How would you design a URL shortening service like Bitly?

A good URL shortener design covers requirements, data model, and scaling. **Requirements:** create short links, redirect quickly, analytics (optional), high availability. **Key design choices:** - **ID generation:** Base62 encode a unique ID (Snowflake/sequence) to avoid collisions. - **Storage:** Key-value mapping `shortKey -> longUrl` (plus createdAt, expiry, owner). - **Caching:** Cache popular keys in Redis to reduce DB reads. - **Redirects:** Use 301 (permanent) or 302 (temporary) depending on needs. - **Scale:** Partition by `shortKey`, add read replicas, and front with CDN for edge redirects. **Risk areas:** abuse/spam, link expiration, and analytics write amplification.