Top 30 Most Common Lld Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Introduction
Preparing for technical interviews can be a daunting task, especially when they involve Low-Level Design (LLD). While High-Level Design (HLD) focuses on the overall architecture of a system, LLD delves into the nitty-gritty details of how individual components are implemented. A strong grasp of LLD is crucial for translating broad system ideas into practical, maintainable, and efficient code. Interviewers use LLD questions to assess your ability to structure code, apply object-oriented principles, utilize design patterns effectively, and consider aspects like scalability, maintainability, and error handling at a granular level. This guide covers 30 common LLD interview questions, offering insights into why they are asked, how to approach them, and example answers to help you build a solid foundation for your preparation. Mastering these topics will significantly boost your confidence and performance in LLD interviews, demonstrating your readiness to tackle complex coding challenges.
What Are Lld Interview Questions?
LLD interview questions focus on designing the detailed internal workings of specific software components or smaller systems. Unlike HLD, which deals with the interaction of major services and infrastructure, LLD involves defining classes, interfaces, methods, data structures, and the relationships between them. These questions test your understanding of object-oriented programming (OOP) principles, design patterns, clean code practices, and how to build modular, flexible, and maintainable codebases. They often involve designing systems like a parking lot, a library, or an elevator controller, requiring you to think about the interactions between objects and the flow of data within a constrained scope. Successfully answering LLD questions demonstrates your ability to write production-ready code that is easy to understand, test, and extend.
Why Do Interviewers Ask Lld Questions?
Interviewers ask LLD questions to evaluate your practical coding and design skills beyond just writing algorithms. They want to see how you structure code, apply software engineering principles, and make design decisions that impact maintainability and scalability. These questions assess your ability to translate abstract requirements into concrete class structures and interactions. They also reveal your proficiency in using design patterns appropriately and your understanding of core concepts like encapsulation, inheritance, polymorphism, and composition. A strong performance in the LLD section indicates that you can contribute to a codebase effectively, collaborating with other engineers to build robust and well-structured software. It's a critical part of evaluating whether you can deliver high-quality, production-ready code.
Preview List
Why is Low-Level Design important?
How do you prepare for LLD interviews?
What are the key stages in solving an LLD problem in interviews?
What is the SOLID principle and why is it important?
How do you write clean and maintainable code in LLD?
How do you handle scalability in Low-Level Design?
How do you design a parking lot system?
How to design a library management system?
How do you design a hotel booking system?
How do you design a real-time collaborative document editing system?
How do you design an elevator control system?
What design patterns are most common in LLD?
What is the difference between High-Level Design (HLD) and Low-Level Design (LLD)?
How do you explain technical designs to non-technical audiences?
How do you stay current with industry developments related to system design?
How would you design a notification system?
How do you manage database design in LLD?
How do you approach designing an API in LLD?
How do you design a chat application?
What are some best practices for code review in LLD?
How do you handle error handling and logging in your designs?
How do you design a cache system?
How do you design a file storage system?
How do you design a rate limiter?
What is your approach to designing a system for URL shortening?
How would you design an online food ordering system?
How do you design a system to find the top K frequent words in a book?
How do you design an access control system?
How do you design a dealing machine software?
How do you ensure your LLD is extensible?
1. Why is Low-Level Design important?
Why you might get asked this:
Evaluates understanding of LLD's foundational role in the software development lifecycle, its connection to coding quality and project success.
How to answer:
Explain how LLD translates architecture into implementable details, ensuring the system is robust, efficient, maintainable, and scalable through careful component design.
Example answer:
LLD is crucial because it bridges the gap between high-level ideas and actual code. It defines the internal structure of components, detailing classes, data structures, and interactions, which is essential for building a maintainable, scalable, and efficient system.
2. How do you prepare for LLD interviews?
Why you might get asked this:
Assesses your systematic approach to learning and practicing design skills, showing initiative and structured preparation methods.
How to answer:
Describe a multi-faceted approach: studying principles (SOLID), design patterns, practicing common problems, drawing diagrams, and understanding requirement clarification.
