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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Forgetting to invalidate the cache on delete, only handling update

// Wrong: delete forgets to clear the cache
const deleteProduct = async (id) => {
  await db.products.delete(id);
  // missing: await redis.del(`product:${id}`);
};

// Correct
const deleteProduct = async (id) => {
  await db.products.delete(id);
  await redis.del(`product:${id}`);
};

The Solution //

Teams often wire up cache invalidation for updateProduct() but forget deleteProduct() entirely, so a deleted item keeps 'existing' in the cache and gets served to users until its TTL finally expires. Every code path that mutates a row — create, update, and delete — needs the matching cache invalidation call, not just the most common one.

The Error //

Rebuilding a hot cache key with no lock, causing a thundering herd on expiry

// Wrong: every concurrent miss queries the DB
let data = await redis.get('home');
if (!data) { data = await db.query(); await redis.set('home', data); }

// Correct: only one request rebuilds, others wait
if (!data) {
  const locked = await redis.setnx('lock:home', '1');
  if (locked) { data = await db.query(); await redis.set('home', data); await redis.del('lock:home'); }
  else { await sleep(50); data = await redis.get('home'); }
}

The Solution //

When a popular key's TTL expires, every concurrent request sees a cache miss and hits the database at once, which can take an otherwise healthy database down in seconds under real traffic. Use a lock (e.g. Redis SETNX) so only the first request rebuilds the cache while the rest wait briefly and re-read the now-populated cache.