3 ms·

That's usually solved with traditional database transactions. Even if you have a complex long-running multistep orchestration problem, you can break it down in

That's usually solved with traditional database transactions. Even if you have a complex long-running multistep orchestration problem, you can break it down into simpler transactions. Eg you could start with a "lock the resources" txn. But 99% of these conversations around idempotence are simple POST operations like "create order" that regular old database concurrency management handles just fine.

pdonis 6mo ago
> That's usually solved with traditional database transactions. That doesn't answer my question. What response do you return to the client in the case I described?
- matsemann 6mo ago
  The lock would normally make the second request wait, aka not return a response, until the first one is done. Then it sees it's a duplicate and returns that. Or it times out and returns an error. Then the client hopefully have some exponential back off strategy, so the third attempt doesn't suffer the same fate.
  - MattDaEskimo 6mo ago
    If a transaction is locked then subsequent requests would return a 409, ideally with an error message indicating that it's currently being processed.
- stickfigure 6mo ago
  This is just normal concurrent programming? If two requests come in for the same idempotency key/customer reference id, only one will succeed. Use standard database transaction isolation. So one will complete with 200, one will complete with 409. It doesn't matter which. That said, there's something odd about the way you phrased this question. If the original request hasn't gotten a response yet, why is it sending a retry? What you're asking is more general: What happens when two conflicting requests come in? This is something we've been solving with RDBMSes since the 1970s.
  - pdonis 6mo ago
    > If the original request hasn't gotten a response yet, why is it sending a retry? Um, because connections over the Internet aren't 100% always on? Because packets can get lost? Because computers sometimes have to reboot? You're assuming that the client will always receive whatever response your server finally sends, and that the client will wait indefinitely to receive a response. Neither of those things are true. So the client can be in a state where it sends a retry because it got no response and doesn't know why. And that means a retry request could come in while the first one is still being resolved--because the client had a timeout or it rebooted or something else happened that made it lose the connection state it previously had. That's the case I'm asking about.
    - stickfigure 6mo ago
      I'm not assuming anything. Let me try to reframe this for you. The case of "client sends a retry with the same idempotency key" generalizes to "multiple requests come in for the same idempotency key". These can come in spread out over time (like a traditional loop), or they could come in at once. The solution is the same either way. The problem of "how do we deal with multiple conflicting requests coming in at once" is something we have been dealing with for decades. We have databases with transactions and isolation levels. If I said in an interview "make an endpoint that inserts a value in a database and returns an error if the value is a duplicate", any competent backend web developer should be able write it without Claude's help. Concurrency is part of our life. Whether you want to return 409 or replay the success is irrelevant to this question. You must serialize the idempotent operation on the server, because you can have multiple requests coming in simultaneously. If you put the operation in a database transaction with an appropriate isolation level, you are most of the way there.
      - pdonis 6mo ago
        > I'm not assuming anything. Sure you are. You said: "Retries will only receive 409 if the original request was successful. If the original request failed, the server performs the operation as normal on the second request. It doesn't replay failures." I understand all that just fine; you don't need to keep trying to "reframe" it. But what you said that I just quoted above assumes, implicitly, that if you get a second request with the same idempotency key, the original request has either failed or succeeded--because you don't even address the case where neither of those things are true. I'm asking you to address that case. If your answer is "that will never happen", I disagree, and I explained why in response to your question about why the client would send a retry if it hasn't received a response to the original request. You could answer, I guess, that you still think that would never happen--and I would still disagree. But at least that would be an answer. So far all you've done is "reframe" something that I already understand and wasn't asking about.
hn_throwaway_99 6mo ago
To be honest, I liked your original response about returning a 409 - it's not something I'd done before and I like how it keeps things simpler. But your follow up responses here are making me rethink. Now you have to have all these special cases where the original request is still in process. I think or assertion of "99% are simple POST operations" is bullshit. For the times where idempotency is hard and really matters, often times you're calling a third party API, like a payment processing API. I would think a better approach would be to always return a 409 on a subsequent request, regardless of whether it passed or failed, and then have a separate standard API that lets you get the result of any request by its idempotency key.
- stickfigure 6mo ago
  > special cases where the original request is still in process This isn't a special case, and it's the same problem if you want to replay the original response on conflict. If the original request isn't complete, what are you going to replay?
  - pdonis 6mo ago
    > If the original request isn't complete, what are you going to replay? Who says you have to replay? If you get a second request with the same idempotency key, and the original request is still in process, why not just send the client a response that says so?
    - deleted 6mo ago
      [deleted]
    - stickfigure 5mo ago
      It's not a terrible question. It would be complicated to implement and isn't more useful than using the database's consistency model, so nobody does it that way. Long running transactions create all sorts of problems, so transactions are generally expected to be short. The actual work behind "create payment" or "create order" is generally fairly trivial - more or less insert a row in a table. There's no good reason to make the API complicated... you either "win" at concurrency or you lose, and the difference is generally sub-millisecond. The only meaningful thing you need to communicate to the client is "you're done" (for both the win and lose cases) or "you need to try again" (for the "something unexpected went wrong" case). Complicated workflows can certainly have multiple steps, with "fetch the current status" calls in between. But somewhere near the beginning of every complicated workflow there will be a call to "create workflow" and it will need to have sort of mechanism which allows clients to call it idempotently. Otherwise you end up with multiple starts. I've literally received duplicate products in the mail because of this kind of problem. I've also sent multiple products in the mail because services I relied on didn't offer the necessary idempotency mechanisms.