Example answer:
Preparation involves understanding core OOP principles and design patterns. I practice common LLD problems like parking lots and library systems, focusing on requirement gathering, drawing class/sequence diagrams, and applying SOLID principles for maintainable code.
3. What are the key stages in solving an LLD problem in interviews?
Why you might get asked this:
Checks your problem-solving methodology for design tasks, ensuring you have a structured approach rather than jumping straight to coding.
How to answer:
Outline the steps: requirement clarification, identifying core objects/classes, defining relationships and interactions (diagrams), and considering edge cases/details.
Example answer:
The key stages are: thoroughly clarifying requirements, identifying core entities and their relationships (drawing class diagrams), defining interactions between components (sequence diagrams), and detailing methods and attributes, considering constraints.
4. What is the SOLID principle and why is it important?
Why you might get asked this:
A fundamental test of understanding core object-oriented design principles that promote maintainable and flexible code.
How to answer:
Define each letter (Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion) and explain how they contribute to robust, flexible, and maintainable codebases.
Example answer:
SOLID is an acronym for five design principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion. They are important because they guide developers in creating software that is more understandable, flexible, and maintainable.
5. How do you write clean and maintainable code in LLD?
Why you might get asked this:
Evaluates practical coding habits that lead to high-quality, long-lasting software, crucial for team environments.
How to answer:
Discuss adherence to principles like SOLID, using meaningful names, modular design, minimizing complexity, avoiding duplication, and writing tests.
Example answer:
Clean and maintainable code in LLD involves following SOLID principles, using clear and descriptive naming conventions, creating modular components with limited responsibilities, avoiding code duplication, and ensuring adequate test coverage.
6. How do you handle scalability in Low-Level Design?
Why you might get asked this:
Tests awareness of performance and growth considerations at the component level, bridging LLD with system-wide concerns.
How to answer:
Explain designing loosely coupled components, choosing appropriate data structures, considering caching, and designing for easy extension without major refactoring.
Example answer:
Scalability in LLD is handled by designing components that are loosely coupled, selecting efficient data structures and algorithms, planning for potential caching layers, and ensuring the design allows for future extensions and load increases gracefully.
7. How do you design a parking lot system?
Why you might get asked this:
A very common LLD problem testing your ability to identify entities, define relationships, and model state changes and interactions.
How to answer:
Identify key classes (ParkingLot, Floor, ParkingSpot, Vehicle, Ticket), define their attributes/methods, outline the process flow (entry, parking, exit, payment), and discuss spot allocation logic.
Example answer:
Design involves classes like ParkingLot, Floor, ParkingSpot (different types), Vehicle (Car, Bike, Truck), Ticket, and Payment. The system manages spots, allocates them upon entry, tracks vehicles, and processes payment upon exit, handling spot availability updates.
8. How to design a library management system?
Why you might get asked this:
Another classic LLD problem focusing on managing entities, states (checked out/in), relationships, and business logic.
How to answer:
Define classes like Book, Member, Librarian, Catalog, Loan, Fine. Explain features like searching, checking out/in, managing members, handling overdue books, and notifications.
Example answer:
Key classes include Book, Member, Librarian, Catalog, Loan, and Fine. The design manages book inventory, tracks member loans, handles borrowing and returning processes, calculates fines for overdue books, and facilitates searching the catalog.
9. How do you design a hotel booking system?
Why you might get asked this:
Tests handling resource availability, reservations, user management, and transactional aspects in a domain-specific context.
How to answer:
Outline classes like Hotel, Room, Booking, Customer, Payment. Explain search functionality, booking process, room availability management, handling cancellations, and preventing double bookings.
Example answer:
Design includes classes for Hotel, Room, Customer, Booking, Payment. It handles searching rooms by criteria, creating and managing bookings, updating room availability, processing payments, and using mechanisms to prevent issues like double bookings.
10. How do you design a real-time collaborative document editing system?
Why you might get asked this:
A complex problem assessing understanding of concurrent operations, state synchronization, and real-time updates.
How to answer:
Discuss handling concurrent edits using techniques like Operational Transformation (OT) or CRDTs, backend architecture for processing and broadcasting changes, and frontend for real-time updates via websockets.
