What the parent poster described isn’t what makes Kafka’s “exactly once” semantics work. It’s the use of an idempotency token associated with each publication, which effectively turns “at-least-once” semantics into effectively “exactly once” via deduplication.
Re:Kafka is Fast – I'll use Postgres
by saberience on Monday November 03, 2025 @11:38AM
(#45800998)
We used Flink and S3 compatible backends (for the Flink state) to ensure exactly once processing.
I didn't say Kafka magically solves these problems for you, but it was required for the scalability we needed.
The trivial solution is to use Kafka. They're clearly saying that Kafka makes it trivial, not that it's trivial to solve from scratch.