URL Shortener at Scale

The URL shortener is the quintessential system design interview question, and for good reason — it touches on hashing strategies, database selection, read-heavy workload optimization, and horizontal scaling. In this challenge, you will design a service that accepts long URLs and returns short, unique aliases, then redirects users with sub-10ms latency. You will evaluate trade-offs between hash collision strategies (base62 encoding vs. pre-generated key pools), choose between DynamoDB and ElastiCache for the redirect lookup layer, and implement a cache-aside pattern using CloudFront and ElastiCache to handle bursty read traffic. The architecture must support analytics tracking (click counts, referrers, geo data) without adding latency to the redirect path. You will also design the expiration and cleanup strategy for short URLs using DynamoDB TTL and Lambda-based garbage collection. This challenge teaches you to think about read/write ratio optimization, consistent hashing, and the real cost implications of different storage tiers on AWS.