Caching

Overview

Cache aggressively, but always have an invalidation strategy.

Caching improves performance dramatically, but stale data causes bugs. Every cache needs a plan for freshness.

When to Use

• Same data fetched repeatedly
• Expensive computations
• Slow database queries
• External API rate limits
• Asked to "just add caching"

The Iron Rule

NEVER add a cache without defining its invalidation strategy.

No exceptions:

• Not for "it rarely changes"
• Not for "TTL is enough"
• Not for "we'll figure it out later"
• Not for "users can refresh"

Detection: Cache Without Strategy Smell

If cache has no invalidation plan, STOP:

// ❌ VIOLATION: Cache without invalidation strategy
const cache = new Map();

async function getUser(id: string) {
  if (cache.has(id)) {
    return cache.get(id);  // Could be stale forever!
  }
  
  const user = await db.users.findById(id);
  cache.set(id, user);  // When does this expire? When user updates?
  return user;
}

Problems:

• User updates name → cache shows old name
• No memory limit → memory leak
• No TTL → stale forever
• Distributed system → multiple stale copies

The Correct Pattern: Cache with Strategy

// ✅ CORRECT: Cache with TTL and invalidation

interface CacheEntry<T> {
  data: T;
  expiresAt: number;
}

class UserCache {
  private cache = new Map<string, CacheEntry<User>>();
  private TTL_MS = 5 * 60 * 1000; // 5 minutes
  
  async get(id: string): Promise<User> {
    const cached = this.cache.get(id);
    
    if (cached && cached.expiresAt > Date.now()) {
      return cached.data;
    }
    
    const user = await db.users.findById(id);
    this.set(id, user);
    return user;
  }
  
  set(id: string, user: User): void {
    this.cache.set(id, {
      data: user,
      expiresAt: Date.now() + this.TTL_MS
    });
  }
  
  // Explicit invalidation on updates
  invalidate(id: string): void {
    this.cache.delete(id);
  }
  
  invalidateAll(): void {
    this.cache.clear();
  }
}

// Usage with invalidation on write
async function updateUser(id: string, data: UpdateUserDto) {
  const user = await db.users.update(id, data);
  userCache.invalidate(id);  // Clear stale cache
  return user;
}

Cache Invalidation Strategies

1. Time-Based (TTL)

// Good for: data that can be slightly stale
const TTL = 60 * 1000; // 1 minute
cache.set(key, value, { ttl: TTL });

2. Write-Through

// Good for: data you control writes for
async function updateProduct(id, data) {
  const product = await db.products.update(id, data);
  await cache.set(`product:${id}`, product);  // Update cache on write
  return product;
}

3. Event-Based

// Good for: distributed systems
eventBus.on('user.updated', (userId) => {
  cache.delete(`user:${userId}`);
});

eventBus.on('product.priceChanged', (productId) => {
  cache.delete(`product:${productId}`);
});

4. Cache-Aside (Lazy)

// Good for: read-heavy, tolerance for staleness
async function getProduct(id) {
  let product = await cache.get(`product:${id}`);
  if (!product) {
    product = await db.products.findById(id);
    await cache.set(`product:${id}`, product, { ttl: 300 });
  }
  return product;
}

What to Cache

Redis Example

import Redis from 'ioredis';

const redis = new Redis();

class ProductCache {
  private prefix = 'product:';
  private ttl = 300; // 5 minutes
  
  async get(id: string): Promise<Product | null> {
    const cached = await redis.get(this.prefix + id);
    return cached ? JSON.parse(cached) : null;
  }
  
  async set(id: string, product: Product): Promise<void> {
    await redis.setex(
      this.prefix + id,
      this.ttl,
      JSON.stringify(product)
    );
  }
  
  async invalidate(id: string): Promise<void> {
    await redis.del(this.prefix + id);
  }
  
  async invalidatePattern(pattern: string): Promise<void> {
    const keys = await redis.keys(this.prefix + pattern);
    if (keys.length) await redis.del(...keys);
  }
}

Pressure Resistance Protocol

1. "It Rarely Changes"

Pressure: "This data almost never updates"

Response: "Almost never" still means sometimes. When it does, stale cache = bugs.

Action: Add TTL at minimum. Add invalidation on write.

2. "TTL Is Enough"

Pressure: "We'll just expire after 5 minutes"

Response: 5 minutes of stale data might be unacceptable. User updates profile, sees old data.

Action: TTL + write-through invalidation.

3. "We'll Figure It Out Later"

Pressure: "Just add caching, we'll handle staleness if it's a problem"

Response: Staleness bugs are hard to debug. Design invalidation upfront.

Action: No cache without invalidation strategy defined.

Red Flags - STOP and Reconsider

• Cache with no TTL
• No invalidation on data updates
• "Users can refresh to see new data"
• In-memory cache in distributed system
• Cache without memory limits

All of these mean: Define invalidation strategy.

Quick Reference

Common Rationalizations (All Invalid)

The Bottom Line

Every cache needs: TTL, size limit, and invalidation strategy.

Cache aggressively for performance. But always know how the cache gets invalidated when data changes. "It rarely changes" is not a strategy.