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What is idempotency and why does it matter?

Q: What is idempotency and why does it matter?

## Idempotency An operation is **idempotent** if performing it **many times has the same effect as performing it once**. `GET`, `PUT`, and `DELETE` are idempotent; **`POST` is not** (two POSTs = two resources). It matters because the network is unreliable: a client times out and **retries**, or a queue delivers **at-least-once**. Without idempotency a retry double-charges a card or creates duplicate orders. The fix is an **idempotency key**: the client sends a unique key per logical operation; the server records it and returns the **saved result** for any duplicate. Client <text cl

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idempotency

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Idempotency-key handler

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processed = {}   # key -> saved result (use Redis + TTL in production)

def charge(idempotency_key, amount):
    if idempotency_key in processed:
        print("duplicate", idempotency_key, "-> returning saved result")
        return processed[idempotency_key]
    # ... actually charge the card exactly once here ...
    result = {"status": "charged", "amount": amount}
    processed[idempotency_key] = result
    print("charged", amount, "for", idempotency_key)
    return result

charge("k1", 4999)   # charges
charge("k1", 4999)   # retry -> no double charge
charge("k2", 1200)   # different op -> charges

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