Design a scalable URL shortener

System Design: Scalable URL Shortener

Design a highly scalable URL shortener that converts long URLs into short links and supports redirection.

Core requirements

• Create a short URL for a given long URL.
• Redirect : when a user visits the short URL, redirect to the original long URL.
• Short codes must be:
  • Globally unique .
  • Reversible in the sense that the system can always retrieve the original URL from the short code (via lookup).
• The system should be high availability and handle high QPS (reads/redirects typically much higher than writes).

Design topics to cover

1. APIs
   • Example endpoints for creating short links and performing redirects.
2. Data model & storage
   • What data you store for each short link (e.g., short_code , long_url , created_at , expiry , owner_id , etc.).
   • Choice of datastore(s): KV store / relational DB / both, and why.
3. Short code generation strategy
   • Hash-based vs ID-based (e.g., Snowflake/auto-increment) and Base62 encoding.
   • How you ensure uniqueness and performance under concurrency.
4. Caching & performance
   • Use of in-memory cache (e.g., Redis) and/or CDN for hot links.
   • Cache key design, TTL, and cache-miss behavior.
5. Scalability & reliability
   • Partitioning/sharding strategy.
   • Handling replication, failover, and consistency trade-offs.
6. Operational concerns
   • Abuse prevention (rate limits), observability/metrics, link expiration, and analytics logging (optional).

Follow-up

• If you use a hash of the long URL to generate the short code, how do you handle hash collisions ?
  • Provide at least two approaches and their trade-offs.