Idempotency, Retries, Backoff — Simple

Problem statement (interviewer prompt)

Design an idempotency story for a payments API. The same request may be retried multiple times because of network ambiguity. Explain idempotency keys, the server-side state machine, and the retry policy (backoff + jitter + budget) that prevents both lost payments and double charges.

flowchart LR
  C([Client])
  S[Server]
  K[(Idempotency key store)]
  C -->|key=abc op=charge $10| S
  S --> K
  K -. exists -> return cached result .-> S
  S --> C

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class C client;
    class S service;
    class K datastore;

Same logical request can be retried any number of times without side-effects multiplying.