Building Scalable APIs

Best practices for designing and building APIs that can handle millions of requests.

Building APIs that scale is both an art and a science. Here are the key principles I've learned.

Design Principles

1. Statelessness

Each request should contain all information needed to process it. Don't rely on server-side sessions.

// Good: Token contains all auth info
app.get('/api/user', authenticate, (req, res) => {
  const userId = req.user.id; // From JWT
  // ...
});

2. Pagination

Never return unbounded lists:

interface PaginatedResponse<T> {
  data: T[];
  nextCursor?: string;
  hasMore: boolean;
}

3. Rate Limiting

Protect your API from abuse:

const rateLimit = require('express-rate-limit');

const limiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100 // limit each IP to 100 requests per window
});

app.use('/api/', limiter);

Caching Strategies

CDN caching for static responses
Redis for session and frequently accessed data
Database query caching for expensive queries

Monitoring

You can't improve what you don't measure:

Request latency (p50, p95, p99)
Error rates
Throughput
Database query times

Conclusion

Scalability isn't an afterthought—it should be designed in from the start.