Low-Level Design Interview Questions

Regardless of the position for which you are applying in software engineering, you will probably run into low-level design interview questions. They evaluate not only your theoretical knowledge but also how you think and the extent to which you know how to apply the theory to practical, real-world scenarios.

In this guide, we have compiled the best low-level design interview questions for freshers and experienced engineers in their LLD preparation. Let’s take a look.

Table of Content:

• Introduction to Low-Level Design (LLD)
• Why Low-Level Design is Important in Interviews
• How to Prepare for LLD Interviews
• Low-Level Design Interview Questions for Freshers
• Low-Level Design Interview Questions For Experienced
• LLD Interview Questions on Design Principles and Best Practices
• Tips to Prepare for Low-Level Design Interview Questions
• Conclusion

Introduction to Low-Level Design (LLD)

• High-Level Design (HLD) can be described as the “big picture” design. It describes the modules, components, and how they come together and interact.
• Low-Level Design (LLD) can be described as the “details” or specifications of classes, methods, objects, data structures, and the relationships of these classes.

LLD interview questions reveal something a coding test cannot: how you think about complexity. Can you break a problem into parts? Can you think about edge cases? Will your design work in the real world?

Why Low-Level Design is Important in Interviews

LLD rounds go a step up from standard coding problems. Instead of offering you a problem to solve, LLD cases ask you to design a solution to the problem that will be practical, scalable, and maintainable. An ideal answer will show the interviewer you can:

• Effectively use object-oriented principles.
• Organize classes and methods logically and understandably.
• Consider scaling, performance, and maintainability from day one.

Strong LLD skills signal to employers you can do more than solve toy problems. For freshers, it is an opportunity to showcase your skills above and beyond other candidates, and for experienced developers, it is perhaps the next differentiating factor between good candidates and the one who will be hired. This is why companies rely on Low-Level Design interview questions to test candidates beyond simple coding problems.

How to Prepare for LLD Interviews

Here is the reality: you can’t cram patterns and memorize class diagrams. You need to think like a software architect.

• OOP is your foundation

You have to be comfortable with inheritance, encapsulation, abstraction, and polymorphism. While it might sound theoretical, it all comes down to these same concepts when deciding how to break down a system:

• Know the patterns that matter

Focus on Singleton, Factory, Observer, Strategy, and Decorator. Know when and why to use them. Don’t memorize definitions.

• Practice real problems

Take small, real-world systems (like a parking lot or a library) and work on why or how you would design them, drawing them out. Think about responsibilities and dependencies.

• Keep scalability in mind

Your design should accommodate growth without requiring major rewrites or introducing bottlenecks. If a new feature pops up, can your design absorb it without breaking?

• Draw diagrams

A simple class diagram, or a simple sequence diagram, is probably worth more than 10 lines of your code, because it is actually your thinking and intentionality that you are carrying out to provide future direction.

• Learn by example

Consider studying how someone or some group might have solved problems; compare and contrast would be best, and then see if there is something there to copy or adapt.

When practicing low-level Design interview questions, focus less on memorization and more on thinking like a software architect.

Low-Level Design Interview Questions for Freshers

Typically, freshers are assessed on small-scale but still practical problems. This section of the interview covers approximately entry-level LLD questions, such as how to design a parking lot, a library system, or a stack, to assess how well you can apply the fundamentals.

1. Design a Parking Lot System

A parking facility is more than just painted lines. You have vehicles, bicycles, and maybe large, heavy trucks. You need to know who has parked where, and things like tickets are your lifeline.

So, naturally, we split responsibilities:

• ParkingLot keeps the big picture. It knows how many spaces are free, who’s parked where.
• ParkingSpace? Simple. It’s either empty or occupied.
• Vehicle becomes a base class—Car, Bike, Truck extend it.
• Ticket ties a vehicle to a spot. No ticket, no parking. That’s the rule.

Operations? Just the essentials: parkVehicle(), leaveVehicle(), maybe isAvailable() to check for free spots.

2. Implement a URL Shortener

At first glance, URL shorteners may seem like a straightforward system. However, when you reach millions of records, you may run into collisions, and the application’s performance could begin to degrade.

Core Components:

• encode(longUrl): generates a short, unique code. Usually a hash or base62 conversion.
• decode(shortUrl): retrieves the original URL from storage.

You can use some sort of HashMap or dictionary to store the mappings.

