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0 pointsluuio10mo ago0 comments

> why wouldn't doing the same write twice produce an idempotent result

you can imagine this:

```

var thing = KVStore.Get<MyThing>(...);

if (things.Checkpoints.Contains(myUuid) == false) {

  thing.Counter += 1;

}

KVStore.Update(thing); ```

having an etag doesn't help you with retries, where we expect that `thing` could be mutated by another flow between your retries.

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jrochkind110mo ago

If the thing was mutated between your retries, then wasn't the etag changed by that mutation? So if you know the etag you started with, your conditional update on etag fails due to changed etag. So you fetch it again and start over. Which is the general optimistic locking algorithm.

i may be missing something though?

luuioOP10mo ago

let's say you have an object like this when you started: {count: 10, etag: 1}. then for some reason, something failed.

when you retry and load the object, you get {count: 12, etag: 3}. how do you know if your previous attempt had successfully persisted or not, or if the updates to the object came from other processes/requests?

you're mixing up conflict handling vs. idempotency

jrochkind110mo ago

Ah, the system is not capable of giving you a confirmation that your update succeeded, you don't really have a way to know without external "checkpoint" system?

Anyway, okay, I get that I don't get it, and I get that it does sound terrible, agreed!

luuioOP10mo ago

Look up the two general problem on youtube. Unless the entire end to end operation is wrapped inside a giant transaction, no system in the world can give you the confirmation.

Imagine this: you issued a write, a few things can happen: 1. The callsite crashed, maybe due to an out of memory issue or whatever. You don't know if it succeeded

2. The database returned an acknowledgement, but then the callsite crashed before storing the acknowlwedgement for the next step.

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jrochkind110mo ago

i may be missing something though?

luuioOP10mo ago

let's say you have an object like this when you started: {count: 10, etag: 1}. then for some reason, something failed.

you're mixing up conflict handling vs. idempotency

jrochkind110mo ago

Ah, the system is not capable of giving you a confirmation that your update succeeded, you don't really have a way to know without external "checkpoint" system?

Anyway, okay, I get that I don't get it, and I get that it does sound terrible, agreed!

luuioOP10mo ago

Look up the two general problem on youtube. Unless the entire end to end operation is wrapped inside a giant transaction, no system in the world can give you the confirmation.

Imagine this: you issued a write, a few things can happen: 1. The callsite crashed, maybe due to an out of memory issue or whatever. You don't know if it succeeded

2. The database returned an acknowledgement, but then the callsite crashed before storing the acknowlwedgement for the next step.

1 more reply

j / k navigate · click thread line to collapse