First design walkthrough (URL shortener) in HLD - System Design Guide

Problem Statement

Users want to share long URLs easily, but long links are hard to remember, prone to errors when typed, and visually cluttered. Without a system to shorten URLs, sharing links in messages, social media, or print becomes inconvenient and error-prone.

Solution

A URL shortener creates a unique short code for each long URL and redirects users from the short URL to the original long URL. It stores the mapping in a database and uses a simple lookup to forward requests quickly, making links easier to share and remember.

Architecture

User Client

(Browser/App)

→URL Shortener

↓

This diagram shows a user sending a request to the URL shortener service, which stores or retrieves the long URL from a database and redirects the user to the original URL.

Trade-offs

✓ Pros

→

Simplifies sharing by converting long URLs into short, easy-to-remember links.

→

Reduces errors when typing or copying URLs.

→

Enables tracking and analytics on link usage.

✗ Cons

→

Requires storage and management of URL mappings, which can grow large.

→

Potential for collisions if short codes are not generated carefully.

→

Adds a dependency on the shortener service's availability for redirects.

Use when your application needs to share or distribute long URLs frequently, especially if user experience benefits from shorter links and you expect at least hundreds of requests per day.

Avoid if your system has very low URL sharing needs (less than a few dozen per day) or if you cannot maintain the availability of the redirect service reliably.

Real World Examples

Bitly

Bitly shortens URLs for marketing campaigns, enabling easy sharing and click tracking across social media and email.

Twitter

Twitter uses its own URL shortener to fit links within the character limit and track link clicks.

Google

Google's former URL shortener helped users share links easily and provided analytics on link usage.

Alternatives

Hash-based URL shortening

Generates short codes by hashing the original URL instead of random or sequential IDs.

Use when: Choose when you want deterministic short codes that always map the same long URL to the same short code.

Custom alias URL shortening

Allows users to specify their own short code instead of auto-generated ones.

Use when: Choose when user-friendly or branded short URLs are required.

Distributed URL shortening

Uses multiple servers and databases to scale URL shortening for very high traffic.

Use when: Choose when expecting millions of URL shortening requests and redirects per day.

Summary

URL shorteners solve the problem of sharing long, unwieldy URLs by creating short, easy-to-share links.

They work by storing a mapping from a short code to the original URL and redirecting users accordingly.

This pattern improves user experience but requires careful design to handle scale, collisions, and availability.

Practice

1. What is the primary purpose of a URL shortener system?

easy

A. To generate random passwords for users

B. To increase the length of URLs for security

C. To store user passwords securely

D. To convert long URLs into short, easy-to-share links

Solution

1. Step 1: Understand the function of a URL shortener

   A URL shortener takes a long URL and creates a shorter alias that redirects to the original URL.

2. Step 2: Identify the correct purpose among options

   Only To convert long URLs into short, easy-to-share links describes this function correctly; others describe unrelated tasks.

3. Final Answer:

   To convert long URLs into short, easy-to-share links -> Option D

4. Quick Check:

   URL shortener purpose = convert long URLs to short [OK]

Hint: Focus on what URL shorteners do: shorten URLs [OK]

Common Mistakes:

• Confusing URL shortening with security features
• Thinking it stores passwords
• Assuming it generates random data unrelated to URLs

2. Which component is essential for generating unique short codes in a URL shortener?

easy

A. Cache server

B. Load balancer

C. Short code generator

D. User authentication module

Solution

1. Step 1: Identify the role of each component

   Short code generator creates unique short strings; load balancer distributes traffic; cache stores data temporarily; authentication manages users.

2. Step 2: Match component to short code creation

   Only the short code generator is responsible for creating unique short URLs.

3. Final Answer:

   Short code generator -> Option C

4. Quick Check:

   Unique code creation = short code generator [OK]

Hint: Short codes come from the short code generator [OK]

Common Mistakes:

• Confusing load balancer with code generation
• Thinking cache generates codes
• Assuming authentication creates short URLs

3. Given a URL shortener system, what happens when a user clicks a short URL http://short.ly/abc123?

medium

A. The system looks up abc123 in storage and redirects to the original long URL

B. The system generates a new short code for abc123

C. The system deletes the short URL from the database

D. The system asks the user to enter the long URL manually

Solution

1. Step 1: Understand the redirection process

   When a short URL is accessed, the system finds the original URL mapped to the short code and redirects the user.

2. Step 2: Evaluate each option

   The system looks up abc123 in storage and redirects to the original long URL correctly describes this lookup and redirect. Others describe incorrect or unrelated actions.

3. Final Answer:

   The system looks up abc123 in storage and redirects to the original long URL -> Option A

4. Quick Check:

   Click short URL = lookup and redirect [OK]

Hint: Clicking short URL triggers lookup and redirect [OK]

Common Mistakes:

• Thinking system creates new codes on click
• Assuming deletion happens on access
• Believing user must input original URL manually

4. In a URL shortener design, which issue arises if the short code generator does not check for duplicates?

medium

A. The system runs out of storage space quickly

B. Multiple long URLs map to the same short code causing incorrect redirects

C. Users cannot create new accounts

D. The system slows down due to too many cache hits

Solution

1. Step 1: Understand duplicate short code impact

   If duplicates exist, different long URLs share the same short code, causing wrong redirection.

2. Step 2: Analyze options for relevance

   Multiple long URLs map to the same short code causing incorrect redirects correctly identifies this problem. Others describe unrelated issues.

3. Final Answer:

   Multiple long URLs map to the same short code causing incorrect redirects -> Option B

4. Quick Check:

   Duplicate codes cause wrong redirects [OK]

Hint: Duplicates cause wrong URL redirects [OK]

Common Mistakes:

• Confusing storage issues with duplicate codes
• Thinking duplicates affect user accounts
• Assuming cache hits cause this problem

5. To scale a URL shortener for millions of users, which design choice improves performance and availability?

hard

A. Use distributed storage with caching and load balancing

B. Store all data on a single server to simplify access

C. Generate short codes sequentially without randomness

D. Disable caching to ensure fresh data every time

Solution

1. Step 1: Identify scalability needs

   Millions of users require fast access, high availability, and fault tolerance.

2. Step 2: Evaluate design choices

   Distributed storage spreads data load; caching speeds up lookups; load balancing distributes traffic. This combination supports scale.

3. Step 3: Reject poor choices

   Single server limits scale; sequential codes risk collisions; disabling cache slows response.

4. Final Answer:

   Use distributed storage with caching and load balancing -> Option A

5. Quick Check:

   Scaling needs distributed storage + cache + load balancer [OK]

Hint: Scale with distributed storage, caching, and load balancing [OK]

Common Mistakes:

• Choosing single server for large scale
• Ignoring caching benefits
• Using sequential codes risking collisions