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| Message lost when interrupting application |
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| rgarcia |
 Posted: Tue Nov 12, 2002 1:49 am Post subject: Message lost when interrupting application |
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Newbie
Joined: 12 Nov 2002
Posts: 5
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Hi, I'm pretty new to MQSeries programming and I've met with a serious problem:
I am doing a Java application under Linux that works with a Solaris MQSeries 5.1 Server. The problem is the following: if I am locked in a MQGet call and I Ctrl+C the application (thus abnormally finishing it), next time I run the application the first message on the queue gets lost. 
This is happening always. Is there a programming technique or method to avoid this?
Thanks in advance  |
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| mgrabinski |
| Posted: Tue Nov 12, 2002 3:40 am Post subject: |
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Master
Joined: 16 Oct 2001
Posts: 246
Location: Katowice, Poland
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The message did not get lost - it was consumed by your application. You can deal with this in two ways:
1) MQGET with MQGMO_BROWSE_NEXT option (message stays in queue after being read)
2) MQGET with MQGMO_SYNCPOINT option (getting a message within a transaction, the message will stay in the queue until you issue MQCMIT, if you abend your app, it will cause internal MQBACK resulting in the message being left in the queue)
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Marcin Grabinski <>< |
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| rgarcia |
| Posted: Tue Nov 12, 2002 3:58 am Post subject: |
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Newbie
Joined: 12 Nov 2002
Posts: 5
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Thanks, that really sounds interesting, but the fact is that when I Ctrl+C my app, the queue is empty, then I run the app and send a message to the queue. Is this first message that gets lost, as if nothing has happened. The second one works fine. Any more clue, please?  |
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| nimconsult |
| Posted: Tue Nov 12, 2002 4:34 am Post subject: |
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Master
Joined: 22 May 2002
Posts: 268
Location: NIMCONSULT - Belgium
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If I understand well, your app is waiting on MQGET (with MQGMO_WAIT option) when you interrupt it. After you have interrupted the application, the first message you put on the queue gets lost.
If this is the description of your problem, then the reason is that the MQ Series agents linked to your application has not realized that your application is interrupted. When you put a first message on the queue, the agent retrieves the message, then fails in attempting to pass it back to the application and dies. When you put a second message on the queue, the agent is dead so the second message (and subsequent messages) remains on the queue.
How can you solve the issue? Implement your MQGET with MQGMO_SYNCPOINT. If you work under syncpoint, the agent will issue a rollback if it cannot pass the message to your application, so the message will go back into the queue.
I have already met this problem and I also find it frustrating. I don't like with this problem the side effect that the message sequence can be broken.
Nicolas
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Nicolas Maréchal
Senior Architect - Partner
NIMCONSULT Software Architecture Services (Belgium)
http://www.nimconsult.be |
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| mgrabinski |
| Posted: Tue Nov 12, 2002 4:46 am Post subject: |
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Master
Joined: 16 Oct 2001
Posts: 246
Location: Katowice, Poland
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I'm not sure I get you correctly:
- you try to MQGET a message (with MQGMO_WAIT I assume)
- the queue is empty, so you abend your app
- you try to MQPUT two messages
- the first one gets lost
Is the expiry value set for the first mesasge? Are you puting with SYNCPOINT? Are you sure that nobody reads the queue in the meantime?
I know that those questions seem trival..
Can you post most crucial (from MQ point of view) parts of your code?I'd like to see put and get options especialy.
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Marcin Grabinski <>< |
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| rgarcia |
| Posted: Tue Nov 12, 2002 7:05 am Post subject: |
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Newbie
Joined: 12 Nov 2002
Posts: 5
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YES!  By activating MQGMO_SYNCPOINT in the MQGet options the message gets not longer lost. Lots of thanks!!! |
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| PeterPotkay |
| Posted: Wed Nov 13, 2002 6:22 pm Post subject: |
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Poobah
Joined: 15 May 2001
Posts: 7722
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Marcin,
You have a Get with wait, and the queue is empty. If you abend your app, the app goes away, but the QM may not realize it right away. You can see this beacuse even though your app is gone, IPROCS count on the queue is still 1 for a while (undocumented by IBM as to how long. "About 10 seconds" is the best answer I got).
Now 2 messages land on the queue. As soon as the first message lands in the queue, the QM sees IPROC > 0, and sends the message to the "app". When it gets there, it says "Ooops, there is no one here! Should the message be rolled back? Well, there was no syncpoint associated with this last GET, so toss it. Oh, and fix that IPROCS count now." The second message does not get shipped to the phantom GET.
If the original GET was done with syncpoint, that 1st message would have been replaced back to the queue.
This is how it has been explained to me. I think I will test this whe I get a chance. For one thing, I am curious if the MQMD_BACKOUTCOUNT will go up by one on that 1st message when no syncpoint is used. And if I wait until IPROCS goes to 0, will the 1st message not get lost?
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Peter Potkay
Keep Calm and MQ On |
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| nimconsult |
| Posted: Wed Nov 13, 2002 11:41 pm Post subject: |
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Master
Joined: 22 May 2002
Posts: 268
Location: NIMCONSULT - Belgium
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Hi Peter,
I have performed some of these experiences some times ago.
You are right, it's a question of a pending application agent running under MQ that does not realize that the application has gone away.
After the first message hits the queue, the agent gets it from the queue and dies in absence of the application. It's not really that MQ "fixes" the IPPROCS, but simply that the agent stops, which decreases the IPPROCS by 1.
If the original get was done within syncpoint, the first message goes back to the queue and the backout count gets increased by 1, as usual. After the first message, the agent is gone, so the second message will not disappear.
One side effect of this ghost application is that you can have a broken message sequence. Let me try to illustrate this:
- I start MYSERVER.EXE, which performs a get wait under syncpoint.
- For one reason or another, the application process gets killed.
- I restart the application MYSERVER.EXE
- I post MESSAGE1 and MESSAGE2 on the queue, in this sequence.
- MESSAGE1 is retrieved by the "ghost" agent
- MESSAGE2 is retrieved by MYSERVER.EXE
- the "ghost" agent puts MESSAGE1 back on the queue and dies
- MESSAGE1 is retrieved by MYSERVER.EXE
_________________
Nicolas Maréchal
Senior Architect - Partner
NIMCONSULT Software Architecture Services (Belgium)
http://www.nimconsult.be |
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| mgrabinski |
| Posted: Thu Nov 14, 2002 12:38 am Post subject: |
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Master
Joined: 16 Oct 2001
Posts: 246
Location: Katowice, Poland
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Peter, Nicholas - thanks for your thorough explanation. Althouth I've never seen that in my experience, but it's good to know.
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Marcin Grabinski <>< |
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