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| Ordering messages across multiple instances |
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| crossland |
 Posted: Mon May 14, 2012 4:58 am Post subject: Ordering messages across multiple instances |
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Master
Joined: 26 Jun 2001
Posts: 248
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The article http://www.ibm.com/developerworks/websphere/library/techarticles/0311_dunn/dunn.html covers message sequencing as follows:
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Message sequencing is controlled by the Order Mode property on the Advanced Properties tab of the MQInput node. There are three possible values:
Default
Messages are retrieved in the order defined by the queue attributes, but this order is not guaranteed as the messages are processed by the message flow.
User Id
Messages that have the same UserIdentifier in the MQMD are retrieved and processed in the order defined by the queue attributes, and this order is guaranteed to be retained when the messages are processed. Therefore, a message associated with a particular UserIdentifier that is being processed by one thread will be completely processed before the same thread, or another thread, can start to process another message with the same UserIdentifier. No other ordering is guaranteed to be preserved.
Queue order
Messages are retrieved and processed by this node in the order defined by the queue attributes, and this order is guaranteed to be retained when the messages are processed. This behaviour is identical to the behaviour exhibited if the Additional Instances property of the message flow is set to 0. |
This seems to suggest that the only option for message sequencing across multiple instances is to order by userid (as the two other alternatives are either "order not guaranteed" or "identical to the behaviour exhibited if Additional Instances = 0"). However, many sites do not allow the userid to be set for security reasons.
Has anybody managed to achieve message sequencing across multiple instances, by a means other than setting order mode = userid? After a long trawl through this site and various others, I have been unable to find another alternative. |
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| lancelotlinc |
| Posted: Mon May 14, 2012 5:10 am Post subject: |
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Jedi Knight
Joined: 22 Mar 2010
Posts: 4941
Location: Bloomington, IL USA
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If message ordering is your goal, this implies that you intend to process messages sequentially. What's the point of having multiple instances when you need to process messages sequentially?
If you want multiple sequential streams, set a CorrelId and assign that CorrelId to a specific instance.
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| crossland |
| Posted: Mon May 14, 2012 6:19 am Post subject: |
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Master
Joined: 26 Jun 2001
Posts: 248
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The manual states the following for order by userid"
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| Messages that have the same UserIdentifier in the MQMD are retrieved and processed in the order that is defined by the queue attributes; this order is guaranteed to be preserved when the messages are processed. |
Are you saying that assigning the CorrelId to a specific instance would also result in the order being guaranteed to be preserved when the messages are processed? |
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| lancelotlinc |
| Posted: Mon May 14, 2012 6:26 am Post subject: |
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Jedi Knight
Joined: 22 Mar 2010
Posts: 4941
Location: Bloomington, IL USA
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| crossland wrote: |
The manual states the following for order by userid"
| Quote: |
| Messages that have the same UserIdentifier in the MQMD are retrieved and processed in the order that is defined by the queue attributes; this order is guaranteed to be preserved when the messages are processed. |
Are you saying that assigning the CorrelId to a specific instance would also result in the order being guaranteed to be preserved when the messages are processed? |
No.
Flow 1 MQInput node with CorrelId set to 1. Flow 2 MQInput node with CorrelId set to 2. Flow 3 MQInput node with CorrelId set to 3.
Each CorrelId represents a unique stream where the messages are utilizing sequence Id within that stream:
http://www.ibm.com/developerworks/websphere/library/techarticles/0602_currie/0602_currie.html
The point is, you are illogical in your premise. You say you want multiple instances of your flow, but that the messages must be processed sequentially. This is illogical (unless you follow the grouping as I have described).
What's the point of having multiple instances when you must process the messages sequentially anyway?
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| kimbert |
| Posted: Mon May 14, 2012 11:32 am Post subject: |
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Jedi Council
Joined: 29 Jul 2003
Posts: 5542
Location: Southampton
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lancelotinc said:
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| What's the point of having multiple instances when you must process the messages sequentially anyway? |
Perhaps because lower latency can be achieved by splitting the workload for a single batch of messages across multiple instances, and then re-sequencing the results.
I'm just theorising, by the way. I'll leave the rest to others who have implemented actual solutions to this problem. |
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| lancelotlinc |
| Posted: Mon May 14, 2012 11:58 am Post subject: |
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Jedi Knight
Joined: 22 Mar 2010
Posts: 4941
Location: Bloomington, IL USA
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| kimbert wrote: |
lancelotinc said:
| Quote: |
| What's the point of having multiple instances when you must process the messages sequentially anyway? |
Perhaps because lower latency can be achieved by splitting the workload for a single batch of messages across multiple instances, and then re-sequencing the results.
I'm just theorising, by the way. I'll leave the rest to others who have implemented actual solutions to this problem. |
Yes, Sir Kimbert, a good use of this design. Would this be more akin to Fan-Out-Fan-In? And since the recomposition is done in the Fan-In flow, sequentializing the Fan-Out is no longer important (meaning that the OP's original premise is not valid since the logic of recomposition is done outside of the original transaction).
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