First design walkthrough (URL shortener) in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a URL shortener system?

A URL shortener converts a long URL into a shorter, easy-to-share link that redirects users to the original long URL.

beginner

Name two key components of a URL shortener system.

1. A service to generate and store short URLs mapping to long URLs. 2. A redirect service that sends users from the short URL to the original long URL.

intermediate

Why is it important to handle collisions when generating short URLs?

Because two different long URLs might generate the same short URL code, causing incorrect redirection. Handling collisions ensures each short URL uniquely maps to one long URL.

intermediate

What is a common technique to generate short URL codes?

Using a base62 encoding of a unique ID or hashing the long URL and encoding part of the hash to create a short, alphanumeric code.

advanced

How can a URL shortener system scale to handle millions of requests?

By using caching for popular URLs, load balancing across servers, distributed databases for storage, and asynchronous processing for analytics.

What does a URL shortener primarily do?

A Converts long URLs into shorter ones for easy sharing

B Encrypts URLs for security

C Blocks malicious URLs

D Stores user passwords

A URL shortener creates short links that redirect to long URLs.

Which encoding is commonly used to create short URL codes?

A Base10

B Base62

C Base16

D Base8

Base62 uses digits, uppercase and lowercase letters, making short codes compact.

What is a key challenge when generating short URLs?

A Handling collisions

B Encrypting URLs

C Compressing URLs

D Translating URLs

Collisions occur when two URLs map to the same short code, which must be avoided.

Which component redirects users from short URLs to original URLs?

A Database service

B Authentication service

C Analytics service

D Redirect service

The redirect service handles user requests and sends them to the original URL.

How can caching help a URL shortener system?

A By encrypting URLs

B By compressing URLs

C By storing popular URL mappings to reduce database load

D By blocking spam URLs

Caching popular URLs speeds up redirection and reduces database queries.

Explain the main components and flow of a URL shortener system.

Think about how a long URL becomes a short one and how users get redirected.

Describe strategies to ensure scalability and reliability in a URL shortener design.

Consider how to handle many users and keep the system fast and accurate.

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