1import Redis from 'ioredis';
2 
3const redis = new Redis(process.env.REDIS_URL);
4 
5// Cache-Aside Pattern with TTL and error handling
6async function getUser(id: string): Promise<User> {
7  const cacheKey = `user:${id}`;
8 
9  // 1. Check cache first
Check cache first — fast path returns immediately without touching DB
10  const cached = await redis.get(cacheKey);
11  if (cached) {
12    return JSON.parse(cached);
13  }
14 
15  // 2. Cache miss — load from database
16  const user = await db.users.findById(id);
17  if (!user) throw new Error('User not found');
18 
Always set TTL. Redis with no TTL = memory leak. 1 hour is reasonable for user profiles
19  // 3. Populate cache with TTL
20  await redis.setex(cacheKey, 3600, JSON.stringify(user)); // 1 hour TTL
21 
22  return user;
23}
24 
25// Cache invalidation on update
Invalidate on write — simpler and safer than updating the cached value
26async function updateUser(id: string, data: Partial<User>): Promise<User> {
27  const user = await db.users.update(id, data);
28 
29  // Invalidate cache — force next read to go to DB
30  await redis.del(`user:${id}`);
31 
32  return user;
33}
34 
35// Thundering herd prevention with distributed lock
36async function getUserWithLock(id: string): Promise<User> {
37  const cacheKey = `user:${id}`;
Thundering herd: 1000 requests hit cache miss simultaneously → 1000 DB queries. Lock prevents this
38  const lockKey = `lock:user:${id}`;
39 
40  const cached = await redis.get(cacheKey);
41  if (cached) return JSON.parse(cached);
NX = set only if Not eXists. EX 5 = 5-second lock timeout prevents deadlocks
42 
43  // Only one process refills cache at a time
44  const acquired = await redis.set(lockKey, '1', 'EX', 5, 'NX');
45 
46  if (!acquired) {
47    // Another process is loading — wait briefly and retry
48    await new Promise(r => setTimeout(r, 50));
49    return getUserWithLock(id);
50  }
51 
52  try {
53    const user = await db.users.findById(id);
54    await redis.setex(cacheKey, 3600, JSON.stringify(user));
55    return user;
56  } finally {
57    await redis.del(lockKey);
58  }
59}