Design a Highly Available URL Shortener

# Design a Highly Available URL Shortener Design a distributed service that creates short URLs and redirects them at large scale. Cover APIs, code generation, persistence, caching, partitioning, collision handling, expiration, failure recovery, availability, and horizontal growth. ### Constraints & Assumptions - Redirect traffic is much heavier than creation traffic. - A short code must resolve to at most one active destination. - Expired links must stop redirecting according to a stated consistency policy. ### Clarifying Questions to Ask - May users request custom aliases? - Are destination changes allowed after creation? - What latency, retention, and regional availability are required? ```hint Protect the mapping invariant Choose a generation strategy only after explaining how the system prevents one code from mapping to two destinations. ``` ### What a Strong Answer Covers - Create and redirect API contracts. - Unique-code generation and collision behavior. - Storage keys, partitions, caches, expiration, and invalidation. - Read scaling, replication, regional failure, observability, and abuse controls. ### Follow-up Questions 1. How would custom aliases alter the hot partition and reservation design? 2. How would you migrate the code-generation scheme without breaking old links?