster) that can direct incoming requests to the appropriate server that hosts the primary session state. There are many ways in which you can accomplish this:
A hardware load balancer can be placed in front of the cluster. In this case, all client requests to the cluster must pass through the load balancer. The load balancer can distribute requests across the available members of the cluster, and direct requests involved in a session to the appropriate server holding the primary state.
A web server can be augmented with a proxy plug-in, a piece of software supplied with WebLogic that will redirect certain requests (for example, those to servlets and JSPs) through to the cluster. Clients accessing the web server then can be served up static content by the web server directly and transparently proxied through to the cluster behind the web server for the dynamic content.
HttpClusterServlet is an alternative to a
proxy plug-in. A WebLogic instance can be configured to host the
servlet, which can forward requests across to the cluster in the same
way as the proxy plug-in.
The main ways in which you can architect your cluster and web tiers are discussed in much more detail in Chapter 14. Here we will give a brief introduction ...