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
