System Design Interview Preparation: A Practical Framework

The 4-Step Framework

System design interviews test your ability to design complex systems under constraints. Here's the framework we teach our engineers.

Step 1: Requirements (5 minutes)

Clarify functional and non-functional requirements before drawing anything.

# Design a URL Shortener
Functional: Shorten URLs, Redirect, Custom aliases, Analytics
Non-Functional: 100M URLs/day, 10:1 read:write ratio, 99.9% uptime
Scale: 1,160 writes/sec, 11,600 reads/sec

Step 2: High-Level Design

Draw the major components and their interactions.

[Client] → [Load Balancer] → [API Server] → [Database]
                                    ↓
                              [Cache Layer]

Step 3: Detailed Design

Dive deep into the most critical components.

function generateShortUrl(longUrl) {
  const id = generateUniqueId();
  return base62Encode(id);
}

Common Patterns

Rate Limiting

class RateLimiter {
  async isAllowed(userId) {
    const key = `ratelimit:${userId}:${Math.floor(Date.now() / 60000)}`;
    const count = await this.redis.incr(key);
    return count <= 100;
  }
}

Practice Problems

Conclusion

Practice the framework, not just the solutions. Interviewers want to see your thought process, not a memorized architecture.