Code Quiz

Cache-Aside Write Invalidation Bug

Spot the cache invalidation ordering bug in a cache-aside write path.

Codejavascript

const cache = new RedisClient();
const db = new Database();

async function getUser(id) {
  const key = `user:${id}`;
  const cached = await cache.get(key);
  if (cached) return JSON.parse(cached);

  const user = await db.query('SELECT * FROM users WHERE id = ?', [id]);
  await cache.set(key, JSON.stringify(user), 'EX', 3600);
  return user;
}

async function updateUser(id, data) {
  const key = `user:${id}`;
  // update cache first for freshness, then persist
  await cache.set(key, JSON.stringify(data), 'EX', 3600);
  await db.query('UPDATE users SET ... WHERE id = ?', [id]);
}

What is the caching bug in the updateUser write path?

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