Re: bug in JGStore

Davide Rossi wrote:
> So that you can add it to your own list.
> Session are replicated just when "touched" but to check if a session has 
> been touched you just look at the session itself and not into the 
> contents of the objects referenced by the session.
> In my case I have a list of items stored in the session, if I add or 
> remove items from the list (i.e. the reference in the session is the 
> same but the referenced object has been modified) the session does not 
> get propagated.
>
> Reagards,
>     Davide.

Yes,

This is a big problem with HttpSession replication.

It boils down to the fact that HttpSessions were initially designed as 
local objects, then had remote semantics grafted on to them with the 
addition of <distributable/> webapps.

You have three choices when trying to resolve this:

1). always replicate the whole session - you cannot know whether its 
contents have changed, so assume the worst case scenario - very expensive.

2). make a contract with the application that if they change an 
attributes value, they will call call session.setAttribute() and only 
replicate when you know the session is dirty - much more efficient, but 
does not handle change by reference - your problem.

3). use aspects/byte-code-engineering in some way to alter the app-space 
objects held in the session so that they will notify you upon change. 
Then you will know when the session is dirty and what to replicate. - 
would be highly efficient, but I don't see how it can be done and have 
yet to find anyone who has done it successfully.

Of course, you can use various levels of checks and caching to mitigate 
the effect of the above, but I think these are the basic solutions to 
the problem.

wadi.codehaus.org, my latest attempt to perfect the art of bomb-proof 
HttpSessions will provide both (1) and (2). The Jetty distributable 
session manager only provides (2). The best workaround for you is 
probably just to call s.setAttribute("X", s.getAttribute("X")) every 
time you know you have changed a session attribute value. This may add 
to the complexity of your code, but will give you a working solution.

Does that help ?


cheers,



Jules


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