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Guaranteeing “exactly once” semantics by using idempotency keys

A few weeks ago I had a discussion with a colleague about the importance of idempotency.

From http://www.restapitutorial.com/lessons/idempotency.html:

From a RESTful service standpoint, for an operation (or service call) to be idempotent, clients can make that same call repeatedly while producing the same result. In other words, making multiple identical requests has the same effect as making a single request. Note that while idempotent operations produce the same result on the server (no side effects), the response itself may not be the same (e.g. a resource's state may change between requests).

A good example where you can get into trouble is when your API withdraws some money from a customer account. If the user accidently calls your API twice the customer is double-charged, which I don’t think they’ll like very much…

A solution for this problem is the usage of idempotency keys. The idea is that the client generates a unique key that is send to the server along with the normal payload. The server captures the key and stores is together with the executed action. If a second requests arrives with the same key, the server can recognize the key and take the necessary actions.

What are situations that can happen?

  • Situation 1 – The request didn’t made it to the server; in this case when the second request arrives the server will not know the key and just process the request normally
  • Situation 2 –The request made it to the server but the operation failed somewhere in between; in this case when the second request arrives the server should pick up the work where it failed previously. This behavior can of course vary from situation to situation.
  • Situation 3 – The request made it to the server, the operation succeeded but the result didn’t reach the client; in this case when the second request arrives the server recognizes the key and returns the (cached) result of the succeeded operation.

Note: Idempotency keys get important when you are running systems that are not ACID compliant. If you are running an ACID transactional system, you can just re-execute the same operation as the previous operation should be rolled back(or at least that’s the theory Winking smile).

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