pdonis 6mo ago

> That's usually solved with traditional database transactions. That doesn't answer my question. What response do you return to the client in the case I described?

matsemann 6mo ago
The lock would normally make the second request wait, aka not return a response, until the first one is done. Then it sees it's a duplicate and returns that. Or it times out and returns an error. Then the client hopefully have some exponential back off strategy, so the third attempt doesn't suffer the same fate.
- MattDaEskimo 6mo ago
  If a transaction is locked then subsequent requests would return a 409, ideally with an error message indicating that it's currently being processed.
stickfigure 6mo ago
This is just normal concurrent programming? If two requests come in for the same idempotency key/customer reference id, only one will succeed. Use standard database transaction isolation. So one will complete with 200, one will complete with 409. It doesn't matter which. That said, there's something odd about the way you phrased this question. If the original request hasn't gotten a response yet, why is it sending a retry? What you're asking is more general: What happens when two conflicting requests come in? This is something we've been solving with RDBMSes since the 1970s.
- pdonis 6mo ago
  > If the original request hasn't gotten a response yet, why is it sending a retry? Um, because connections over the Internet aren't 100% always on? Because packets can get lost? Because computers sometimes have to reboot? You're assuming that the client will always receive whatever response your server finally sends, and that the client will wait indefinitely to receive a response. Neither of those things are true. So the client can be in a state where it sends a retry because it got no response and doesn't know why. And that means a retry request could come in while the first one is still being resolved--because the client had a timeout or it rebooted or something else happened that made it lose the connection state it previously had. That's the case I'm asking about.
  - stickfigure 6mo ago
    I'm not assuming anything. Let me try to reframe this for you. The case of "client sends a retry with the same idempotency key" generalizes to "multiple requests come in for the same idempotency key". These can come in spread out over time (like a traditional loop), or they could come in at once. The solution is the same either way. The problem of "how do we deal with multiple conflicting requests coming in at once" is something we have been dealing with for decades. We have databases with transactions and isolation levels. If I said in an interview "make an endpoint that inserts a value in a database and returns an error if the value is a duplicate", any competent backend web developer should be able write it without Claude's help. Concurrency is part of our life. Whether you want to return 409 or replay the success is irrelevant to this question. You must serialize the idempotent operation on the server, because you can have multiple requests coming in simultaneously. If you put the operation in a database transaction with an appropriate isolation level, you are most of the way there.
    - pdonis 6mo ago
      > I'm not assuming anything. Sure you are. You said: "Retries will only receive 409 if the original request was successful. If the original request failed, the server performs the operation as normal on the second request. It doesn't replay failures." I understand all that just fine; you don't need to keep trying to "reframe" it. But what you said that I just quoted above assumes, implicitly, that if you get a second request with the same idempotency key, the original request has either failed or succeeded--because you don't even address the case where neither of those things are true. I'm asking you to address that case. If your answer is "that will never happen", I disagree, and I explained why in response to your question about why the client would send a retry if it hasn't received a response to the original request. You could answer, I guess, that you still think that would never happen--and I would still disagree. But at least that would be an answer. So far all you've done is "reframe" something that I already understand and wasn't asking about.

hn_throwaway_99 6mo ago

To be honest, I liked your original response about returning a 409 - it's not something I'd done before and I like how it keeps things simpler. But your follow up responses here are making me rethink. Now you have to have all these special cases where the original request is still in process. I think or assertion of "99% are simple POST operations" is bullshit. For the times where idempotency is hard and really matters, often times you're calling a third party API, like a payment processing API. I would think a better approach would be to always return a 409 on a subsequent request, regardless of whether it passed or failed, and then have a separate standard API that lets you get the result of any request by its idempotency key.

stickfigure 6mo ago
> special cases where the original request is still in process This isn't a special case, and it's the same problem if you want to replay the original response on conflict. If the original request isn't complete, what are you going to replay?
- pdonis 6mo ago
  > If the original request isn't complete, what are you going to replay? Who says you have to replay? If you get a second request with the same idempotency key, and the original request is still in process, why not just send the client a response that says so?
  - deleted 6mo ago
    [deleted]
  - stickfigure 5mo ago
    It's not a terrible question. It would be complicated to implement and isn't more useful than using the database's consistency model, so nobody does it that way. Long running transactions create all sorts of problems, so transactions are generally expected to be short. The actual work behind "create payment" or "create order" is generally fairly trivial - more or less insert a row in a table. There's no good reason to make the API complicated... you either "win" at concurrency or you lose, and the difference is generally sub-millisecond. The only meaningful thing you need to communicate to the client is "you're done" (for both the win and lose cases) or "you need to try again" (for the "something unexpected went wrong" case). Complicated workflows can certainly have multiple steps, with "fetch the current status" calls in between. But somewhere near the beginning of every complicated workflow there will be a call to "create workflow" and it will need to have sort of mechanism which allows clients to call it idempotently. Otherwise you end up with multiple starts. I've literally received duplicate products in the mail because of this kind of problem. I've also sent multiple products in the mail because services I relied on didn't offer the necessary idempotency mechanisms.