First design walkthrough (URL shortener) in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Identify the main components in a URL shortener system

Which of the following lists correctly identifies the essential components needed to build a scalable URL shortener?

A File Storage, Image Processing, CDN, Video Streaming Service

B Load Balancer, Cache, User Authentication, Payment Gateway

C API Gateway, Database, URL Redirect Service, Analytics Service

D Web Server, Email Service, Notification Service, Logging Service

💡 Hint

Think about what components handle requests, store data, and redirect users.

The core components include an API Gateway to receive requests, a Database to store mappings, a URL Redirect Service to handle redirection, and optionally an Analytics Service to track usage.

❓ scaling

intermediate

Scaling the database for high read traffic

In a URL shortener, most traffic is read-heavy (redirects). Which approach best improves read scalability?

A Add read replicas to the database and use caching for redirects

B Use database sharding to split data by URL hash ranges

C Store all URLs in a single master database with no replicas

D Use a distributed file system to store URL mappings

💡 Hint

Consider how to reduce load on the main database for frequent reads.

Read replicas allow scaling read operations, and caching popular URLs reduces database hits, improving performance and scalability.

❓ tradeoff

advanced

Choosing the URL key generation strategy

Which URL key generation method balances uniqueness, short length, and ease of generation?

A Generate a random 10-character alphanumeric string for each URL

B Use a sequential numeric ID converted to base62 encoding

C Use the full original URL as the key without shortening

D Use the current timestamp as the key

💡 Hint

Think about avoiding collisions and keeping keys short.

Sequential IDs encoded in base62 produce short, unique keys easily without collisions, unlike random strings which risk collisions and longer keys.

🧠 Conceptual

advanced

Handling expired or deleted URLs

What is the best approach to handle URLs that have expired or been deleted in a URL shortener system?

A Redirect to the original URL regardless of expiration

B Return a 404 error when the short URL is accessed

C Automatically generate a new short URL for the expired one

D Redirect to a default homepage or informational page

💡 Hint

Consider user experience when a URL is no longer valid.

Redirecting to a default page informs users the link is invalid and can provide options, better than a plain 404 error which can be confusing.

❓ estimation

expert

Estimating storage needs for URL mappings

Estimate the approximate storage required to store 100 million URL mappings if each original URL averages 100 characters and each short URL key is 7 characters. Assume UTF-8 encoding (1 byte per character) and 20 bytes overhead per record for metadata.

A Approximately 12 GB

B Approximately 10 GB

C Approximately 15 GB

D Approximately 8 GB

💡 Hint

Calculate total bytes per record and multiply by 100 million.

Each record: 100 bytes (original URL) + 7 bytes (short key) + 20 bytes (metadata) = 127 bytes. 127 bytes * 100 million = 12.7 billion bytes ≈ 12 GB.