[Podcast] Message Ordering: Is it Cost Effective?

Ask Udi Podcast – Udi Dahan – The Software Simplist

In this podcast we’ll be discussing the issues around multi-threaded processing of messages by a service, specifically that the processing of message received second may be finished before that of the first. This scenario tends to rear its ugly head at higher levels of load and is critical for correctness in high-scalability environments.

Our long time listener Bill asks:

Hi Udi,

I have a question around processing of messages in proper order. When leveraging multiple threads to process messages in a message queue, it is possible for the second message in the queue to get processed before the first – especially if the first message is considerably larger than the second. I have taken a lot of care to make sure that messages are sent in the correct order, only to find that the receiving system can process them out of order anyway.

Consider a Policy Created notification, which must come before a Policy Approved notification. If both messages are sitting in the queue when the receiving service starts up, the approval message can be processed before the creation message. How can I make sure that message ordering is respected by the receiving system? I am using WCF/MSMQ as the underlying transport by the way. The only way I have found so far is to limit the receiving service to a single thread, which is by no means desirable.

Best Regards,

Bill

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Additional References

• Blog post: In-Order Messaging a Myth?
• Blog post: Handling Messages out of Order

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