Session State Replication of Referenced Objects

Question

Here is question on Clustering and Session state replication:
          Scenario:
          Consider two Session Objects A and B.
          Object A has-a reference to object B.
          So whatever changes done to Object B is reflected in Object A.
          In-memory Session replication happens on every HttpSession.setAttribute() call.
          The question is will the two session objects(A and B) be replicated to the other cluster and will still maintain the reference relationship or will they be two separate objects with same data (in different memory locations).
          regards,
          Jaiganesh

Answer 1

Ok, I have been meaning to dig into session replication in a bit more detail and your question is a good excuse to do it!
          First stop the docs:
          Bea Docs
          The bits that jump out are under:
          Programming Considerations for Clustered Servlets and JSPs
          1) Session Data Must Be Serializable, so both your objects are serializable.
          2) Use setAttribute to Change Session State In an HTTP servlet that implements javax.servlet.http.HttpSession, use HttpSession.setAttribute (which replaces the deprecated putValue) to change attributes in a session object. If you set attributes in a session object with setAttribute, the object and its attributes are replicated in a cluster using in-memory replication. If you use other set methods to change objects within a session, WebLogic Server does not replicate those changes. Every time a change is made to an object that is in the session, setAttribute() should be called to update that object across the cluster.
          Likewise, use removeAttribute (which, in turn, replaces the deprecated removeValue) to remove an attribute from a session object.
          >Will the reference as shown in the step 3 will be >maintained after replication on the other cluster ?? I >guess not!!
          I don't understand "on the other cluster" There is normally one cluster made up of a few servers. The session state is maintained on one primary server and a secondary server. So you have aObj.a2 = bObj, from the docs I would understand that at the momment you called session.setAttribute the aObj and all its fields provided they are seriliazable will be replicated in Mememory.
          >There the aObj and bObj stored in the two different >session objects will not have a reference relationship >in other words if i change bObj on the other cluster >will the reference a2 of aObj be able see it ??
          This is testing the limits of my understanding, but would have thought if the objects are in different sessions they should not be able to reference each other in the manner I think you are suggesting, so if I access aObj.a2 (an instance of bObj) and changed it in one session it should not affet any other bObj unless bObj is a singleton....I think!
          What say you?