LLD interview expectations - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Key Focus Areas in LLD Interviews

Which of the following is NOT typically a key focus area in a Low-Level Design (LLD) interview?

A Writing efficient SQL queries for data retrieval

B Ensuring modularity and reusability in code

C Designing classes and their relationships

D Applying design patterns appropriately

💡 Hint

Think about what LLD interviews mainly test: code structure and design, not database querying.

LLD interviews focus on designing classes, interfaces, and applying design patterns. Writing SQL queries is usually part of database or backend interviews, not LLD.

❓ Architecture

intermediate

Class Diagram for a Simple Library System

In a Low-Level Design interview, you are asked to design a simple library system. Which class relationship best represents the connection between 'Book' and 'Author'?

A Composition: Book is composed of Authors

B Inheritance: Author inherits from Book

C Aggregation: Author contains Books

D Association: Book has one or more Authors

💡 Hint

Consider if a Book can exist without an Author and vice versa.

Books and Authors have an association relationship because a book can have multiple authors, and authors can write multiple books. Neither inherits from the other, and composition or aggregation do not fit well here.

❓ scaling

advanced

Handling Increased Load in LLD Designs

You designed a notification system in your LLD interview. The interviewer asks how you would modify your design to handle a sudden increase in users sending notifications simultaneously. Which approach is the best to scale your design?

A Store all notifications in a single database table with no indexing

B Use a message queue to decouple notification sending from processing

C Add more methods to the Notification class to handle concurrency

D Make the Notification class static to share state across threads

💡 Hint

Think about how to handle many requests without blocking or losing data.

Using a message queue helps to buffer and process notifications asynchronously, improving scalability and reliability. Other options either do not address concurrency properly or can cause bottlenecks.

❓ tradeoff

advanced

Tradeoff Between Flexibility and Complexity

In an LLD interview, you propose a design with many interfaces and abstract classes to allow future flexibility. The interviewer asks about the tradeoff of this approach. What is the main downside?

A It makes the system run faster

B It eliminates the need for unit testing

C It reduces code readability and increases complexity

D It decreases modularity

💡 Hint

More abstraction can make code harder to understand and maintain.

While interfaces and abstractions improve flexibility, they can also make the codebase more complex and harder to read, which is a common tradeoff in design.

❓ estimation

expert

Estimating Capacity for a Chat Application

You designed a chat application in your LLD interview. The interviewer asks: If your system expects 1 million active users sending an average of 10 messages per minute, how many messages per second must your system handle?

A Approximately 166,667 messages per second

B Approximately 10,000 messages per second

C Approximately 166 messages per second

D Approximately 600,000 messages per second

💡 Hint

Calculate total messages per minute and convert to per second.

1 million users * 10 messages/minute = 10 million messages/minute. Dividing by 60 seconds gives about 166,667 messages per second.