May 7, 2026

Design a URL Shortener

Interview notes on short-code generation, redirects, caching, uniqueness, and scaling a Bitly-like service.

Reading notes System design Flow first

The Problem

A URL shortener is mostly a read-scaling problem with a very small write path.

Requirements

Create a short URL from a long URL
Redirect from short URL to original URL
Optional custom aliases and expiration

Core Idea

The short code must be globally unique. A global counter plus Base62 encoding is a simple and reliable way to generate it.

Reads

Redirects should be fast. Cache the shortCode -> originalUrl mapping in Redis and fall back to the database on a miss.

Writes

Writes are cheap, so a standard relational database is enough for persistence and metadata.

Extra Features

Custom aliases and expiration can fit into the same table with conflict checks and TTL handling.

Final Shape

The clean interview answer is: generate unique short codes with an atomic counter, store mappings in a database, cache redirects aggressively, and return 302 for redirects.