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czaszek
PostPosted: Sun Apr 22, 2007 11:53 pm    Post subject: XA and extended transactional client Reply with quote

Apprentice

Joined: 30 May 2006
Posts: 36
Location: Poland

Hi All

In our system we are going to implement failover capabilities, however I am not sure whether they will work. The main problem I see at the moment is XA transaction rollback.
The system:
1. support for XA
2. message affinity (message order has to be kept)

In the gateway layer there is running MQ qmgr (6.0.2...), to this server connects one server from backend layer (single MDB) and process messages from one queue and puts to another. The server (BEA WLS) connects using extended transactional client (with XA support).
This server needs to have a standby instance (which takes over in case of failure of the first one). We plan to run a cronjob which will monitor the queue depth and the last timestamp of get from the queue in the gateway queue manager. In case there is no get within a specified time the cronjob will stop SVRCONN channel 1 (used by first BEA server) and start channel 2 (used by the second BEA server). The second one BEA server will start processing messages.

The problem is when there are ongoing XA transactions, which can not be rolled back because the channel 1 is stopped.
My questions are:
- is there something like XA transaction timeout, which will happen in case transaction manager (BEA server) is not available, and all resource managers (in this case MQ queues) will rollback the transactions.
- how the XA transaction works, I mean: does the message is taken from a queue for processing and in case of failure is put back on the top of the queue or is it only browsed but not removed from the queue until commit is executed?

Thanks in advance for any help
Lukas
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Vitor
PostPosted: Mon Apr 23, 2007 1:23 am    Post subject: Reply with quote

Grand High Poobah

Joined: 11 Nov 2005
Posts: 26093
Location: Texas, USA

It's never a good idea to have message affinity in any MQ system, for reasons including the ones you've discovered. Try and design this out.

If the WLS is the software being failed over, what are you using as the XA transaction coordinator?
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czaszek
PostPosted: Mon Apr 23, 2007 1:38 am    Post subject: Reply with quote

Apprentice

Joined: 30 May 2006
Posts: 36
Location: Poland

I know that message affinity is not a good idea, however we can not change it. Our system will be a re-write of an old system, which is used by many clients and message order is one of critical requirements they have, which can not be changed.

BEA WLS will be a transaction manager of our transactions. That is why I asked whether there is possible transaction timeout/rollback without transaction manager active (after the first server failes, and the second one does not start yet)
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Vitor
PostPosted: Mon Apr 23, 2007 2:23 am    Post subject: Reply with quote

Grand High Poobah

Joined: 11 Nov 2005
Posts: 26093
Location: Texas, USA

czaszek wrote:
BEA WLS will be a transaction manager of our transactions. That is why I asked whether there is possible transaction timeout/rollback without transaction manager active (after the first server failes, and the second one does not start yet)


I've no direct expereince of BEA, but loss of contact with the coordinator should cause MQ to rollback the transaction. How will you register the second instance as the new coordinator with the queue manager?
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czaszek
PostPosted: Mon Apr 23, 2007 3:20 am    Post subject: Reply with quote

Apprentice

Joined: 30 May 2006
Posts: 36
Location: Poland

Honestly I have no idea who WLS does it. I hope it can be done automatically without any manual intervention. When I start the server connection channel WLS will be able to connect and create connection factory and connection pool (single MDB).
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