Difference Between Low-Level Design and System Design Interview Questions

System design interviews are a crucial part of technical hiring for many software roles. These software design interviews typically come in two flavors: High-Level Design (HLD) and Low-Level Design (LLD).

High-level questions (often just called system design questions) focus on the overall architecture of a system, while low-level design questions dive into detailed class and component design.

It’s important to recognize that LLD is not the same as system design – each addresses different scopes and challenges in designing software.

In this post, we’ll discuss the difference between low-level design and system design interview questions, why each is important, provide examples of common questions, and offer tips on how to prepare for both.

What is System Design?

System Design (often referred to as high-level design) is about designing the overall architecture of a software system.

In these questions, you’re asked to think big-picture about how different parts of a system fit together.

This includes considerations of how to split the system into components or services, how those components communicate, and how to ensure the system meets certain non-functional requirements (like scalability or reliability). System design questions test your ability to create a blueprint for a large application or distributed system.

Key aspects of system design include:

Architecture & Components: Defining the major pieces of the system (services, databases, load balancers, etc.) and how they interact. For example, deciding if an application should be monolithic or microservices-based.
Scalability: Scalability is about designing the system to handle increased load. This might involve techniques like load balancing, database sharding, or using distributed systems to scale out.
Data Storage: Choosing the right databases or storage solutions. You might discuss SQL vs NoSQL databases, caching layers (like Redis), and data partitioning strategies.
Performance & Reliability: Ensuring the system is fast and reliable. Consider caching strategies, content delivery networks (CDN) for static content, and redundancy for high availability.
Security & APIs: Designing secure interfaces. Define how different services or clients interact through APIs, and consider authentication, authorization, and data encryption for sensitive data.
Microservices & Integration: If using a microservices architecture, explain how the services will communicate (REST, gRPC, message queues) and manage things like service discovery and failure handling.

In a system design interview, you might be asked to sketch the overall system diagram on a whiteboard or explain the flow of data between components.

The focus is on system design concepts at a high level – you won’t typically write code, but you should discuss how the system meets the requirements (e.g. handling X million users, low latency reads, etc.).

What is Low-Level Design (LLD)?

Low-Level Design (LLD), sometimes called object-oriented design (OOD) or low-level system design (LLSD), focuses on the detailed design and implementation of individual components or modules.

Instead of the big-picture architecture, LLD is about how to design the classes, methods, and interactions within a specific part of the system. In an interview context, low-level design questions test your ability to translate requirements into a logical, maintainable code structure.

When working on LLD, you’ll consider software design principles and patterns to create a blueprint at the code level. Key aspects of LLD include:

Class Design & Relationships: Identify the classes needed and their responsibilities. Define class attributes and methods, and determine relationships like inheritance or composition. (For example, if designing a chess game, what classes do you need? Perhaps Game, Board, Piece, etc. with appropriate relationships.)
Object-Oriented Principles: Apply OOP principles such as encapsulation, abstraction, inheritance, and polymorphism. A good LLD will follow SOLID principles to ensure the design is modular and extensible.
Data Structures & Algorithms: Decide on data structures for each component’s functionality and any algorithms needed. For instance, if you’re designing a parking lot system, you might need an efficient way to track available parking spots (perhaps a max-heap or queue).
Design Patterns: Use relevant design patterns to solve common problems. Patterns like Singleton, Factory, Observer, Strategy, etc., often come up in LLD to handle specific scenarios. Explaining why a particular pattern is suitable demonstrates deeper design insight.
Module Interaction: Define how different classes or modules interact via interfaces or methods. Clarify the flow of information between objects (sometimes with sequence diagrams or simple pseudocode for method calls).
Error Handling & Edge Cases: Plan for exceptions and edge cases at the code level. For example, how does your design handle invalid inputs or system errors? Detailing error-handling mechanisms and considering edge cases (like empty states or maximum capacity reached) is part of a solid low-level design.
Scalability of Code: While system scalability is more of an HLD concern, in LLD you consider code scalability and maintainability. This means designing in a way that adding new features or modifying components won’t require massive rewrites (achieved by clean interfaces and abstraction).

In essence, low-level system design is about zooming in on the internals of a module. You might draw a class diagram or write out pseudocode to illustrate how each piece functions.

The interviewer wants to see that you can create a clean, logical software design that could be handed to a team to implement.

For a deeper understanding of what goes into LLD, you might refer to guides on what low-level system design entails in practical scenarios.

Key Differences Between System Design and Low-Level Design

Now that we’ve defined both, let’s compare system design vs. low-level design. Though both are about design, they operate at different levels of abstraction. Here are the key differences:

Scope: System design (HLD) covers the entire system’s architecture – it’s a broad overview of all major components and their interactions. Low-level design (LLD) zooms into individual parts of the system – designing a specific module or component in detail. In short, HLD is macro, LLD is micro.
Abstraction Level: System design stays high-level and abstract. It focuses on what components exist and how data flows between them. LLD is very detailed and concrete, focusing on how each component actually works and is implemented in code.
Focus & Concerns: System design questions emphasize scalability, reliability, and overall integration of components. You’ll discuss things like network communication, data partitioning, and choice of technology (e.g. using a cache, load balancer, message queue, etc.). LLD questions emphasize software design principles and internal logic – you discuss class responsibilities, method logic, choice of data structures, and design pattern usage.
Output/Artifacts: In system design, the output is usually an architecture diagram or outline of the system’s components and how they interact (plus explanations of decisions made). In low-level design, the output is often a class diagram, pseudocode, or detailed description of how to implement a part of the system. For example, HLD might produce a diagram of a microservices ecosystem, whereas LLD might produce a UML class diagram for one service.
Interview Style: A system design interview feels like an open-ended discussion about a large system. The interviewer expects you to drive the discussion, clarify requirements (throughput, data size, etc.), and come up with a robust architecture. A low-level design interview, on the other hand, feels closer to a collaborative problem-solving session about how to design code for a given feature or scenario. You might start by clarifying requirements and then listing classes and their APIs.
When Each Is Asked: System design (HLD) questions are more common for senior developers or system architect roles (and often for candidates with a few years of experience) because they test large-scale design thinking. LLD questions (sometimes just called object-oriented design questions) can be asked even of mid-level or entry-level candidates, especially to test understanding of OOP and design patterns. Some companies use LLD questions in place of or in addition to coding algorithm questions to assess design skills on a smaller scale.

