July 5th, 2002

Idempotence: avoiding errors in web service interactions

I'm currently sitting in the main hall at TechEd, filling in the gap between the first and second parts of Don Box's 3 part extravaganza presentation. He's talking about the problems that can arise when using web services to handle distributed computing - and thinking about the solutions...

One key point is the concept of idempotent messages: messages that can be sent many times with the same effect - so there is no difference in the result between message 1 and message n.

To explain a bit further, this can be very important if you're in the middle of a transaction and a send-action-response sequence fails to send the response, so you resend, only to trigger another instance of the action... Leaving you to deal with the consequences of send-action-send-action-send-action-response - where the response only reports the last send-action pair...

Now, the idea's clear... but the implementation? Something that could need quite a bit of thought, but one option is the obvious inclusion of a sequence header in the XML message.
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