Data engineering·Time and correctness in streams
the job crashed mid-window, replayed the last two minutes, and the output table still shows each event counted exactly one time.
End-to-end exactly-once
Also calledexactly-once semantics, EOS
The guarantee that each input record affects the output exactly once, despite retries and restarts. It is never achieved by delivering once; it is achieved by replaying at-least-once and making the effect idempotent or transactional, which means it is a property of the sink as much as of the engine. A pipeline claiming it while writing to a plain HTTP endpoint does not have it, and the place to check is always the last hop.