Idempotency for aiohttp servers (and why client retries need it)

[violetta98]

Hi everyone!

Sharing a library and a gotcha that cuts both ways.

On the client side: if you retry a POST, generate the Idempotency-Key once outside the retry loop. Generate it per attempt and the server sees a fresh request every time, which defeats the point.

On the server side: honouring that key is harder than it looks. The obvious version checks whether the key exists and then stores it, and those are two steps, so two retries landing together can both pass the check.

from idemkit import MethodConfig, RedisBackend, idempotent

@idempotent(backend=RedisBackend.from_url("redis://localhost:6379"),
            config=MethodConfig(key_fields=["order_id"]))
async def charge(*, order_id: str, amount: int):
    return await payments.charge(order_id, amount)

Same order_id runs once. A second request arriving while the first is still going waits and gets its result rather than an error.

https://github.com/idemkit/idemkit, Apache-2.0, async-native, zero-dependency core.

If it's useful, a star would really help, it's a new project.

[Dreamsorcerer]

If you're offering idempotency, then you shouldn't be using POST to start with.

I also don't see any example using aiohttp, so not sure why you're posting here? Even the example above looks like it may be for FastAPI, not aiohttp...