Building a Simple URL Shortener with Go and Redis

URL shorteners are everywhere, from social media to analytics platforms. They look simple, but under the hood, they require careful thought: idempotency, collision management, fast lookups, and clean API design.

I recently completed the URL Shortener challenge from Coding Challenges, and although it seems like a tiny project on the surface, it turned into a rich learning experience.

In this article, I walk you through how I built a lightweight, fast, and reliable URL Shortener using:

• Go
• Redis(Memurai on Windows)

Why I Chose Go + Redis

I built the project using:

• Go for its simplicity, speed, and clean standard library
• Redis as an in-memory key–value store for O(1) lookups
• A very small project structure with separate handlers and utilities

What the Service Does

The service exposes two endpoints:

• POST /shorten
  Accepts a long URL and returns a short one.

• GET /{shortCode}
  Redirects the user to the original URL.

The project structure looks like this:

url-shortner/
├─ config/
│  └─ redis.go
├─ handlers/
│  ├─ shorten.go
│  └─ redirect.go
├─ utils/
│  └─ hash.go
├─ main.go
├─ go.mod
└─ go.sum

How It Works

1. Shortening URLs

The ShortenHandlerin handlers/shorten.go handles POST requests to /shorten. The workflow is as follows:

1. Receive request – The API expects a JSON payload with the URL field.
2. Check for existing mapping – If the long URL already exists in Redis, the same short URL is returned (ensuring idempotency).
3. Generate a shortcode – A random alphanumeric code of 6 characters is generated. If a collision occurs, a salted SHA1 hash ensures uniqueness.
4. Store mappings – The short code maps to the long URL, and a reverse mapping is also stored for idempotency.

Example request:

POST /shorten
{
  "url": "https://example.com/very-long-url"
}

Response:

{
  "short_url": "http://localhost:8080/aB9xYz"
}

2. Redirecting Short URLs

The RedirectHandlerin handlers/redirect.go handles GET requests for short URLs:

1. Extract the short code from the request path.
2. Look up the original URL in Redis.
3. Redirect the client to the long URL using HTTP status 302 Found.

3. Generating Short Codes

utils/hash.go provides two functions:

• GenerateShortCode(n int) – Generates a random alphanumeric code.
• GenerateShortCodeWithSalt(url string) – Uses SHA1 hashing and the current timestamp as a salt to avoid collisions in case of duplicates.

Redis Integration

config/redis.go sets up the Redis client:

• Connects to Redis at localhost:6379.
• Pings the server to ensure connectivity.
• Provides a global RedisClientand Ctxfor use across the project.

Redis stores:

• shortCode → longURL(for redirecting)
• longURL → shortCode (for idempotent shortening)

Here’s the full implementation on GitHub

Key Features

• Idempotent URL shortening – Multiple requests for the same long URL return the same short URL.
• Collision handling – Ensures unique short codes.
• Fast in-memory storage – Using Redis for near-instant lookups.
• Simple API – Minimalist design for ease of use.

Future Enhancements

• Add analytics to track URL clicks.
• Implement custom short codes for user-defined aliases.
• Add expiration time for temporary links.