Example answer:
This requires handling concurrent modifications. Techniques like Operational Transformation (OT) or CRDTs manage edits. The system needs a backend to process changes and broadcast them, typically via websockets, ensuring all clients see consistent, real-time updates.
11. How do you design an elevator control system?
Why you might get asked this:
Focuses on state machines, scheduling algorithms, handling requests, and managing multiple moving components.
How to answer:
Identify components like Elevator, Floor, ControlPanel, Scheduler. Explain how requests are received and prioritized using algorithms (e.g., SCAN, LOOK), and how the system manages elevator movement and state.
Example answer:
Design components like Elevator (with state like moving, idle), Floor, ControlPanel (buttons), and a central Scheduler. The scheduler receives requests and uses algorithms (like SCAN) to determine elevator movement sequences to optimize service efficiency.
12. What design patterns are most common in LLD?
Why you might get asked this:
Evaluates familiarity with standard solutions to recurring design problems, indicating ability to use proven techniques.
How to answer:
Mention common patterns like Singleton, Factory, Observer, Strategy, Adapter, Decorator, Command, and briefly explain their typical use cases in LLD context.
Example answer:
Common LLD patterns include Factory for object creation, Singleton for unique instances, Observer for notification systems, Strategy for interchangeable algorithms, Adapter for incompatible interfaces, and Decorator for adding behavior dynamically.
13. What is the difference between High-Level Design (HLD) and Low-Level Design (LLD)?
Why you might get asked this:
Ensures clear understanding of where LLD fits within the overall system design process.
How to answer:
Explain that HLD defines the overall system architecture, major components, and their interactions, while LLD details the internal structure, classes, methods, and relationships within those components.
Example answer:
HLD focuses on the system's macro view: major components, services, and their interactions. LLD is the micro view, detailing individual components' internal structures, classes, methods, data structures, and relationships.
14. How do you explain technical designs to non-technical audiences?
Why you might get asked this:
Tests communication skills, especially the ability to simplify complex concepts for different stakeholders.
How to answer:
Suggest using analogies, simple diagrams, focusing on outcomes and benefits, avoiding jargon, and tailoring the explanation to their understanding and concerns.
Example answer:
I simplify by using analogies, focusing on 'what it does' and 'why it matters' rather than 'how it works' in technical terms. I use simple visuals and relate the design back to their business goals or user experience, avoiding jargon.
15. How do you stay current with industry developments related to system design?
Why you might get asked this:
Checks commitment to continuous learning and awareness of modern design practices and technologies.
How to answer:
Mention reading tech blogs, books, attending conferences/webinars, participating in online communities, practicing problems, and following key industry figures or companies.
Example answer:
I stay current by reading blogs from major tech companies, following industry publications, participating in developer communities online, reading books on design patterns and system architecture, and practicing design problems regularly.
16. How would you design a notification system?
Why you might get asked this:
Evaluates design for asynchronous processing, multiple delivery channels, and managing subscribers.
How to answer:
Describe core components: NotificationService, Subscribers, MessageQueue. Explain supporting channels (email, SMS, push), using queues for decoupling and scalability, and managing user subscriptions.
Example answer:
Design involves a NotificationService that accepts requests. It would interact with specific channel adapters (EmailSender, SMSSender) and likely use a MessageQueue to handle notifications asynchronously, ensuring scalability and decoupling the sending process.
17. How do you manage database design in LLD?
Why you might get asked this:
Tests understanding of data persistence considerations and translating object models to relational or NoSQL schemas.
How to answer:
Explain translating class diagrams to schema, focusing on normalization, defining relationships (one-to-one, one-to-many, many-to-many), and considering indexing for performance.
Example answer:
Database design in LLD involves mapping classes to tables (or documents), defining relationships and cardinalities, ensuring appropriate normalization to reduce redundancy, and considering indexing strategies for query performance based on access patterns.
18. How do you approach designing an API in LLD?
Why you might get asked this:
Checks understanding of interface design, data modeling, and service interaction points.
