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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Assuming a POST endpoint is safe to retry automatically without any idempotency protection

// Wrong: no protection, a retry can create a duplicate
app.post("/orders", async (req, res) => { const order = await createOrder(req.body); res.json(order); });

// Correct: idempotency key prevents a retry from duplicating
const key = req.headers["idempotency-key"];
// check Redis for an existing result under this key first

The Solution //

POST is not naturally idempotent — a client automatically retrying a POST request after a network timeout, unaware of whether the original request actually succeeded, risks creating a duplicate resource (a duplicate order, a duplicate charge) if the original request had in fact already succeeded.

The Error //

Storing an idempotency key's result without also binding it to the exact original request body

// Wrong: doesn't detect a mismatched reuse
const cached = await redis.get(`idempotency:${key}`);
if (cached) return res.json(JSON.parse(cached));

// Correct: detects and flags a mismatch
if (cached.requestHash !== hashRequest(req.body)) return res.status(422).json({ error: "Key reused with different data" });

The Solution //

Without verifying the request body matches, a client accidentally reusing an idempotency key with genuinely different data would silently receive the original, now-mismatched result instead of a clear error — masking what is likely a client-side bug.