System Design

4 deep-dive scenarios · Step-by-step breakdowns · Architecture patterns

System Design Quiz

12 questions · 80% to pass

URL Shortener (Bit.ly)

Medium · 35-40 min

Functional Requirements

  • ✓Given a long URL, generate a short URL (6-7 chars)
  • ✓Given a short URL, redirect to the original URL
  • ✓Support custom short URLs (optional)
  • ✓Track analytics: clicks, geolocation, device

Non-Functional Requirements

  • →100M URLs created/day, 10B redirects/day
  • →Redirect latency < 10ms P99
  • →High availability (99.99% uptime)
  • →URLs should not be predictable (no sequential IDs)

Scale Estimates

  • 100M writes/day = ~1,200 writes/sec
  • 10B reads/day = ~115,000 reads/sec (100:1 read:write ratio)
  • 500 bytes/URL × 100M/day × 365 days × 5 years = ~90TB storage

Define the contract before the architecture. Two endpoints: POST /shorten (body: {long_url, custom_alias?}) → {short_url}. GET /{short_code} → 301/302 redirect to long_url.

Key Decisions

  • • 301 (permanent) vs 302 (temporary) redirect: 301 reduces load (browser caches), 302 allows accurate analytics tracking. Choose based on requirements.
  • • Authentication: public API needs rate limiting; business API needs auth tokens

Key Components

Load Balancer

Distributes 115K req/sec across stateless app servers

ID Generation Service

Snowflake-style unique IDs without coordination

Redis Cache

80%+ cache hit rate makes redirect sub-millisecond

SQL Database (read replicas)

Persistent storage; read replicas handle read scale

Kafka + Analytics DB

Async click tracking without impacting redirect latency

CDN (optional)

Edge caching for 301 redirects reduces origin load globally

Trade-offs to Discuss

  • • 301 vs 302: 301 is faster (browser caches), 302 gives better analytics
  • • SQL vs NoSQL: SQL is simpler and sufficient; NoSQL if multi-region write scale needed
  • • Hashing vs counter: hashing has collisions; counter requires central sequence generator

Expected Follow-up Questions

  • • How would you handle URL expiration?
  • • How do you prevent abuse (spam/malicious URLs)?
  • • How would you design this for 10x the scale?
  • • How do you handle custom domains (company.short.link)?