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Data & Analytics · Advanced

Exactly-Once Semantics (EOS)

A processing guarantee that each record affects the output exactly one time despite failures and retries.

Definition

Exactly-once is achieved with idempotent or transactional sinks plus checkpointed offsets, as in Kafka transactions, Spark Structured Streaming with Delta sinks and Dataflow. Most transport layers are at-least-once, so sinks must deduplicate.

Exam tip

End-to-end exactly-once needs a replayable source, checkpointing and an idempotent or transactional sink.

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