3. Design a Library Management System

In a library management system, the scenarios you can consider are: members borrow books, return, maybe reserve, and fines for late return. When you model this as classes, you get:

• Library: which manages books and members, and tracks transactions.
• Book: with title, author, and availability.
• Member: individuals who can borrow, return, or reserve books;
• Transaction: for logging borrows and returns.

Functions like borrowBook() and returnBook() seem straightforward on the surface, but edge cases are where you really stand out. A reserved book, an overdue return, more than one copy. Interviewers appreciate it when you acknowledge reality’s complications.

4. Design a Shopping Cart System

Designing a shopping cart system is not just about the array of items you include, but rather how you will manage state and total amounts. These are the classes that we could implement.

• Item (the object that stores the name, price, and maybe the discounts)
• ShoppingCart (the object that manages the items, calculates totals, and checks out.)

There are several considerations above and beyond just that:

• Concurrent asks
• Promotions
• Tax
• Discounts

5. Implement a Stack Using Arrays and Linked List

A Stack is a data structure that follows the LIFO principle. Key operations include:

• push(x): insert element
• pop(): remove top element
• peek(): get top element without removing

1. Array-Based Stack

Output:

2. Linked List–Based Stack

Output:

Low-Level Design Interview Questions For Experienced

For more advanced positions, interviewers present complex, real-world problems. For example, you might be asked a question such as, “Design a web crawler,” or “Design a chat app.” These questions will test your ability with coding as well as further design considerations, such as, but not necessarily limited to, scalability, concurrency, and edge cases.

6. Which object-oriented principles do you think are most important in LLD?

While all object-oriented principles have their own place in the development process, the most important and commonly used OOP principles are Single Responsibility and Encapsulation.

• SRP ensures that each class has only one clear purpose.
• Encapsulation hides the internal details and logic and only exposes the necessary behaviour.

7. Explain the YAGNI and DRY principles.

8. What are UML diagrams and how are they classified?

UML diagrams (Unified Modeling Language) are visual diagrams of software structure and behavior. They provide a high-level description of how the code works and how code (or systems) can work together.

• Structural diagrams focus on how the system is organized. Class, Component, and Deployment are examples of structural diagrams.
• Behavioral diagrams focus on the dynamic aspects, meaning how the system behaves at runtime. Sequence, Activity, State, and Use

In LLD, examples of frequently used diagrams include Class Diagrams and Sequence Diagrams.

9. How are design patterns classified?

Creational Patterns

• Purpose: Manage how objects are created while hiding the creation details (making the system less dependent on the specifics of how objects are instantiated).
• Examples: Factory Method, Abstract Factory, Builder, Prototype, Singleton.
• When to use: Helpful when object creation is complicated or we want to control a process, such as managing a pool of database connections or a pool of objects.

Structural Patterns

• Purpose: Specify how classes and objects are combined to create larger, more flexible structures.
• Examples: Adapter, Decorator, Composite, Facade, Proxy, Bridge, Flyweight,
• When to use: Great when relationships are complicated and you are looking for a way to add new functionality without changing existing code, or improve performance.

Behavioral Patterns

• Purpose: Define how the objects interact and share responsibilities.
• Examples: Strategy, Observer, Command, Template Method, Iterator, Mediator, State, Chain of Responsibility, Visitor, Memento, Interpreter.
• When to use: Useful when you want to manage workflows, organize collaboration between objects or manage some behavior change at run-time.

10. What are some common criticisms against design patterns?

Although design patterns provide many benefits, they are not without pitfalls.

• Overusing patterns: When you apply a design pattern to a design that has no need for a pattern, the code quickly becomes unnecessarily complicated.
• Outdated language features: Many of the design patterns were created to deal with restrictions in older programming languages. New programming languages (Java, C#, Python, etc.) have a lot of features baked into the language that mean certain design patterns are no longer needed.
• Poor design camouflage: In some cases, using a design pattern is like putting a band-aid on a design flaw rather than fixing the actual flaw.

11. How to prevent the Singleton pattern from reflection, serialization, and cloning?

In Java, an enum singleton is preferred, as it is immune to reflection and serialization. Alternatively, you can protect the constructor, use readResolve to return the singleton, and/or override clone() to prevent cloning.

