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by pverheggen 38 days ago
I really like this approach, and am going to keep it in my back pocket.

However, it's good for e-commerce where there are a subset of "important" operations, but I'd argue the idempotency key is better for a financial API like Stripe where the majority of your operations need these semantics.

You also run into a problem if PUT/PATCH needs to be exactly-once instead of at least once. Not as common, but again, might be something you run into with a financial API.
1 comments
stickfigure 38 days ago
There's nothing special about any particular HTTP method; it is the inherent nature of distributed systems that you can not have exactly-once semantics with a single HTTP request. To create exactly-once semantics, you need a client which retries until success and a server which prevents duplicates.

The only difference between the approach I'm describing and Stripe's approach is the detail of how the client knows that it's done. "My" mechanism (notify the client that a request is a duplicate) was dominant in financial transaction processing systems before Stripe; I used probably a half dozen of them back in the day.

Stripe came along and from the beginning decided "we want to compete based on making a friendly API". They succeeded, and if you ever look at Paypal's original API, it's easy to see why. It's true that repeating successful API responses makes life slightly easier for clients; they never need to check for a 409 (or whatever) response. It comes at a cost of making life quite a bit more complex on servers. Personally I don't think the tradeoff is worth it, but YMMV. If your single competitive advantage is "easy API" maybe it makes sense. If you're normal B2B, almost certainly not.

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saltcured 37 days ago
FWIW, I enjoyed your engagement throughout the thread above, fighting the good fight.

I think there is also a risk of an "easy API" when it leads to magical thinking and sloppy development. If the naive client programmer starts to think the reliability is handled for them, they may also flub the handling of the idempotence key that remains the crux here. E.g. not persisting them well enough and returning to a situation where the user can accidentally make duplicate payments just like the naive system with no idempotence feature...

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