Cluster aware servlet development

Question

How can one develop a cluster-aware servlet and what is the design criteria for the same?

Answer 1

This isn't a problem which is to be solved at code level, but rather at webserver level. So the Servlet code doesn't need to be aware of being clustered.

Answer 2

As @BalusC said, this is primarily a server configuration task, and how to do it depends very much on which server you're using (and which you don't mention), but here's how to do it with Tomcat 6, for example.

There is one thing to keep in mind at the code side, though, which is that you have to be careful what objects you put into the HTTP session (using HttpSession.setAttribute(). For session replication to work, these objects have to be serializable in order to be transported across the network to the other servers in the cluster. If they are not serializable, then either the server may drop them, or it may throw an exception.

It's not uncommon for developers to use the HTTP session as a place to put large, complex business objects (to allow them to be accessed from JSPs, for example), and these things are very unlikely to be serializable. Other examples for form-binding objects which, while being simple form-data holders, are often not serializable.

Answer 3

The code does not need to be aware of being clustered but the developer needs to be aware that the code may be clustered and the session replicated. Let me explain.

When you mark an webapp in web.xml you are telling the container that that this web-application can be clustered.

If the webapp is deployed on a cluster, each machine in the cluster will run a vm and this webapp inside it. As far as the client is concerned the request it sees one webapp though each request from the client can be serviced by a different vm in the cluster.

So if the webapp is storing any state, it must be made available to all the instances of the vms(in the cluster) running the webapp. How can this be done ? By marking the things that you put into the httpsession object as "Serializable". You are signaling to the container that it should replicate the state to the other vms (if you have setup session replication). It is accomplished in a couple of ways in weblogic. Everytime you use setAttribute() on the session, it triggers a sessionreplication event.

In WL There are two ways of replicating inmemory replication and using database for replication . I would like to hear how this is done in other appservers.