How to answer:
Discuss defining resources, endpoints, HTTP methods (REST), request/response formats, data serialization, authentication, authorization, and clear error handling.
Example answer:
Approach by defining clear resources and endpoints. Use standard HTTP methods (GET, POST, PUT, DELETE) semantically. Define request/response data structures, consider authentication/authorization, pagination, filtering, and consistent error handling formats.
19. How do you design a chat application?
Why you might get asked this:
A common problem involving real-time communication, state management, and handling messages between users/groups.
How to answer:
Identify core classes: User, Message, ChatRoom. Discuss using websockets for real-time communication, message persistence, managing user presence (online/offline), and handling group chats.
Example answer:
Core classes are User, Message, ChatRoom. Real-time communication relies on websockets. Design includes components for sending/receiving messages, message persistence, handling user presence, and managing multiple chat rooms or direct messages.
20. What are some best practices for code review in LLD?
Why you might get asked this:
Evaluates understanding of collaborative development processes and quality assurance at the code level.
How to answer:
Mention checking for adherence to design principles, code readability, modularity, test coverage, potential performance bottlenecks, security issues, and clear documentation.
Example answer:
Best practices include checking for adherence to design principles (SOLID, patterns), code readability and clarity, appropriate modularization, testability, potential performance or security issues, and ensuring the code meets requirements effectively.
21. How do you handle error handling and logging in your designs?
Why you might get asked this:
Checks awareness of robustness, debugging, and monitoring needs in production systems.
How to answer:
Describe using exceptions or clear error codes for error handling, centralized logging with relevant context (timestamps, user, stack traces), and different logging levels.
Example answer:
I centralize error handling, often using exceptions for predictable errors and clear error codes. Logging is crucial: I include context like timestamps, user info, and stack traces for debugging and monitoring, using different log levels.
22. How do you design a cache system?
Why you might get asked this:
Evaluates understanding of performance optimization techniques by storing frequently accessed data.
How to answer:
Discuss choosing cache location (in-memory, distributed), selecting an eviction policy (LRU, LFU), and strategies for cache invalidation to maintain data consistency.
Example answer:
Designing a cache involves choosing between in-memory or distributed layers. I consider eviction policies like LRU (Least Recently Used) or LFU (Least Frequently Used) and plan strategies for invalidating cached data when the source data changes.
23. How do you design a file storage system?
Why you might get asked this:
Focuses on handling binary data, metadata, storage organization, and potentially distribution/replication.
How to answer:
Identify components for managing file metadata and the actual data storage. Discuss handling uploads/downloads, file organization, potential replication for durability, and access control.
Example answer:
Design involves components for managing file metadata (name, size, owner, permissions) and the storage layer for the binary data. This includes APIs for uploading/downloading, potentially chunking large files, and considering data replication for availability.
24. How do you design a rate limiter?
Why you might get asked this:
Evaluates understanding of controlling resource usage to prevent abuse and ensure system stability.
How to answer:
Explain using algorithms like Token Bucket or Leaky Bucket to enforce limits on the number of requests allowed within a specific time window, typically per user or API key.
Example answer:
A rate limiter uses algorithms like Token Bucket or Leaky Bucket. It tracks requests per user or key over time. The system allows requests as long as tokens are available (Token Bucket) or capacity exists (Leaky Bucket), rejecting excess requests.
25. What is your approach to designing a system for URL shortening?
Why you might get asked this:
A practical LLD problem involving mapping, hashing, collision handling, and redirection.
How to answer:
Describe mapping long URLs to unique short codes using hashing or a counter, handling potential collisions, storing the mapping, and implementing efficient redirection for the short URL.
Example answer:
I'd use a strategy to generate unique short codes (e.g., hashing or a sequence). This mapping is stored. When a short URL is accessed, the system looks up the long URL and redirects. Collision handling for hashing is key.
26. How would you design an online food ordering system?
Why you might get asked this:
Tests handling entities, relationships, workflows, and potentially concurrency in a business domain.
How to answer:
Identify classes: User, Restaurant, Menu, Item, Order, Payment, Delivery. Outline user flows (browsing, ordering, payment), order management for restaurants, and integrating delivery logistics.
