MDB Topic & concurrency

Question

The JMS faq say
"For Topics in WebLogic JMS 6.1, there is one JMSSession per bean instance in
the pool. Because of the way Topics work, the session, and thus every bean instance,
receives a copy of each message published on that Topic.
-- snip --
For each kind of MDB listening on the topic, the message is delivered exactly
once (i.e., the message will be delivered exactly once to an instance in each
named MDB pool listening on the topic)."
The two statements seem to condradict each other. One seems to say that each
instance in a pool will get a copy of the message and the otehr syas that the
pool as a whole will get only one copy of the message.
Am I being dense? Is there a version issue? What's going on?
-Gordon

Answer 1

If you have a cluster of N WLS servers and on each server you have a MDB
deployed which listens to topic T. Then an instance on each of the N
servers will receive a message from the topic.
If you send P messages to the topic T, then up to P instances may be
processing the messages in parallel on a given server. However, under
the covers they will all actually be using the same JMS Session. The
EJB container does some "fancy" stuff to get the JMS acks right. This
is one of those things that makes MDBs very attractive since it's a pain
to write this code yourself in JMS.
-- Rob
Gordon Palumbo wrote:
I would agree, but given the FAQ and this statement from an old post
(feb 2001)
"In the upcoming 6.0 service pack, only one instance per deployment
will receive
a copy of a topic message. In 6.0, each deployment listener instance
receives
a copy."
The statement that each instance get's a session (and is thus a
seperate listener)bothered
me.
I got worried about expected behavior or a lingering bug in Weblogic 6.X
I was looking for someone to confirm the actual behavior in WebLogic 6.1
-Gordon
Simon Evans wrote:
it is saying for each bean, there is a pool of instances. the message
gets delivered to each bean, but to only one instance from the pool.
one instance will be picked out of the pool to handle the message.
it would not make sense for each instance of the bean in a pool to
receive the message.
Gordon Palumbo wrote:
The JMS faq say
"For Topics in WebLogic JMS 6.1, there is one JMSSession per bean
instance
in
the pool. Because of the way Topics work, the session, and thus everybean instance,
receives a copy of each message published on that Topic.
-- snip --
For each kind of MDB listening on the topic, the message is deliveredexactly
once (i.e., the message will be delivered exactly once to an instancein each
named MDB pool listening on the topic)."
The two statements seem to condradict each other. One seems to saythat each
instance in a pool will get a copy of the message and the otehr syasthat the
pool as a whole will get only one copy of the message.
Am I being dense? Is there a version issue? What's going on?
-Gordon