12. Design a web crawler system or search engine.

Some of the basic components of a web crawler or search engine are:

• URL Frontier: A FIFO queue that decides which URLs the crawler will visit next; it may implement politeness policies that prevent the crawler from requesting too many pages from one server at any one time.
• Fetchers: The mechanism that downloads web pages from the Internet.
• Parsers: The mechanism that extracts content, links, and metadata from downloaded web pages.
• Deduplication: Prevents the crawler from revisiting the same web pages using techniques like Bloom filters.
• Indexer: Build an inverted index mapping keywords to documents for fast search.
• Storage: The element responsible for efficiently storing downloaded web pages, parsed data, and an inverted index.
• Query Engine: Accepts user queries and finds relevant results.
• Ranking: Offers a way to order results and filter suboptimal or irrelevant results based on relevancy ranking algorithms like PageRank or TF-IDF.

13. Design a traffic control system.

A traffic control system typically includes:

• An intersection controller, which isolates individual intersections where the controller is in charge of signals/inputs to the area.
• Traffic signals, indicating stop/go/wait, which is frequently modeled as a state machine.
• Sensors that detect when vehicles or traffic density, or pedestrian movement, are present to alter the signal timing if need be.
• Scheduler/coordinator, which is either centralized or distributed, either having one controller coordinate multiple intersections or communicate in coordination from intersection to the next, assuring traffic flows through the corridor consistently.

14. Design a Snake and Ladder game using OOP principles.

We can design a simple Snakes and Ladders game using a few simple classes like so:

• Board: Contains a collection of cells, where each cell has either a snake or a ladder that connects to another cell.
• Cell: A representation of a single place on the board; it stores jump (snake/ladder) information.
• Player: Keeps track of the player’s location on the board.
• Die: Simulates rolling a die, returning a value between 1 and 6.
• GameController: Coordinates the game, rolls the die, moves players, implements snake/ladder jumps, manages player turns, and checks for a winner.

OOP Principles in Action:

• Encapsulation: Each class manages its own state (e.g., Player knows its position; Board knows cell mappings).
• Single Responsibility: The GameController manages the game logic; the Board manages the cells; the Player manages the position.
• Testability: The classes are small and separated, making it easy to write unit tests on them.

15. Design data structures and algorithms for an in-memory file system.

Consider representing the file system as a tree, with directories as internal nodes and files as leaves. Use a HashMap from path → node to enable fast lookups. Store the file data in blocks or in contiguous byte arrays and use locks when dealing with concurrent access. To efficiently handle modifications for versioning or snapshots, implement copy-on-write so only the modified part is copied.

16. Design a global file-sharing and storage app.

A file storage and sharing system has several core components:

• Client: Uploads/downloads files.
• API Gateway: Accepts requests from users and routes them to the appropriate service.
• Metadata Service: Manages the details associated with a file, like ownership and location.
• Chunk Store / Object Storage: Stores file data in chunks.
• Index: Provides a mapping from a file to its individual chunks.
• Auth & ACL: Enforces access control.
• Replication Manager: Ensures availability and consistency.

Design Techniques:

• Chunking & Deduplication: Store repeated content only once.
• Content-addressable Storage: Easy identification and retrieval.
• Geo-replication & CDNs: Serve files globally with low latency.
• Eventual Consistency for Metadata: Scales efficiently while keeping access safe.

17. Design a movie ticket booking system.

Essential entities include Theater, Show, Seat, Booking, User, and Payment. Users select seats for a show, and a Booking entity is created upon payment confirmation.

Concurrency Handling:

• Seat-level locks or optimistic locking mechanisms help prevent double booking.
• Add timeouts for reservations in order to quickly expire unconfirmed reservations.

Scalability:

• Sharding inventory by show will allow you to more fairly distribute the loading function.
• Use CQRS: allowing for fast reads for seat availability and reliable writes for the number of bookings and number of users.
• Event sourcing can be utilized to allow users to replay and recover bookings.

18. Design an e-commerce website using microservices for transactions.

Break the system into microservices such as Catalog, Cart, Order, Payment, Inventory, Shipping, and Auth. Each service owns its own data and exposes APIs.

Key Design Considerations:

• Transaction Coordination: We will utilize the SAGA pattern when we want to coordinate onboarding and identify compensating actions as part of the workflow (e.g. not only refund a payment if inventory can’t process).
• Consistency: Leverage event-driven messaging for eventual consistency.
• Reliability: Use idempotency keys to prevent duplicate processing of orders.
• Scalability & Routing: An API Gateway handles requests, and service discovery would allow for scaling and resiliency.

