Tom's question was really about approach rather than solutions and you did
ask for views... You might choose a "simple" scenario such as pure SSBs
first, but I tend to agree with Tom that you should consider the core
building blocks (e.g. DTM, cluster membership, resource location, state
replication, and many more) whilst doing so as in reality none of the
scenarios are actually that simple.
As for applicability of a DTM, one of the goals of a cluster is location
transparency (at least to the application). So consider a cluster with two
nodes, 1 & 2, and two SSBs A & B; SSB A calls a method on SSB B; SSB A is
deployed to node 1, SSB B to node 2.
So if a client invokes SSB A, it is directed by availability to node 1. SSB
A calls SSB B and is directed by availability to node 2. You now have more
than one node involved which requires a distributed transaction.
Now make it more complex, deploy SSB A on both node 1 and node 2 so the
client could be load balanced to either node. If it goes to node 1 then you
have a distributed transaction as above whereas if it is directed to node 2
then the transaction is local to node 2.
Take it further and deploy both beans on both nodes. The client can get
load-balanced to either node; in theory so could the inter-bean call but due
to the overhead of a remote call + DTM you would prefer it was directed to
the local machine.
However, suppose SSB B becomes temporarily unavailable on one node for some
reason; availability would require that the inter-bean call is now directed
to the surviving node. The determination of which node will service the
request is dynamic, based on availability, and as a result so is the
determination of whether the transaction is local or distributed.
If you plan to support scenarios beyond the most basic, you *will* need a
DTM so design it in early.
Jeremy
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of marc fleury
Sent: Tuesday, March 06, 2001 7:18 PM
To: JBoss-Dev
Subject: RE: [jBoss-Dev] CL: Clustering, let's get started
what does a dtm have to do with Stateless session bean load balancing and
high availability....
?
marc
