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Predict the output · Node.js

The Stale Cache Set

Medium Predict the output

A cache-aside read and a price update run concurrently. What does it print? See The Stale-Set Race.

What does this print?

const db = { price: 100 };
const cache = new Map();
const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

async function read() {
  if (cache.has("price")) return cache.get("price");
  const value = db.price;        // cache miss → SELECT
  await sleep(20);               // slow network / GC pause
  cache.set("price", value);     // SET
  return value;
}

async function update() {
  await sleep(5);
  db.price = 80;                 // UPDATE … COMMIT
  cache.delete("price");         // invalidate
}

await Promise.all([read(), update()]);
console.log(db.price, cache.get("price"));

  1. 80 80
  2. 80 100
  3. 80 undefined
  4. 100 100

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Solution

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The output is:

80 100

The reader loaded 100, the writer then committed 80 and deleted the key, and only afterwards did the slow reader set its old value. The cache now holds 100 until its TTL: a stale set. Prevent it with version-checked sets or leases, and bound it with short TTLs. See The Stale-Set Race.