идемпотентность и дедупликация запросов: ментальная модель

Question

Accepted Answer

Idempotent = twice equals once. Failed *response* ≠ failed *request* → **Idempotency-Key** for unsafe writes.

## Разбор

Core problem: **`POST /charge` timeout** — charge may succeed, response lost. Client lose-lose: retry → double charge, don't retry → lost charge.

Idempotency key breaks tie. Mint unique key **once before first attempt**; every retry same key. Server records `k → R`; second `k` replays R.

```
Attempt 1:  POST /charge  Idempotency-Key: k9f…   ─▶ server charges $10, stores k9f→receipt
            ✗ response lost in the network (but the charge HAPPENED)
Attempt 2:  POST /charge  Idempotency-Key: k9f…   ─▶ server sees k9f, REPLAYS receipt
            ✓ exactly one charge, client gets its answer
```

**Same key = same operation.** Per «Pay» click, not per HTTP attempt.

## Итог

Networks duplicate; idempotency keys make POST as safe to retry as PUT for the client.