Designing a URL Shortener at Scale

A URL shortener converts long URLs into short aliases and redirects users when accessed.

Functional Requirements

• Create a short URL from a long URL
• Redirect short URL to original
• Support custom aliases
• Track click analytics

Non-Functional Requirements

• 100M URLs generated per day
• Low latency reads (<10ms)
• High availability (99.99%)

Key Design Decisions

URL Encoding

Use Base62 encoding (a-z, A-Z, 0-9) to generate 7-character short codes.

62^7 = ~3.5 trillion unique URLs

Database Choice

• Write: Use a NoSQL store like Cassandra for high write throughput
• Read: Cache hot URLs in Redis (80% of traffic hits 20% of URLs)

Architecture

Client → Load Balancer → App Servers
                            ↓
                       Redis Cache
                            ↓
                        Cassandra

Redirection Flow

1. Client hits short URL
2. Check Redis cache
3. If miss, query Cassandra
4. Store in cache, redirect

Analytics

Use Kafka to stream click events asynchronously to an analytics pipeline.

Conclusion

A well-designed URL shortener handles billions of redirects with sub-10ms latency using caching and distributed storage.