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Biswas Prasana Swain
Biswas Prasana Swain

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Designing a URL Shortener

#systemdesign #tutorial #career #learning

Imagine your friend sends you a super long link to a cat video:

https://www.youtube.com/watch?v=dQw4w9WgXcQ&ab_channel=RickAstleyOfficial&list=PL1234567890ABCDEFGHIJK
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That’s ugly, right? What if we could turn it into something like:

https://sho.rt/xyz123
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That’s exactly what a URL shortener does — and in this article, you’ll learn how to design such a system, from scratch, even if you’ve never done any system design before.

🌐 What Is a URL Shortener?

A URL shortener is a web service that turns long URLs into short ones. When someone clicks the short link, it redirects them to the original long URL.

Popular examples:

  • bit.ly
  • tinyurl.com
  • t.co (used by Twitter)

🎯 Goal of the System

Design a system that:

  1. Accepts a long URL like https://www.example.com/articles/2025/07/shorten-this-link.
  2. Returns a short URL like sho.rt/abc123.
  3. When someone visits sho.rt/abc123, they’re redirected to the original URL.

🔨 Core Features

Feature Description
Shorten URL Accept a long URL and generate a short one
Redirect Given a short URL, redirect to the original long URL
Analytics (optional) Count how many times a short URL was clicked
Expiration (optional) Make some short URLs valid only for a limited time

🧠 Key Design Considerations

Before we build anything, we must think about:

1. Scalability

Can our system handle millions of links and billions of redirects?

2. Availability

Can people always access their links even if something crashes?

3. Speed

Can the system redirect people in milliseconds?

4. Uniqueness

How do we ensure no two links generate the same short code?

🧱 Basic Components

Let’s break down the system into pieces:

[Client] -> [Load Balancer] -> [Application Servers] -> [Database]
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  1. Client: The user who wants to shorten or open a link.
  2. Load Balancer: Distributes incoming requests to servers.
  3. Application Server: Handles logic (generate/redirect).
  4. Database: Stores mapping between short and long URLs.

🔢 How to Generate Short URLs

Let’s say we want to shorten this:

https://example.com/my-very-long-article-about-space
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We can generate a short URL code in several ways:

Option 1: Hashing

  • Hash the original URL using a function like SHA-256.
  • Take the first 6–8 characters of the hash.
  • Map that to a short code.

✅ Simple
❌ Risk of collision (two URLs might get same short code)

Option 2: Auto-Increment ID + Base62

  • Store each new URL in the database with a numeric ID (like 123456).
  • Convert the ID to Base62 (0-9 + a-z + A-Z): 
  123456 → "abc123"
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✅ Unique, Compact
✅ Easy to reverse
❌ Not random (easy to guess next links)

Option 3: Random String

  • Generate a random string of 6-8 characters.
  • Check in DB if it's already taken. If not, save it.

✅ Harder to guess
❌ Can collide, so we need retries

📦 Data Model (Database Schema)

Let’s use a simple table:

Field Type Description
short_code VARCHAR(10) The generated code, like abc123
long_url TEXT The original URL
created_at DATETIME When the short URL was created
expiry_date DATETIME (Optional) Expiration date
click_count INT (Optional) Number of times clicked

🚦 Redirect Flow

When someone visits:

https://sho.rt/abc123
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Here’s what happens:

  1. User’s browser sends a request to your server with code abc123.
  2. Server looks up the code in the database.
  3. If found, it sends a 301 redirect to the original long URL.
  4. Browser takes the user to the long URL.

🏗️ Putting It Together (Step-by-Step)

1. URL Shortening API

POST /shorten

Request:

{
  "long_url": "https://example.com/article/123"
}
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Response:

{
  "short_url": "https://sho.rt/abc123"
}
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2. Redirection API

GET /abc123

Action:

  • Find abc123 in the database.
  • If found, return a 301 redirect to the long URL.

⚙️ Scaling the System

If the system grows big, here’s how we keep it fast:

1. Caching

Use Redis or Memcached to cache popular short codes in memory.
This avoids hitting the database on every redirect.

2. Sharding

Split the database into multiple machines to handle billions of entries.

3. Rate Limiting

Prevent abuse by limiting how many links a user can create per minute.

4. Load Balancing

Distribute traffic across multiple servers using a load balancer like Nginx or AWS ELB.

🛡️ Security and Edge Cases

Problem Solution
Malicious URLs (phishing, malware) Use a URL scanner like Google Safe Browsing
Abuse (spamming) Rate limit users, add CAPTCHA
Expired links Add TTL (Time-To-Live) logic
Editable links Let users log in and manage their links

🏭 Diagram

URL Shortener Architecture

📊 Optional Features

  • Analytics Dashboard: Show users how many clicks, locations, devices, etc.
  • Custom Short Codes: Let users choose their code (e.g., sho.rt/mybrand)
  • Link Previews: Show metadata when sharing links on social media
  • QR Codes: Auto-generate QR for each short link

✅ Summary

Concept Description
Goal Shorten and redirect URLs
Short Code Can be generated using hashing, Base62, or random strings
Storage Store code-to-URL mapping in a database
Redirect Lookup short code and return 301 redirect
Scaling Use cache, sharding, and load balancing
Security Scan for malicious URLs, rate limiting, CAPTCHA
Extra Features Analytics, custom codes, link expiration

🧠 Final Thought

Even though a URL shortener seems simple on the surface, designing it at scale teaches you many real-world software engineering concepts: databases, caching, hashing, APIs, distributed systems, and security.

Whether you're building for a side project or preparing for a system design interview — mastering the design of a URL shortener is a perfect starting point.

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