How do you implement idempotency in a Node.js API?

Question

Accepted Answer

**Idempotency** — same request produces same result (important for retries).

```javascript
const redis = require("redis").createClient();

async function idempotentHandler(req, res, next) {
    const idempotencyKey = req.headers["idempotency-key"];

if (!idempotencyKey) {
        return res.status(400).json({
            error: "Idempotency-Key header required",
        });
    }

const key = `idempotency:${idempotencyKey}`;
    const existing = await redis.get(key);

if (existing) {
        const parsed = JSON.parse(existing);
        return res.status(parsed.statusCode).json(parsed.body);
    }

// Store response
    const originalSend = res.send.bind(res);
    res.send = async (body) => {
        await redis.setEx(key, 86400, JSON.stringify({
            statusCode: res.statusCode,
            body: JSON.parse(body),
        }));
        return originalSend(body);
    };

next();
}

app.post("/payments", idempotentHandler, async (req, res) => {
    const payment = await processPayment(req.body);
    res.json(payment);
});
```

**Key points:**
- Client sends unique Idempotency-Key header.
- Server stores response for replay.
- Expire keys after 24-48 hours.

Node.js: How do you implement idempotency in a Node.js API?