Example answer:
Design includes classes for User, Restaurant, Menu, MenuItem, Order, Payment, DeliveryPartner. It manages user profiles, restaurant menus, order creation, status updates, payment processing, and coordinating with delivery services.
27. How do you design a system to find the top K frequent words in a book?
Why you might get asked this:
Evaluates data structure and algorithm selection for frequency counting and selection under constraints.
How to answer:
Explain using a hash map to count word frequencies during a single pass through the book, then using a min-heap (priority queue) of size K to efficiently track the top K words seen so far.
Example answer:
I would use a hash map to store word frequencies by iterating through the book. Then, use a min-heap of size K. Iterate through the hash map; for each word, add it to the heap. If the heap exceeds size K, remove the minimum element.
28. How do you design an access control system?
Why you might get asked this:
Focuses on managing permissions, roles, users, and implementing authorization logic.
How to answer:
Describe defining Users, Roles, and Permissions. Explain implementing Role-Based Access Control (RBAC) where permissions are assigned to roles, and users are assigned roles, with clear authentication and authorization components.
Example answer:
Design involves defining Users, Roles, and Permissions. A common approach is RBAC (Role-Based Access Control), where permissions are granted to roles, and users are assigned roles. The system authenticates users and authorizes actions based on their assigned roles and permissions.
29. How do you design a dealing machine software?
Why you might get asked this:
Tests understanding of state management, fairness, and algorithms in a game or simulation context.
How to answer:
Focus on components for managing the deck (shuffling, dealing), player hands, game state, and ensuring random/fair distribution of cards.
Example answer:
Design components for a Deck (with shuffle/deal methods), Player hands, and Game state. The core is ensuring the shuffle is genuinely random and dealing distributes cards correctly to players while managing the remaining deck state.
30. How do you ensure your LLD is extensible?
Why you might get asked this:
Evaluates forward-thinking in design to accommodate future changes and features without breaking existing code.
How to answer:
Discuss applying principles like the Open/Closed Principle, using interfaces or abstract classes to define contracts, and designing components to be loosely coupled and replaceable.
Example answer:
Extensibility is achieved by adhering to principles like the Open/Closed Principle (open for extension, closed for modification). Using interfaces and abstract classes creates clear contracts, and designing loosely coupled components allows replacing or adding new parts easily.
Other Tips to Prepare for a LLD Interview
Beyond practicing specific problems, immerse yourself in design principles and patterns. As Erich Gamma famously said about design patterns, "They provide a common vocabulary and understanding." Review the SOLID principles regularly. Draw lots of diagrams – class, sequence, and state diagrams are your best friends in visualizing your design. Think out loud during the interview process; explain your rationale behind design choices. Don't be afraid to ask clarifying questions about requirements. Consider using the Verve AI Interview Copilot at https://vervecopilot.com for personalized practice sessions that adapt to your responses and provide instant feedback on your LLD approach. The Verve AI Interview Copilot can help you refine your communication and problem-solving structure. Utilizing a tool like Verve AI Interview Copilot can simulate real interview pressure and highlight areas for improvement in explaining your designs. Practice makes perfect, and varied practice with resources like Verve AI Interview Copilot is most effective.
Frequently Asked Questions
Q1: What's the difference between LLD and OOD? A1: LLD is a broader concept, incorporating OOD (Object-Oriented Design) principles, patterns, and implementation details for specific components.
Q2: Should I code in LLD interviews? A2: Often, focus is on design diagrams and discussion. Pseudocode or small code snippets might be required to illustrate key parts.
Q3: How detailed should LLD be? A3: Detailed enough to show classes, methods, and relationships, but not necessarily implementation of every minor getter/setter.
Q4: Are diagrams mandatory? A4: Highly recommended. Diagrams clarify your thought process and design structure much better than just talking.
Q5: How do I handle ambiguities? A5: Ask clarifying questions to refine requirements. State assumptions you are making if clarification isn't possible.
Q6: What if I forget a design pattern? A6: Explain the concept or solution the pattern provides. Focus on solving the problem logically rather than naming the pattern.
