Design a URL Shortener

Design a URL Shortener

What You Will Learn

Design a simple URL shortener from requirements through API, database, caching, and scaling decisions.

Core Idea

A URL shortener converts a long URL into a short identifier.

Example:

https://example.com/articles/very-long-title
                |
                v
          https://sho.rt/aB92x

Core requirements:

• Create a short URL.
• Redirect users to the original URL.
• Optionally support expiration.
• Handle many redirects.

A simple architecture is:

Client
  |
Load Balancer
  |
API Servers
  |
  +--> Redis Cache
  |
  +--> Database

The database stores the mapping. Redis can cache popular short codes.

A Simple Mental Model

Client
  |
  v
Application
  |
  +--> Database
  |
  +--> Cache
  |
  +--> Background Worker

Do not worry about building every component at once. System design is mainly about understanding why a component exists, what problem it solves, and how components communicate.

Practical Example

Example APIs:

POST /urls
GET /:code

Database:

short_code
original_url
created_at
expires_at

Redirect flow:

GET /aB92x
     |
     v
Redis
  | hit
  v
Original URL

Cache miss -> Database -> Redis -> Redirect

A key design issue is generating unique short codes. A counter, random identifier, or encoded ID can be used depending on requirements.

Key Points

['Start with a simple relational database.', 'Cache frequently accessed redirects.', 'Use TTLs for expiring links.', 'Consider collisions and abuse prevention.', 'Measure redirect traffic before adding complex infrastructure.']

Common Beginner Mistakes

• Adding technologies before understanding the problem.
• Designing for millions of users when the requirements do not justify it.
• Ignoring failure cases.
• Treating every system as if it needs the same architecture.
• Focusing on diagrams instead of explaining the decisions behind them.

Quick Summary

The goal of this lesson is to build a simple mental model that you can reuse in future designs.

Next, the examples become more interactive with chat and notification systems.