It seems to me that there is very little discussion of clustering on
this newsgroup. I am curious about the following issues.
1. Have you used successfully implemented and deployed a clustered
system that uses ejb in a real world applications?
a) If so what server did you use?
b) Did you use entity beans and session beans?
c) Did you have to modify or redesign or program in order to achieve
correctness of the program?
Part c above relates to another question I have. In the case of
weblogic your program which was designed for a single server may no
longer work in the clustered case for several reasons. This is contrary
to what I would expect or like. In other words I would like the fact
of whether the system is clustered to be totally transparent to the
programmer. I should be able to write my program without thinking about
clustering and it should then work in a clustered system. This is not
the case for weblogic for the following reasons:
1) Clustering is transparent only for idempotent stateless session
beans. (Idempotent means that the same method can be called multiple
times without sacrificing integrity of the data. Or that calling it more
than once has the same effect as calling it once.)
2) For entity beans no automatic failure is provided. You need to wrap
your method calls around a try, catch(RemoteException) block and retry
the call in a loop.
3) I am unclear about stateful beans using weblogic clustering.
4) In single server mode you can assume pessimistic concurrency is
used. In a clustered mode you must assume optimistic concurrency is
used.
5) You cannot use dbIsShared=false and take advantage of caching of the
entity beans.
The bottom line is that when you design your system for weblogic you
should be thinking about clustering up front. Many weblogic designs are
based on pessimistic concurrency. If you do not you may wind up having
to make significant design changes afterwards.
I would like to see a discussion and comparison of how other ejb servers
handle clustering.
dan
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