Exactly-once processing

The effect of each message happens once even if it is delivered twice. Systems get there with idempotent writes, deduplication keys or transactional offsets, never by trusting the network.

Where it shows up in real systems

  • Stream processor

    Windows such as the last five minutes turn an endless stream into results: counts, fraud scores, trending topics. Checkpoints let the job restart without losing or double-counting events.

    In production: Apache Flink, Kafka Streams, Spark Structured Streaming, Amazon Managed Service for Apache Flink

See it break, then fix it

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