How would you design a URL shortener?

Direct answer

What a strong answer covers

How it works

The core is a mapping from a short code to a long URL with a read path that must be fast and highly available and a write path that is rare by comparison. Codes come from a base62 encoding of a counter or from random IDs checked for collision; the mapping lives in a key-value store or a simple table with the code as primary key; hot codes are cached at the edge; the redirect is a 301 or 302 depending on whether analytics need every hit. Abuse controls scan destinations, rate-limit creation, and allow takedown. Analytics are written asynchronously so a slow pipeline never delays a redirect. Capacity is estimated from write rate, read rate, and retention.

Example

For 100 million links a year and 10 billion redirects, a 7-character base62 code space is ample; writes go to a partitioned key-value store with the code as key; a CDN caches redirects for popular codes; each hit publishes an event to a stream for counting. Creation requires an API key with per-key quotas, and new destinations pass through a safe-browsing check before the link becomes active.

Common mistakes

• Using an auto-incrementing integer directly as the code so links are enumerable.
• Returning a 301 while promising per-click analytics that browsers will then skip.
• Putting the analytics write on the redirect path and making every click wait for it.

A useful answer structure

Answer from your own work.

Do not memorise a script: use only examples you can explain honestly in follow-up questions.