The Race Condition in Your Cache

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

hardNode.jsAsync/AwaitPerformance⏱ 10-15 min

Attempts

892

Solved

21%

Avg time

7min

Published

2024-02-01

// the code

Here's what passed code review.

A standard pattern. Nothing in this code is obviously wrong. It works perfectly in dev. It works perfectly in staging. It even works in production — for a while.

cache.jsjavascript

class UserCache {
  constructor() {
    this.cache = new Map();
  }

  async getUser(userId) {
    // Check cache first
    if (this.cache.has(userId)) {
      return this.cache.get(userId);
    }

    // Fetch from database
    const user = await database.fetchUser(userId);

    // Store in cache
    this.cache.set(userId, user);

    return user;
  }

  async updateUser(userId, data) {
    // Update database
    await database.updateUser(userId, data);

    // Invalidate cache
    this.cache.delete(userId);
  }
}

// This cache implementation has a race condition
// that can serve stale data indefinitely.
// Can you find it?

// stuck? progressive hints

Reveal hints one at a time.

Each hint nudges you closer without giving it away. Use as many as you need.

hint_01Start here →+

What happens if getUser() and updateUser() are called simultaneously?

hint_02Need another nudge?+

Consider the timing of async operations.

hint_03Need another nudge?+

The cache might be repopulated with stale data.

// submit your guess

Want more code challenges? Subscribe.

New challenge every two weeks. Production bugs that passed review. ~5 min reads.

See all challenges/challenges Book a 30-min chatcalendly