Idempotency in APIs

Idempotent operations (e.g., GET, PUT with full resource) can be retried safely. Non-idempotent (POST creating resources) need dedup keys or client-generated ids.

Pattern: client sends idempotency-key header; server deduplicates requests using that key.

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POST /charge
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Idempotency-Key: abc-123
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{
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  "amount": 100
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}

Server stores the key → response mapping for short TTL and returns previous result on duplicate.