Exactly Once

Replayable source, checkpointed state, transactional or idempotent sink — what the phrase means and where it stops

Exactly-once state semantics are Flink's; end-to-end exactly-once needs the source and the sink to cooperate. The sink writes a cost record, the TaskManager dies before the checkpoint completes, the record is written again after recovery, and the billing consumer double-bills. Two fixes: a transactional Kafka sink with read_committed consumers, or an idempotent upsert keyed by tenant and window.

Estimated time: 90 minutes

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