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You can get exactly once processing, but not exactly once delivery. https://bravenewgeek.com/you-cannot-have-exactly-once-delivery/ https://bravenewgeek.com/yo

You can get exactly once processing, but not exactly once delivery. https://bravenewgeek.com/you-cannot-have-exactly-once-delivery/ https://bravenewgeek.com/you-cannot-have-exactly-once-delive...

lisper 2y ago
That seems like a distinction without a difference to me. Why should I care if the thing I get exactly one of is called "processing" or "delivery"?
- jerf 2y ago
  Because you need to understand that your processing code is constrained by the fact that you can't get exactly-once delivery. You must write your processing code to handle it one way or another. There's some libraries that try to wrap the abstraction of processing exactly once around the code, but those libraries still impose constraints on the sort of code you can write. They can make it easier but they can't fully remove the need to think about how your processing code works. It isn't exactly the same. This is why people like me insist it's important to understand that you can not have exactly-once delivery. There is no library that can make that just go away, they can only shuffle around exactly where the lumps under the carpet live, and if one programs with the mistaken idea that these libraries really do solve "exactly once" delivery, one will get into deep trouble. Possibly the "my architecture is fundamentally broken and can't be rescued" sort of trouble.
  - lisper 2y ago
    > Because you need to understand that your processing code is constrained by the fact that you can't get exactly-once delivery. Why do I need to understand that? Why can I not put an abstraction layer that provides me with the illusion of exactly-once delivery? > There is no library that can make that just go away Well, this is the thing that I dispute. I believe that there is a library I can write to make it go away if I have at-least-once delivery. In fact, I claim that writing such a library is an elementary exercise. The TCP protocol is an existence proof. Where is the flaw in my reasoning?
    - pton_xd 2y ago
      > Why can I not put an abstraction layer that provides me with the illusion of exactly-once delivery? You can do that. You can implement a video conferencing system ontop of TCP, and it will even work, technically. It will just have terrible performance characteristics that you'll never be able to fix. You might even call it fundamentally broken.
      - lisper 2y ago
        OK, I don't dispute that, but that is a very different claim than "you can't get exactly-once delivery." You can (if you have at-least-once delivery).
        sethammons 2y ago
        Read the two general's problem. It is proven that exactly once _delivery_ is a physical and mathematical impossibility. Like you said, you can simulate it with exactly once _processing_. But to do that, you have to know to do that. Others are saying "you can't divide by zero" and you are saying "yeah, but if I detect a zero and the do something different, then it is the same thing." No, knowing you have to do something is the very point of acknowledging you can't divide by zero. Because you can't have exactly once delivery, you have to deal with it. One trick is duplicate checks or idempotent writes. This gives exactly once processing. This also takes additional overhead and why audio and video stream processing doesn't typically do the additional checks. I have fixed many bugs written by people who believe the network is reliable. I even hired one when during the interview and we talked about this kind of issue that they realized why they were getting duplicated writes reading from sqs or sns at $existing_job. They were green but smart and she became a real asset to the team.
        lisper 2y ago
        What makes you thin the 2GP is relevant here? 2GP has to do with coordination and consensus, not exactly-once-delivery. > you can simulate it with exactly once _processing_ What exactly do you think is the difference between "simulating" exactly-once delivery and actually having exactly-once delivery? What do you think "delivery" means in the context of a computer network?
- shepherdjerred 2y ago
  I don't know enough to answer this. I'm sure there is plenty of writing on this subject from people more qualified than me.

You can get exactly once processing, but not exactly once delivery. https://bravenewgeek.com/you-cannot-have-exactly-once-delivery/ https://bravenewgeek.com/you-cannot-have-exactly-once-delive...

lisper 2y ago

That seems like a distinction without a difference to me. Why should I care if the thing I get exactly one of is called "processing" or "delivery"?

jerf 2y ago
Because you need to understand that your processing code is constrained by the fact that you can't get exactly-once delivery. You must write your processing code to handle it one way or another. There's some libraries that try to wrap the abstraction of processing exactly once around the code, but those libraries still impose constraints on the sort of code you can write. They can make it easier but they can't fully remove the need to think about how your processing code works. It isn't exactly the same. This is why people like me insist it's important to understand that you can not have exactly-once delivery. There is no library that can make that just go away, they can only shuffle around exactly where the lumps under the carpet live, and if one programs with the mistaken idea that these libraries really do solve "exactly once" delivery, one will get into deep trouble. Possibly the "my architecture is fundamentally broken and can't be rescued" sort of trouble.
- lisper 2y ago
  > Because you need to understand that your processing code is constrained by the fact that you can't get exactly-once delivery. Why do I need to understand that? Why can I not put an abstraction layer that provides me with the illusion of exactly-once delivery? > There is no library that can make that just go away Well, this is the thing that I dispute. I believe that there is a library I can write to make it go away if I have at-least-once delivery. In fact, I claim that writing such a library is an elementary exercise. The TCP protocol is an existence proof. Where is the flaw in my reasoning?
  - pton_xd 2y ago
    > Why can I not put an abstraction layer that provides me with the illusion of exactly-once delivery? You can do that. You can implement a video conferencing system ontop of TCP, and it will even work, technically. It will just have terrible performance characteristics that you'll never be able to fix. You might even call it fundamentally broken.
    - lisper 2y ago
      OK, I don't dispute that, but that is a very different claim than "you can't get exactly-once delivery." You can (if you have at-least-once delivery).
      - sethammons 2y ago
        Read the two general's problem. It is proven that exactly once _delivery_ is a physical and mathematical impossibility. Like you said, you can simulate it with exactly once _processing_. But to do that, you have to know to do that. Others are saying "you can't divide by zero" and you are saying "yeah, but if I detect a zero and the do something different, then it is the same thing." No, knowing you have to do something is the very point of acknowledging you can't divide by zero. Because you can't have exactly once delivery, you have to deal with it. One trick is duplicate checks or idempotent writes. This gives exactly once processing. This also takes additional overhead and why audio and video stream processing doesn't typically do the additional checks. I have fixed many bugs written by people who believe the network is reliable. I even hired one when during the interview and we talked about this kind of issue that they realized why they were getting duplicated writes reading from sqs or sns at $existing_job. They were green but smart and she became a real asset to the team.
        lisper 2y ago
        What makes you thin the 2GP is relevant here? 2GP has to do with coordination and consensus, not exactly-once-delivery. > you can simulate it with exactly once _processing_ What exactly do you think is the difference between "simulating" exactly-once delivery and actually having exactly-once delivery? What do you think "delivery" means in the context of a computer network?
shepherdjerred 2y ago
I don't know enough to answer this. I'm sure there is plenty of writing on this subject from people more qualified than me.