# Idempotent Consumer

> A design pattern in which a consumer records each message it has processed, so a redelivered message has no second effect.

Record: `architecture:idempotent-consumer`
Kind: pattern
Layer: [Execution Core](https://banes-lab.com/records/layer/execution-core.md)
Severity: contextual
Scope: service, messaging, correctness
Canonical: https://banes-lab.com/ontology#architecture-idempotent-consumer

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Dead-Letter Queue](https://banes-lab.com/records/architecture/dead-letter-queue.md) and before [Competing Consumers](https://banes-lab.com/records/architecture/competing-consumers.md).

## Repair

- Refactored by: Make the Consumer Idempotent
- Detected by: duplicate effects under at-least-once delivery
- Violated by: a redelivered message applied twice
- Measured by: duplicate-processing incident rate
- Enforced by: messaging design review

## Requires

- [Deduplication Key](https://banes-lab.com/records/lexicon/deduplication-key.md)

## Reinforces

- [Eventual Consistency](https://banes-lab.com/records/architecture/eventual-consistency.md)
- [At-Least-Once Delivery Safety](https://banes-lab.com/records/lexicon/at-least-once-delivery-safety.md)

## Enables

- [Safe Message Redelivery](https://banes-lab.com/records/lexicon/safe-message-redelivery.md)

## Conflicts with

- [Duplicate Side Effects](https://banes-lab.com/records/lexicon/duplicate-side-effects.md)

## In tension with

- [State Overhead](https://banes-lab.com/records/lexicon/state-overhead.md)

## Tensions

- [Idempotent Consumer / State Overhead](https://banes-lab.com/records/tension/idempotent-consumer-state-overhead.md)

## Severity

- [contextual](https://banes-lab.com/records/vocabulary/severity-contextual.md)

## Category

- [Event / Messaging / Asynchronous Architecture](https://banes-lab.com/records/architecture-category/event-messaging-asynchronous-architecture.md)

## Linked from

- [Posting and waiting are one operation](https://banes-lab.com/disciplined-methodology/collaborate/posting-and-waiting-are-one-operation.md)
- [Anti-patterns](https://banes-lab.com/ontology/principles/architecture-category-anti-patterns.md)
- [Event / Messaging / Asynchronous Architecture](https://banes-lab.com/ontology/principles/architecture-category-event-messaging-asynchronous-architecture.md)
- [Event Messaging Async](https://banes-lab.com/ontology/lexicon/lexicon-category-event-messaging-async.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)