The Race Condition in Your Cache

This caching logic seems bulletproof but has a subtle race condition.

Node.jsAsync/AwaitPerformance

The Code

This code passed code review and works in development. But it has a bug that will cause problems in production.

cache.js

javascript

1class UserCache {
2  constructor() {
3    this.cache = new Map();
4  }
5 
6  async getUser(userId) {
7    // Check cache first
8    if (this.cache.has(userId)) {
9      return this.cache.get(userId);
10    }
11 
12    // Fetch from database
13    const user = await database.fetchUser(userId);
14 
15    // Store in cache
16    this.cache.set(userId, user);
17 
18    return user;
19  }
20 
21  async updateUser(userId, data) {
22    // Update database
23    await database.updateUser(userId, data);
24 
25    // Invalidate cache
26    this.cache.delete(userId);
27  }
28}
29 
30// This cache implementation has a race condition
31// that can serve stale data indefinitely.
32// Can you find it?

// need a hint?

Progressive Hints

0 of 3 revealed

Hint 1 locked

Hint 2 locked

Hint 3 locked

// ready for the answer?

Ready for the Solution?

Enter your email to get the full solution with explanation, the fix, and why this bug is so common.

// more challenges

Keep Debugging

easy3-5 min

The Event Listener Memory Leak

This WebSocket handler looks clean but will slowly consume all your memory.

medium5-10 min

The Database Connection Leak

This query function works perfectly in testing but will bring down production.

View All Challenges →