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| JohnRodey |
 Posted: Mon May 16, 2005 1:01 pm Post subject: Swapping messages based on priority to a full queue |
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Centurion
Joined: 13 Apr 2005
Posts: 103
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If a queue is full and an application wishes to place a high priority message on the queue, is there a way to tell mq to trash a low priority message and put this message in place of it.
I would like to find way to do the deletion of the low priority message and the put of the high priority message simultaneously, because if I manually delete a message from the queue another request might come in to place a message on the queue (and be successful, since now there is an empty slot) prior to my message being posted.
Thanks for any help in advance |
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| fjb_saper |
| Posted: Mon May 16, 2005 3:05 pm Post subject: |
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Grand High Poobah
Joined: 18 Nov 2003
Posts: 20756
Location: LI,NY
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| What is your message delivery option for the queue: priority or FIFO ?? |
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| JohnRodey |
| Posted: Tue May 17, 2005 4:37 am Post subject: |
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Centurion
Joined: 13 Apr 2005
Posts: 103
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| JT |
| Posted: Tue May 17, 2005 6:25 pm Post subject: |
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Padawan
Joined: 27 Mar 2003
Posts: 1564
Location: Hartford, CT.
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| Quote: |
| If a queue is full and an application wishes to place a high priority message on the queue, is there a way to tell mq to trash a low priority message and put this message in place of it. |
AFAIK, no.
| Quote: |
| I would like to find way to do the deletion of the low priority message and the put of the high priority message simultaneously, because if I manually delete a message from the queue another request might come in to place a message on the queue (and be successful, since now there is an empty slot) prior to my message being posted |
I'm not 100% sure, but wouldn't this work if you put the get and the put in the same unit of work, and then issued the MQCMIT? |
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| JohnRodey |
| Posted: Fri May 20, 2005 3:35 am Post subject: |
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Centurion
Joined: 13 Apr 2005
Posts: 103
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Thanks JT for your response.
I thought what you said made sense so I tried it. I issued a get and put prior to disconnecting from the manager (with a sleep between the get and put). I figured either 2 things could happen.
Since the "got" message could still be backed out to the Q I wasn't sure if it (1) remained on the Q and was just blocked from other applications trying to get messages or if (2) it was physically taken off the Q (leaving an empty spot) and if a backout was called then just placed back on Q in the empty spot.
if (1) was true then could the put find a spot on the queue to place its message? And if (2) then could any application fill that spot?
I tried it and it turns out 2 is true. When an application pulls these messages from the queue there is an empty spot created on the queue that any application can place a message on. So, even if prior to disconnecting from the QM (or committing) doing a get then a put, after a get there is a slice of time (granted a very small slice) in which other puts can come along and steal that open spot on the Q.
This now makes me wonder...
If you do a get on a poison message and have to backout that message but that queue could now be full since its spot could have been overwritten, it seems the backout would most likely fail. I haven't tried this scenario, but if that's true It doesn't seem like the desired fundtionality. |
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| jefflowrey |
| Posted: Fri May 20, 2005 4:05 am Post subject: |
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Grand Poobah
Joined: 16 Oct 2002
Posts: 19981
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| JohnRodey wrote: |
This now makes me wonder...
If you do a get on a poison message and have to backout that message but that queue could now be full since its spot could have been overwritten, it seems the backout would most likely fail. I haven't tried this scenario, but if that's true It doesn't seem like the desired fundtionality. |
I would be surprised if the second put succeeded. Messages in transaction still count in queue depth - and probably for just this reason.
So if you have a queue that is 100% full, and you get a message in transaction, the queue depth will still be 100% - and so a put should fail.
_________________
I am *not* the model of the modern major general. |
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| JohnRodey |
| Posted: Fri May 20, 2005 11:05 am Post subject: |
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Centurion
Joined: 13 Apr 2005
Posts: 103
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| I agree jeff and I thought that as well, although I tried this and it does seem to allow the put even though the get wasn't committed. Although it is always possible that I did something wrong in my test, so if anyone finds different results then I would be interested in hearing them. |
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