name: caching
description: Use when same data is fetched repeatedly. Use when database queries are slow. Use when implementing caching without invalidation strategy.

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.