This design ensures the system remains scalable, fault-tolerant, and consistent while supporting complex e-commerce workflows.

19. Design a chess game.

You can design a chess game by breaking it down into a few core classes:

• Board: Represents the 8×8 grid and the position of the pieces.
• Piece(abstract): Base for King, Queen, Rook, Bishop, Knight, Pawn, with each defining its own movement rules.
• Move: From-square (starting position) to-square (ending position) and actions such as capture, castling, promotion.
• GameController: manages turns, validates moves, and checks for check/checkmate.

Design Ideas:

• Pieces generate possible moves, and a moveValidator ensures that the move is legal.
• Keep a move history or an immutable record to allow undo.
• Handle special rules like castling, en passant, and pawn promotion in the rules engine.

OOP Principle: 

• Encapsulation: Each piece knows how to move.
• Polymorphism: different piece types override move generation.
• Single Responsibility: board stores state, controller enforces rules.

20. Design a feed posting system for a social media app.

The feed is at the core of the user experience when it comes to social media applications. At a high-level view, we need to manage the actual posting, mode of distribution, and display of content in a streamlined manner.

Components:

• Post Service: Responsible for storing posts for users.
• Follow Graph: Responsible for keeping track of who follows whom.
• Feed Assembler: Responsible for building the user feed from the relevant posts.
• Ranking Engine: Responsible for ordering posts.

Patterns:

• Fan-out on write(push): When a user posts something, we immediately copy it into their followers’ feeds.
• Fan-out on read(pull): Followers fetch posts at read-time. Used for users with millions of followers to avoid massive writes.
• Hybrid: mix both strategies depending on scale.

Additional considerations:

• We can put background workers to deal with heavy fan-out (asynchronous).
• Cached timelines for quick delivery.
• Idempotency (no duplicate posts appear in any given user’s feed).

This design balances scalability (handles millions of users), low latency (fast feed loading), and flexibility (different user types).

21. Design a mobile chat application.

When it comes to designing a mobile chat application, the focus should be on real-time delivery, reliability, and offline support.

Key Classes:

• User: Represents a specific individual, stores personal profile and contact information.
• Message: Content, sender, receiver, timestamp, status (sent, delivered, read).
• ChatSession: Facilitates and manages the conversation between two or more users. Will contain a list of messages.
• ConnectionManager: Maintains persistent WebSocket/TCP connections between message endpoints.
• MessageService: Sends, receives, retries, and maintains order with acknowledgment messages.
• NotificationService: Handles push notifications for offline users.
• StorageService: Persists chat history.

Relationships

• A User can be a part of multiple chat sessions.
• A ChatSession contains many messages.
• MessageService interacts with ConnectionManager and StorageService.

LLD Principles to Highlight

• Encapsulation: Each class is responsible for and manages its own state.
• Single Responsibility: MessageService handles delivery while StorageService handles persistence.
• Extensibility: Easy to add group chat, media attachments, or encryption later.

22. Design a food delivery system.

The objective of creating a food delivery system is to cover the aspects of managing order placement, restaurant preparation, delivery partner assignment, and payments in an efficient manner.

Key Classes:

• User / Customer: keeps track of their profile, addresses, order history, and payment information.
• Restaurant: Restaurant contains Menu, MenuItems and availability on offer.
• Order: Order has the customer, restaurant, Wait Time, items, and status (PLACED, CONFIRMED, PICKED_UP, DELIVERED, CANCELED, FAILED).
• DeliveryPartner: DeliveryPartner has the location and availability.
• DispatchService: DispatchService assigns orders to the delivery partners by using proximity and availability.
• Payment: Payment manages the operations to process payments, refunds, and payment status.
• OrderLifecycleManager: OrderLifecycleManager manages the events of order placement, order confirmation, preparation of the order, pickup, and delivery of the order.

Relationships:

• A Customer can have multiple Orders.
• Restaurant can have multiple MenuItems.
• An Order can reference a DeliveryPartner once it is assigned.
• A DispatchService may want to use Order and DeliveryPartner.

LLD Considerations

• Atomic operations: avoid double-assignment of delivery partners.
• Event-driven order lifecycle: allows asynchronous updates for preparation, pickup, and delivery.
• Extensibility: easy to add features like ratings, promotions, or multi-restaurant orders.

