First design walkthrough (URL shortener) in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main component responsible for generating short URLs.

HLD

The core component that generates the short URL is called the [1] service.

Drag options to blanks, or click blank then click option'

ALoad Balancer

BURL Resolver

CURL Shortening

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the URL Resolver with the URL Shortening service.

Thinking the database generates the short URL.

The URL Shortening service is responsible for creating the short version of the original URL.

2fill in blank

medium

Complete the code to describe the key-value store used for mapping short URLs to original URLs.

HLD

The system uses a [1] to store the mapping between short URLs and original URLs for fast lookup.

Drag options to blanks, or click blank then click option'

Arelational database

Bkey-value store

Cfile system

Dmessage queue

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing relational database which is slower for simple key lookups.

Choosing message queue which is for asynchronous processing.

A key-value store is ideal for fast retrieval of the original URL using the short URL as the key.

3fill in blank

hard

Fix the error in the request flow description for redirecting short URLs.

HLD

When a user clicks a short URL, the system [1] the original URL from the database and redirects the user.

Drag options to blanks, or click blank then click option'

Areads

Bdeletes

Cwrites

Dupdates

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking the system writes or updates during redirect.

Confusing read with delete operation.

The system reads the original URL from the database to redirect the user correctly.

4fill in blank

hard

Fill both blanks to complete the description of the short URL generation logic.

HLD

To generate a unique short URL, the system uses a [1] to create a unique ID and then [2] it into a short string.

Drag options to blanks, or click blank then click option'

Acounter

Bhashes

Cdatabase

Dencodes

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'hashes' instead of 'encodes' for the second blank.

Using 'database' instead of 'counter' for the first blank.

The system uses a counter to generate a unique ID and then encodes it into a short string for the URL.

5fill in blank

hard

Fill all three blanks to complete the description of scaling the URL shortener system.

HLD

To handle high traffic, the system uses a [1] to distribute requests, multiple [2] for storage, and [3] caching to speed up lookups.

Drag options to blanks, or click blank then click option'

Aload balancer

Bdatabases

Cin-memory

Dsingle server

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'single server' which does not scale well.

Confusing caching with database storage.

A load balancer distributes incoming requests, multiple databases store data reliably, and in-memory caching speeds up data retrieval.

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