Exactly Once

Exactly Once

for message queue system, message is generated from producer to queue and will be consumed by consumer. there’s 3 delivery semantics:

• at least once. (message is delivered once or more)
• at most once. (message is ignored or just once)
• exactly once. (?? it depends on how to define this term, though we expect this is message is only delivered once.)

basically, exactly-once is application level guarantee, inside it’s implemented by:

• at least once
• retry with idempotent
• transaction support

but, if there’s outside system or non-deterministic system, exactly-once is not support. for example:

• how to send email exactly once
• if the consumer’s result depends on outside system but the result is time sensitive (that is the system is non-deterministic), retry would introduce in incorrect result

some essays:

• official doc about delivery semantics
• Exactly-Once Semantics Are Possible: Here’s How Kafka Does It
• You Cannot Have Exactly-Once Delivery
• At most once, at least once, exactly once

some talk about exactly once:

• https://news.ycombinator.com/item?id=34986995
• https://news.ycombinator.com/item?id=15602841