# Idempotency

> A design rule that repeating an operation with the same input has the same effect as running it once.

Record: `architecture:idempotency`
Kind: principle
Layer: [Atomic Boundary](https://banes-lab.com/records/layer/atomic-boundary.md)
Severity: mandatory
Scope: API, command, message handler
Canonical: https://banes-lab.com/ontology#architecture-idempotency

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Root Spine Files](https://banes-lab.com/records/architecture/root-spine-files.md) and before [Atomicity](https://banes-lab.com/records/architecture/atomicity.md).

## Repair

- Refactored by: Add Idempotency Key, Add Dedup Store
- Detected by: side-effectful handlers without deduplication
- Violated by: duplicate charges/orders/messages on retry
- Measured by: duplicate-effect defect rate
- Enforced by: retry tests, API policy

## Requires

- [Idempotency Key or Deterministic Operation](https://banes-lab.com/records/lexicon/idempotency-key-or-deterministic-operation.md)

## Reinforces

- [Retry Safety](https://banes-lab.com/records/lexicon/retry-safety.md)
- [Event-Driven Architecture](https://banes-lab.com/records/architecture/event-driven-architecture.md)

## Enables

- [Safe Retries](https://banes-lab.com/records/lexicon/safe-retries.md)

## Conflicts with

- [Non-Repeatable Side Effects](https://banes-lab.com/records/lexicon/non-repeatable-side-effects.md)

## In tension with

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

## Tensions

- [Idempotency / State Tracking](https://banes-lab.com/records/tension/idempotency-state-tracking.md)

## Severity

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

## Category

- [Transactions / State / Concurrency](https://banes-lab.com/records/architecture-category/transactions-state-concurrency.md)

## Enforced by

- [rules/eslint/no-raw-file-write.eslint.rule.ts](https://banes-lab.com/source/governance/rules/eslint/no-raw-file-write.eslint.rule.ts.md)

## Required by

- [Eventual Consistency](https://banes-lab.com/records/architecture/eventual-consistency.md)
- [Saga Pattern](https://banes-lab.com/records/architecture/saga-pattern.md)
- [Retry Pattern](https://banes-lab.com/records/architecture/retry-pattern.md)

## Enabled by

- [Canonicalization](https://banes-lab.com/records/architecture/canonicalization.md)

## Linked from

- [Detect, log, fix](https://banes-lab.com/disciplined-methodology/build/detect-log-fix.md)
- [Posting and waiting are one operation](https://banes-lab.com/disciplined-methodology/collaborate/posting-and-waiting-are-one-operation.md)
- [Every record has a kind](https://banes-lab.com/software-architecture/principles/every-record-has-a-kind.md)
- [Computation and resource](https://banes-lab.com/software-architecture/principles/computation-and-resource.md)
- [Event / Messaging / Asynchronous Architecture](https://banes-lab.com/ontology/principles/architecture-category-event-messaging-asynchronous-architecture.md)
- [Error Handling / Resilience](https://banes-lab.com/ontology/principles/architecture-category-error-handling-resilience.md)
- [Schema / Canonical Data / Semantics](https://banes-lab.com/ontology/principles/architecture-category-schema-canonical-data-semantics.md)
- [Quality Attributes](https://banes-lab.com/ontology/lexicon/lexicon-category-quality-attributes.md)
- [Transactions / State / Concurrency](https://banes-lab.com/ontology/lexicon/lexicon-category-transactions-state-concurrency.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)

## Evidence in the code

- [rules/eslint/no-raw-file-write.eslint.rule.ts](https://banes-lab.com/source/governance/rules/eslint/no-raw-file-write.eslint.rule.ts.md)