идемпотентность и дедупликация запросов: пример кода

// CLIENT: send a stable key per logical action; reuse it across retries.
async function pay(cartId, amount) {
  const key = crypto.randomUUID();          // once, BEFORE any attempt
  return fetchRetry('/charge', {
    method: 'POST',
    headers: { 'Idempotency-Key': key, 'Content-Type': 'application/json' },
    body: JSON.stringify({ cartId, amount }),
  }); // every retry inside fetchRetry reuses `key` → server dedupes
}

// CLIENT dedupe: coalesce concurrent identical requests into one promise.
const inflight = new Map();
function dedupe(key, fn) {
  if (inflight.has(key)) return inflight.get(key);       // reuse the in-flight call
  const p = fn().finally(() => inflight.delete(key));    // clear when settled
  inflight.set(key, p);
  return p;
}
// Double-click → ONE network request, both callers await the same promise.
btn.onclick = () => dedupe('checkout', () => pay(cartId, amount));

// SERVER sketch: lock, process once, replay thereafter.
async function handleCharge(req, res) {
  const key = req.headers['idempotency-key'];
  if (!key) return res.status(400).json({ error: 'Idempotency-Key required' });
  const existing = await store.get(key);
  if (existing?.status === 'done') return res.status(200).json(existing.response); // replay
  if (existing?.status === 'locked') return res.status(409).json({ error: 'in progress' });
  await store.set(key, { status: 'locked' });            // claim the key
  const receipt = await chargeOnce(req.body);            // the real, unsafe work
  await store.set(key, { status: 'done', response: receipt });
  res.status(201).json(receipt);
}