Idempotency Keys in API Design
Design bulletproof payment and mutation APIs: Client-generated keys, atomic DB locks, status caching, and RFC 9457 error contracts.
01.The Network Failure Hazard & Need for Idempotency
In distributed networks, communication between clients and servers is fundamentally unreliable (The Two Generals Problem).
The Double-Charge Disaster:
- A mobile client sends
POST /v1/chargesto purchase a $500 flight ticket. - The server receives the request, deducts money from the bank account, and commits the transaction.
- As the server sends
200 OKback to the client, the client's mobile cellular connection drops. - The client encounters a network timeout. Not knowing if the charge succeeded, the client's automated retry logic re-sends the exact same
POST /v1/chargesrequest. - Without Idempotency: The server charges the customer a second time ($1,000 total) and creates duplicate flight reservations!
What is Idempotency?
An operation is idempotent if applying it multiple times produces the exact same server state as applying it once:
f(f(x)) = f(x)
While HTTP GET, PUT, and DELETE are idempotent by specification, HTTP POST is non-idempotent. Idempotency Keys make POST mutations safe to retry indefinitely.
Idempotency Key State Machine & Safe Network Retry Workflow 🔒
Idempotency Key State Machine & Safe Network Retry Workflow 🔒
Protecting against double-execution during network connection drops using atomic database locks and response caching.
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