BitlyDifficulty: Beginner

How to Design a URL Shortener (Bitly)

A classic system design interview question. The goal is to take a long URL, generate a unique short alias, and redirect users quickly when they visit the short alias.

High-Level Architecture

[User] -> (POST /create) -> [API] -> (Base62 Encode) -> [DB]
[User] -> (GET /alias) -> [API] -> [Cache] -> (Redirect 301)

Database Design

A simple relational database (PostgreSQL) or NoSQL (MongoDB) mapping `short_hash` -> `original_url`. Include expiration dates and user IDs.

Caching Strategy

Cache heavily using Redis. If a short link goes viral, the DB would crash without caching.

Scaling & APIs

Scaling: Use a pre-generated ID system (like Snowflake or Zookeeper) to ensure multiple servers don't generate the same short alias concurrently. Convert the Base-10 ID to Base-62 to get the short string.

API Design: REST API with rate limiting to prevent malicious users from creating millions of links.

Real-World Challenges

•Preventing hash collisions.
•Generating unpredictable hashes for security.
•Handling 301 (Permanent) vs 302 (Temporary) redirects for analytics.