Idempotency Is Easy Until the Second Request Is Different

mmillin•30 minutes ago

This is an excellent article, I’ve seen almost all of the issues it calls out in production for various APIs. I’ll be saving this to share with my team.

I’ve seen two separate engineers implement a “generic idempotent operation” library which used separate transactions to store the idempotency details without realizing the issues it had. That was in an organization of less than 100 engineers less than 5 years apart.

One other thing I would augment this with is Antithesis’ Definite vs Indefinite error definition (https://antithesis.com/docs/resources/reliability_glossary/#...). It helps to classify your failures in this way when considering replay behavior.

shiandow•about 1 hour ago

This seems to assume retrying a command should result in the same response, but I am not sure I agree.

Idempotency is about state, not communication. Send the same payment twice and one of them should respond "payment already exists".

Jolter•3 minutes ago

I don’t know if we’re reading the same article? The linked one states very plainly:

”Idempotency is about the effect

An operation is idempotent if applying it once or many times has the same intended effect.”

cocoto•36 minutes ago

In your example, idempotency means same request + same state = same response. State becomes part of the request, that’s why it is hard.

shiandow•29 minutes ago

That's just deterministic behaviour.

For idempotency you literally just want f(state) = f(f(state)). Whether you achieve this by just doing the same thing twice (no external effects) or doing the thing exactly once (if you do have side effects) is not important.

But if you have side effects and need something to happen exactly once it seems a lot more useful to communicate this, rather than pretending you did the thing.

zinkem•about 1 hour ago

Idempotency is easy if you don't use mutable state in your middleware.

Auth, logging, and atomicity are all isolated concerns that should not affect the domain specific user contract with your API.

How you handle unique keys is going to vary by domain and tolerance-- and its probably not going to be the same in every table.

It's important to design a database schema that can work independently of your middleware layer.

chaz6•44 minutes ago

You keep the hash of the request so that you can reject a subsequent request with a different body. This has helped me surface bugs and data issues in other systems.

stavros•about 1 hour ago

Half of the mentioned issues are issues of atomicity, not idempotency. If I make a request, and the server crashes midway and doesn't send some crucial events, that's an issue whether or not I send a second request.

From a cursory read, only the part up to "what if the second request comes while the first is running" is an idempotency problem, in which case all subsequent responses need to wait until the first one is generated.

Everything else is an atomicity issue, which is fine, let's just call it what it is.

Idempotency Is Easy Until the Second Request Is Different

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