LLD Interview Questions on Design Principles and Best Practices

In addition to systems and classes, employers want to know if you write code that is clean, maintainable, and can last the test of time. In this section, we will investigate SOLID principles, coding practices, and how to address design problems in a step-by-step manner.

23. Tell me about some code-writing best practices for LLD.

• Favor small, cohesive classes with a single responsibility.
• Give classes, methods, and variables meaningful names.
• Encapsulate internal state and expose only behavior that needs to be exposed.
• Use proper error handling and validation.
• Keep dependencies minimal to reduce coupling.
• Write code with testability in mind, small units, well-defined interfaces, and predictable behavior.
• Comment only when necessary, and comment to clarify intent rather than code that does not need some explanation.

24. What do you know about the SOLID principles?

SOLID is a set of five principles for object-oriented design:

• S: Single Responsibility – Each class should only have one purpose.
• O: Open/closed – code should be open for extension but closed for modification.
• L: Liskov Substitution – subclasses should replace base classes without breaking behavior.
• I: Interface Segregation – prefer many small, specific interfaces over one large, general-purpose interface.
• D: Dependency Inversion – depend on abstractions, not concrete implementations.

Applying SOLID improves flexibility, maintainability, and makes code easier to understand and test.

25. Walk me through your process of solving a standard LLD problem.

Understand the requirements: Make sure that you have a solid understanding of the input, output, and any constraints before you start designing.

• Identify core entities: Determine the main classes and identify what each will be responsible for. Also, try to form some notion of how they relate to each other.
• Define interfaces and methods: Decide what methods each class should provide and how other parts of the system will use these methods.
• Apply design principles: Use best practices such as the Single Responsibility Principle, encapsulation, and composition. Use design patterns when it makes sense.
• Sketch diagrams: Draw simple class or sequence diagrams to visualize how the system flows and how objects interact.
• Consider scalability and maintainability: Design your system so it can grow, stay modular, and be extended easily.
• Validate edge cases and errors: Think about limits, concurrency issues, and failure scenarios.
• Refactor and review: Clean up code that does not need to be there, simplify dependencies, and make sure your design is easy to test.

26. What do you think is the goal of efficient low-level design?

The goal of LLD is to translate requirements into a structured, modular design. It also ensures that classes are cohesive, interactions are clear, and changes can be made without cascading failures.

Tips to Prepare for Low-Level Design Interview Questions

Design interviews can feel overwhelming, but a structured approach makes all the difference. Let’s have a look at some practical LLD preparation tips, from breaking down systems to choosing data structures and thinking about edge cases.

1. Understand the Requirements

Prior to diving into class diagrams or patterns, take a moment to clarify the problem. Oftentimes, the interviewer purposefully leaves the requirements open. Ask questions, confirm assumptions, and understand what the system is supposed to do before designing anything.

2. Break Down the System

Large problems can make you feel dazed or lost. A good approach is to simply break them down into smaller and logically distinct components. For example, when designing a parking lot, consider things like parking spaces, tickets, and vehicles as separate building blocks. This shows a stepwise approach to solving the problem and keeps your design modular.

3. Focus on Scalability and Efficiency

Designing a system that just works for one or two users is not complex enough. You should be thinking of how your system could grow. Discuss what your system could look like when user activity increases or provide some ideas on how you could optimize certain operations (search, caching, database lookups, etc). This shows you are not only solving the problem, but helping to think about future use.

4. Use Appropriate Data Structures

Choosing the right data structure often makes the difference between a clumsy design and an elegant one. For example, a hash map may work well for quick lookups, while a queue might be better for scheduling tasks. Make sure to justify your choices during the discussion.

5. Consider Edge Cases and Error Handling

Interviewers want to see how you think about failure. What happens if the system gets invalid input? How do you prevent double booking in a ticketing system? By addressing edge cases and error handling, you show that your design is robust and production-ready.

Conclusion

Mastering Low-Level Design Interview Questions is less about memorizing definitions and more about showing how you think as an engineer. The difference from interviewing a recent graduate with less experience is their ability to design clean, modular, and scalable solutions to common problems. By working through real-world problems, making use of OOP principles, and thinking about scale, candidates will not only excel in these situations but also become better engineers overall.