Design TinyURL: a URL shortener with fast redirects

Design a URL shortener like TinyURL. It turns a long URL into a short link (for example https://s.example.com/aZ9k2Qx) and redirects visitors to the original URL. The workload is read-heavy: billions of redirects a day, far fewer new links. Users can optionally choose a custom alias and an expiry date.

Requirements

• Functional: create a short link for a long URL; optional custom alias; optional expiry; redirect a short code to its URL; optional click counts.
• Non-functional: billions of redirects a day with low latency worldwide; high availability for redirects; short codes unique and hard to guess if required; links last for years.

Clarifying questions

• How many new links per day, and what read/write ratio?
• How short must codes be, and may they be guessable (sequential)?
• Does the same long URL always get the same code, or a new one each time?
• Permanent (301) or temporary (302) redirects? (It matters for analytics.)
• Are click analytics and abuse prevention (phishing links) in scope?

What a strong answer covers

• Back-of-the-envelope numbers: links per day, codes needed over ten years, storage, redirect QPS; base62 length (7 characters give about 3.5 trillion codes).
• Code generation options with trade-offs: a counter or ID ranges handed to each server and base62-encoded (no collisions, but guessable unless scrambled), random codes with a uniqueness check, or hashing the URL with collision handling.
• Custom aliases as a uniqueness check on insert in the same keyspace.
• Data model: code to long URL, owner, created and expiry times, in a key-value store partitioned by code.
• The redirect path: CDN and in-memory cache in front of the store, with cache-aside reads; 301 versus 302.
• Expiry handled lazily on read plus background cleanup; deleted codes not reused.
• Analytics recorded asynchronously from redirect logs so the redirect stays fast; rate limiting and malicious-URL scanning on creation.

Follow-ups

• A link goes viral and gets a million hits a minute.
• Generate unique IDs across regions without a single coordinator.
• Let users edit where a short link points, safely.
• Detect links that turn malicious after they were created.