Exactly-once processing.

also: exactly-once · exactly once · exactly-once delivery · exactly-once semantics · effectively once

Each message's effect happens once; in practice at-least-once delivery plus idempotent or transactional processing.

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Very hard to guarantee end to end across networks.
Approach 1: idempotent producer (retries keep the same ID) plus a consumer that ignores IDs it has processed.
Approach 2: transactions, such as Kafka's atomic produce + consume, or the outbox pattern.
Record the processed ID in the same transaction as the side effect, or a crash in between double-processes.

also: exactly-once · exactly once · exactly-once delivery · exactly-once semantics · effectively once

Each message's effect happens once; in practice at-least-once delivery plus idempotent or transactional processing.

Very hard to guarantee end to end across networks.
Approach 1: idempotent producer (retries keep the same ID) plus a consumer that ignores IDs it has processed.
Approach 2: transactions, such as Kafka's atomic produce + consume, or the outbox pattern.
Record the processed ID in the same transaction as the side effect, or a crash in between double-processes.