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Idempotency And Exactly Once

Design operations to be safely retryable and handle duplicate execution correctly. Use when building APIs, message consumers, or any operation that may be retried or delivered more than once.

itsual Updated
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Idempotency and Exactly-Once Semantics

Overview

In distributed systems, “exactly-once” is hard. Most practical systems aim for at-least-once delivery + idempotent handlers.

When to Use

  • Payment or state-changing APIs
  • Message / event consumers
  • Any operation that clients or infrastructure may retry

Techniques

  • Idempotency keys
  • Natural idempotency (set-based operations, upserts)
  • Deduplication stores with appropriate TTLs
  • Outbox pattern for reliable publishing
  • Transactional messaging where supported

Principles

  • Assume duplicates will happen
  • Make the side effects of duplicate execution safe or detectable
  • Document the idempotency guarantees of your APIs

Verification

  • Retrying the same request/message does not produce incorrect duplicate effects
  • Idempotency behavior is tested

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