⚡ Level 4 — Caching

Caching is one of the highest-impact optimizations in system design.

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4.1 What is Caching?

Caching involves storing frequently-accessed data in a fast access layer to reduce latency and database load.

Visualizing Cache Speed Improvement

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4.2 Caching Layers

Caching can happen at multiple levels in a modern system.

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4.3 Cache Writing Strategies

Cache-Aside (Lazy Loading)

The application is responsible for reading from and writing to the cache.

💻 JS Example: Cache-Aside Pattern

async function getUserWithCache(userId) {
  const cacheKey = `user:${userId}`;

  // 1. Try to get from cache
  const cachedUser = await redis.get(cacheKey);
  if (cachedUser) {
    console.log("Cache Hit!");
    return JSON.parse(cachedUser);
  }

  // 2. If miss, get from database
  console.log("Cache Miss, querying DB...");
  const user = await db.users.findById(userId);

  // 3. Store in cache for next time (with 1 hour TTL)
  if (user) {
    await redis.setex(cacheKey, 3600, JSON.stringify(user));
  }

  return user;
}

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4.4 Cache Pitfalls

Cache Stampede (Thundering Herd)

When a popular key expires, thousands of concurrent requests might hit the database at once.

🛠️ Solution: Mutex Locking (JS Logic)

async function getWithLock(key) {
  let val = await redis.get(key);
  if (!val) {
    // Acquire a simple lock (or wait for one)
    if (await acquireLock(key)) {
      val = await db.fetch(key);
      await redis.set(key, val);
      await releaseLock(key);
    } else {
      // Wait and retry
      return getWithLock(key);
    }
  }
  return val;
}

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4.5 Redis vs Memcached

Feature	Redis	Memcached
Data Types	Complex (Lists, Sets, Hashes)	Strings only
Persistence	Yes (optional)	No (In-memory only)
Replication	Yes (Primary-Replica)	No
Best For	Leaderboards, Pub/Sub, Objects	Simple Page Caching

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✅ Checklist Before Moving On

• [ ] I can explain cache-aside vs write-through
• [ ] I know what LRU eviction means
• [ ] I can describe cache stampede and how to fix it
• [ ] I know when to use Redis vs Memcached

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➡️ Next: Level 5 — Messaging & Queues