What is Idempotency? — UK Software Development Guide
Direct Answer
Idempotency is a property of an operation where performing it multiple times produces the same result as performing it once. An idempotent API endpoint produces the same outcome whether called once or many times with the same parameters. HTTP PUT and DELETE are idempotent by definition; POST is not. Idempotency keys are used to make POST requests idempotent.
Idempotency in the UK
Idempotency is critical for UK FinTech and payment software. Payment API calls must be idempotent — if a payment request times out and is retried, it must not result in duplicate charges. Stripe uses idempotency keys (a unique string per payment attempt) to prevent duplicate charges on retried requests. NHS FHIR API submissions benefit from idempotent design — if a clinical record submission times out and is retried, the patient record must not be duplicated. UK GDPR: idempotent delete operations are particularly important for right to erasure — repeated deletion requests for the same data must succeed without error (not fail because the data is already deleted).
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