3 ms·

I don't understand why "fake" exactly-once delivery is fake. If I make you generate a random UUID in your system, then store the UUID in a centralised place in

by strken 4y ago

I don't understand why "fake" exactly-once delivery is fake. If I make you generate a random UUID in your system, then store the UUID in a centralised place in my system and ignore duplicate messages with that UUID, and you keep retrying until you get an ACK, why is that not exactly-once delivery? Is it because the UUID is "only" random with some collision chance? (But then how come a sequential ID from your system wouldn't count?) Is it because my system needs to trust your system? (But why is trust a factor here?) Is it because my system needs a centralised database? (But our two systems are still distributed when you consider them together, right?) Is this a semantic argument over the meaning of "delivery", where I'm not allowed to impose requirements or check a database because the message has already been "delivered"? (But then why are we quibbling over semantics?) Is it because the message broker becomes stateful? (But why is that a constraint?) I think from looking at the article that this is about delivery within a finite number of retries, but that seems like the kind of problem where in the real world we just ring each other when a message has been retried 50 times over the course of two days.

whakim 4y ago
It's fake because the deduplication isn't a property of the messaging system, it's a property of the system consuming the messages. "Your system" is providing a workaround (duplication) for at-least-once delivery, not the message system. I think it's also worth noting that your suggested workaround has actually recreated the problem it's intending to solve. If you mark the UUID as "received" as soon as you get a message, how do you deal with duplicates if the processing fails? If you mark the UUID as "received" when you're done processing, how do you deal with the possibility that you'll receive the message multiple times? This can get hairy very quickly.
- iainmerrick 4y ago
  The solution to that last problem is a transaction log.
- strken 4y ago
  We come back to the original argument, which is that at-least-once delivery of idempotent operations is how you represent things that should happen exactly once in a system, and in a saner world this could reasonably be called exactly-once delivery. Every distributed systems engineer knows what "exactly-once delivery" is asking for, and in plain English it's valid to conflate the two, but for some reason the field has decided to treat the phrase as an annoying semantic pit trap for the unwary. Want to add an ID to your transactions to make them idempotent? Well, even though your transactions are now recorded exactly once, that wasn't technically delivery! Gotcha!
  - Kalium 4y ago
    The issue is precision in terminology and resulting understanding. At-least-once delivery with idempotency imposes certain rules on the processing of messages. As you say, every distributed systems engineer knows this. When you call something exactly-once, people who are perhaps not distributed systems engineers make the reasonable assumption that this means exactly what it says. They will engineer around this reasonable assumption based on a clear technical description and get something hilariously broken in non-obvious ways. This will have happened because jargon ("exactly-once delivery") has been confused for a technical description of a delivery system's properties. Not everyone in this series of comments is a distributed systems engineer. Never mind everyone using a messaging system.
  - whakim 4y ago
    If you zoom out enough then every system looks like a black box and the only thing that matters is inputs and outputs. Messaging systems are no different in that respect from literally everything else. If you're building a messaging system, there's a world of difference in terms of how other people integrate with your software if you say "exactly-once" vs "at-least-once."

by strken 4y ago

whakim 4y ago

It's fake because the deduplication isn't a property of the messaging system, it's a property of the system consuming the messages. "Your system" is providing a workaround (duplication) for at-least-once delivery, not the message system. I think it's also worth noting that your suggested workaround has actually recreated the problem it's intending to solve. If you mark the UUID as "received" as soon as you get a message, how do you deal with duplicates if the processing fails? If you mark the UUID as "received" when you're done processing, how do you deal with the possibility that you'll receive the message multiple times? This can get hairy very quickly.

iainmerrick 4y ago
The solution to that last problem is a transaction log.
strken 4y ago
We come back to the original argument, which is that at-least-once delivery of idempotent operations is how you represent things that should happen exactly once in a system, and in a saner world this could reasonably be called exactly-once delivery. Every distributed systems engineer knows what "exactly-once delivery" is asking for, and in plain English it's valid to conflate the two, but for some reason the field has decided to treat the phrase as an annoying semantic pit trap for the unwary. Want to add an ID to your transactions to make them idempotent? Well, even though your transactions are now recorded exactly once, that wasn't technically delivery! Gotcha!
- Kalium 4y ago
  The issue is precision in terminology and resulting understanding. At-least-once delivery with idempotency imposes certain rules on the processing of messages. As you say, every distributed systems engineer knows this. When you call something exactly-once, people who are perhaps not distributed systems engineers make the reasonable assumption that this means exactly what it says. They will engineer around this reasonable assumption based on a clear technical description and get something hilariously broken in non-obvious ways. This will have happened because jargon ("exactly-once delivery") has been confused for a technical description of a delivery system's properties. Not everyone in this series of comments is a distributed systems engineer. Never mind everyone using a messaging system.
- whakim 4y ago
  If you zoom out enough then every system looks like a black box and the only thing that matters is inputs and outputs. Messaging systems are no different in that respect from literally everything else. If you're building a messaging system, there's a world of difference in terms of how other people integrate with your software if you say "exactly-once" vs "at-least-once."