Clustering mutable objects

[Sri Sankaran <Sr...@sas.com>]

I was reading Tapestry In Action and did not understand the implication of clustering a mutable object. Hope you can explain.
 
To set the stage, the section states that an app server -- such as Web Logic -- broadcasts a change to all servers in a cluster in response to an invocation of the setAttribute method of an HttpSession.  The article goes on differentiate mutable and immutable objects placed in a session.  It says that depending on the sequence of the steps, the value of a session attribute on server in a cluster can be stale.  For example:
 
session.setAttribute("some token", x);
x.setSomeValue("some value");
 
In this case, since the object referred to by 'x' is being modified *after* a call to setAttribute, it may not get propagated to all servers in a cluster.  The cagey *may not* is because we may get lucky if the app server hasn't got around to serializing 'x' and broadcasting it, before the application invokes the setSomeValue method.
 
My question is: How is this any different if x were an immutable object -- such as a String?  Wouldn't we have the same problem?
 
Sri