How would you design a URL shortener like bit.ly?

Requirements: shorten URLs, redirect to original, analytics. Design: Generate short code (base62 encoding of incremented ID or hash), store mapping in database (short_code -> original_url), redirect service (lookup, 301/302 redirect). Scaling: read-heavy (cache mappings in Redis), database sharding by short_code, CDN for redirects. Short code generation: counter service, random with collision check. Features: custom aliases, expiration, click analytics. Handle: high read throughput, URL validation, abuse prevention.