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Comment by BeeOnRope | Hacker News Reader

In a sense it's just kicking the can down the road: how you ensure the consumer reads the message only once? It may fail at some point after it has read the message but updated some state to reflect that.

So it's not just a pointless argument about the semantics of the term "delivery": the fact that no communication channel can have exactly once delivery means that these systems are much more difficult to implement. For example, you can't just chain together 2 "exactly once" delivery systems into some longer one with a stateless middle node: instead you need some concept of idempotency that spans both systems or the middle node itself needs to de-duplicate (statefully) the first link and forward messages on to the second link.

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Yes, idempotency is critical to achieve this but my point is that a message which can be recorded in an idempotent can be delivered exactly once. Yes, it is tricky but it is possible.

Same with two-phase commit, it can be implemented perfectly given certain reasonable constraints and assumptions. The consumer process could crash, then on restart, it would use timestamps to check which record it processed last and resume processing starting with the next unprocessed one. If the consumer uses input messages to produce output messages somewhere else, the consumer could also itself perform a two-phase commit on its own outputs to account for the possibility of itself crashing part-way through processing an input message. It can coordinate both inputs and outputs perfectly.

If we define 'processed' as fully committed, then messages ca be processed exactly once. If it can be processed once, it can be delivered exactly once if you accept the simplest definition of that word.

What is often suggested when people say "exactly once delivery is impossible" is that duplicates cannot be avoided and this is usually a cop out.

> If we define 'processed' as fully committed, then messages ca be processed exactly once. If it can be processed once, it can be delivered exactly once if you accept the simplest definition of that word.

I don't see how. Let's take a simple example of a the receiving end of a messaging system, where the messaging system can do whatever it wants to enforce idempotency or whatever semantics it wants, and calls, in-process, some `handler()` method of the application.

The useful processing happens somewhere in `handler()`. No matter what point you identify as the point where processing happens inside handler() it won't have delivered/processed-once semantics: it will potentially be called multiple times. The fact that the messaging system will internally have a commit step after handler() completes which is idempotent is irrelevant: as a user of the system you care how many times handler() is called.

If the sender is writing a message with a UUID and/or a unique incrementing sequence order (to maintain the order) to some kind of remote database using the UUID as the ID; they would be written to the DB in-order and unique. Then the receiver would receive the messages by reading them from the database sorted by their sequence number. The receiver would mark off messages in the DB as processed whenever it receives them. If it fails, it resumes from the last unprocessed one based on sequence number.

If we have a chain of senders and receivers and absolutely wanted to avoid double processing instead of also maintaining idempotence down the line as described above. It is also technically possible (but I guess not worth the complexity); we could make it so that the receiver would add a flag 'about_to_process' on each record just before it starts processing the record and then it would change it to 'committed' once it finishes. If the receiver crashes before completing processing a message, it would see the 'about_to_process' flag and in that case it could ask the next process/receiver in the sequence if they already received a packet with that UUID and only send the output for that message if they have not.

It's very ugly but it's possible.

I guess we are probably in violent agreement on most of this.

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