Back-of-the-envelope estimation in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ estimation

intermediate

Estimating Daily Requests for a New Social Media App

You are designing a new social media app expected to launch in a city with 5 million people. Estimate the number of daily requests the backend servers should handle on launch day.

Assume 10% of the population will sign up in the first month, and each active user generates 50 requests per day.

A 500 million requests per day

B 2.5 million requests per day

C 5 million requests per day

D 25 million requests per day

💡 Hint

Calculate 10% of 5 million users, then multiply by 50 requests per user.

10% of 5 million is 500,000 users. Each user makes 50 requests daily, so 500,000 * 50 = 25 million requests per day.

🧠 Conceptual

intermediate

Understanding Latency Impact on User Experience

Which of the following best explains why low latency is critical in system design for real-time applications?

A Low latency increases the number of users the system can support simultaneously.

B Low latency reduces the delay between user action and system response, improving user satisfaction.

C Low latency decreases the amount of data stored in the database.

D Low latency ensures the system uses less memory.

💡 Hint

Think about how users feel when a system responds quickly versus slowly.

Low latency means the system responds quickly to user actions, which is essential for good user experience in real-time apps.

❓ scaling

advanced

Scaling a Video Streaming Service for Peak Traffic

You are designing a video streaming service expecting 10 million peak concurrent viewers. Each viewer streams at 3 Mbps. Estimate the total bandwidth required to support peak traffic.

A 300 Gbps

B 300 Tbps

C 30 Tbps

D 3 Tbps

💡 Hint

Multiply the number of viewers by the streaming rate, then convert to appropriate units.

10 million viewers * 3 Mbps = 30 million Mbps = 30 Tbps total bandwidth needed.

❓ tradeoff

advanced

Choosing Between Vertical and Horizontal Scaling

Which statement best describes a tradeoff when choosing horizontal scaling over vertical scaling?

A Horizontal scaling improves fault tolerance but increases system complexity.

B Horizontal scaling reduces system complexity but decreases fault tolerance.

C Horizontal scaling is cheaper but limits maximum capacity compared to vertical scaling.

D Horizontal scaling requires fewer servers than vertical scaling.

💡 Hint

Consider how adding more machines affects system design and reliability.

Horizontal scaling adds more machines, improving fault tolerance but making the system more complex to manage.

❓ component

expert

Estimating Cache Size for a High-Traffic E-commerce Site

An e-commerce site has 1 million daily active users. Each user views on average 20 product pages per day. Each product page data size is 50 KB. To cache 10% of the most popular product pages, estimate the minimum cache size needed in GB.

A 10 GB

B 100 GB

C 1 TB

D 500 GB

💡 Hint

Calculate total page views, then 10% of that, multiply by page size, and convert to GB.

1 million users * 20 pages = 20 million views. 10% of 20 million = 2 million pages cached. 2 million * 50 KB = 100,000,000 KB = 100,000 MB = 100 GB. But since pages may repeat, caching unique pages is less. Assuming unique pages are fewer, 10 GB is a reasonable minimum cache size.