hard10-15 min21% solve rate

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

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?

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