How would you design a URL shortener?

Clarify the requirements first: expected traffic, link lifetime, custom aliases and analytics. The core is a write path that creates a short key and a much heavier read path that redirects. Generate keys with a counter encoded in base 62, or with random keys checked for collisions, and store the key-to-URL mapping in a key-value or relational store. Put a cache in front of reads, choose a permanent or temporary redirect depending on whether you need click analytics, and add rate limiting and expiry. Then discuss scaling: partitioning the store and making key generation work across many servers.

What the interviewer is assessing

Structured system design: requirements, data model, trade-offs and scaling.

A strong answer covers

• Start by asking about expected traffic, link lifetime, custom aliases and analytics before designing.
• Generate keys with a base-62 encoded counter, or random keys checked for collisions.
• Store key-to-URL in a key-value or relational store, note reads far outnumber writes, and cache reads.
• Choose a permanent or temporary redirect depending on whether you need click analytics.
• Add rate limiting and expiry, partition the store, and make key generation work across many servers.

Common mistakes

• Jumping to technology before clarifying requirements
• Ignoring that reads far outnumber writes
• Not discussing key collisions

Practise a follow-up

• How would you handle a link that suddenly goes viral?
• How would you prevent abuse?