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Yeah the duplicate email thing is a classic problem, but I’m not sure it’s one of “idempotence”. This can happen in any (intended to be) transactional operation that creates a side affect.

Hit an error, roll-back, side-affect can’t be rolled back. Retry - side-affect happens again.

Wouldn’t the general approach be to have unique message identifiers and queue side-affects? Maybe I’m missing lots of subtleties.

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Email is absolutely something that requires idempotence to avoid sending duplicates. Even if your code is perfect and you don't send emails until after you commit your transaction, the actual http request to the email provider could fail in a way where the email is sent but your system can't tell.

Idempotency (either via a token in the request, or another API to check the result of the previous request) is required to prevent duplicates. And this requires the third party service to support idempotency; there's nothing you can do on your side to enable it if their service doesn't support it.

What if your system is the one actually sending the emails (ie, you are the 3rd party in this scenario)

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