In summary, system design vs LLD can be thought of as architecture vs implementation details. Both are important: HLD ensures the system’s pieces all work together and meet high-level goals, while LLD ensures each piece is well-crafted and fit for its purpose.

(If you’re curious, Design Gurus addresses this distinction in more detail in their Q&A “Is LLD the same as system design?”.)

Common System Design Interview Questions

High-level system design questions can vary widely, but they usually involve designing some well-known large-scale application or service. The key is that these problems are broad and have many components. Here are some typical system design interview questions you might encounter:

Design a URL Shortener (e.g. bit.ly) – How would you design a service that takes a long URL and returns a short URL, which redirects to the original? Discuss generating unique keys, database to store mappings, handling billions of redirects, and analytics. Check Solution
Design a Social Media Feed – For example, design the Twitter timeline or Facebook news feed. Consider how to store user posts, follow relationships, and efficiently retrieve and rank posts for a user’s feed. Discuss databases, caching, and possibly a fan-out service for feeds. [Check Solution] (https://www.designgurus.io/answers/detail/how-to-design-a-twitter-like-application)
Design a Chat/Messaging Application – For instance, a system like WhatsApp or Slack. Think about real-time message delivery, long polling or WebSocket connections, message storage, group chats, and scalability to millions of users. How do you ensure messages are delivered reliably and in order? Check Solution
Design a Ride-Sharing Service (e.g. Uber) – Cover how to match drivers with riders, real-time vehicle location updates, ride pricing, and system components like a dispatch system, location services, and database for rides. Consider scalability for a global user base and low-latency updates on maps. Check Solution
Design an E-Commerce Website – Discuss the architecture of an online shopping platform like Amazon. Consider services like product catalog, user authentication, shopping cart, order processing, payment, inventory management, and recommenders. How do these services interact, and how to handle high traffic during sales. Check Solution
Design a Video Streaming Service (e.g. YouTube/Netflix) – Outline how to handle storing video files, streaming content to users, handling a content delivery network (CDN) for smooth playback, user upload, search and recommendation systems, and managing a large user base concurrently streaming. Check Solution
Design a File Storage/Sharing Service (e.g. Dropbox or Google Drive) – Explain how users upload/download files, how files are stored across distributed storage, syncing across devices, handling conflicting edits, and ensuring reliability and security of data. Check Solution

Each of these questions requires breaking the system into components and discussing choices. For instance, if asked to design a URL shortener, you would talk about an API server, a database for URL mappings (SQL or NoSQL?), how to generate unique IDs (hashing or base62 encoding an auto-increment id), caching popular URLs, and so on. Always clarify requirements first – e.g., how many URLs to handle, or how many active users – as that influences your design decisions (like whether you need multiple database shards, etc.). System design questions are open-ended, and there’s often no single “correct” answer. The interviewer is interested in your thought process, how you reason about trade-offs, and that you cover key areas (scalability, fault tolerance, consistency, etc.) relevant to the problem.

Common Low-Level Design Interview Questions

Low-level design interview questions typically ask you to design the classes and interactions for a specific application or feature. These are often practical scenarios or everyday systems, and they test your ability to apply OOP and design patterns. Some popular LLD interview questions include:

Design a Parking Lot System – Model a parking lot with different vehicle types (bike, car, truck) and parking spot sizes. You’d need classes like ParkingLot, ParkingSpot, Vehicle, etc., and logic for assigning spots, tracking free spots, and handling fees or tickets. Check Solution
Design an ATM Machine – Outline the classes to simulate an ATM. Think in terms of ATM as a central class, CardReader, CashDispenser, Screen, and classes for Account or Transaction. How does the ATM flow for withdrawals, deposits, balance inquiries, and what happens if there’s an error (like insufficient funds)? Check Solution
Design a Library Management System – Create a class design for managing library books, users, and check-outs. For example, classes might include Library, Book, Member, LoanRecord, etc., with methods to search books, check out or return a book, and manage fines for late returns. (This is a classic example; see a detailed Library Management System LLD example for how high-level requirements translate into classes and methods.)
Design an Elevator (Lift) System – Define classes to operate elevators in a building. You might have Elevator, ElevatorController, and maybe Request objects. Consider how to handle multiple elevators, scheduling which elevator responds to a call, and safety conditions. This tests handling of real-time events and state management in design. [Check Solution](https://www.designgurus.io/course-play/grokking-the-object-oriented-design-interview/doc/design-an-atm\)
Design a Restaurant Ordering System – Think of a system for a restaurant (or a Coffee shop). You would have classes like Order, MenuItem, Table, Waiter, etc. The interactions might include placing orders, serving them, generating bills. It checks how you handle state of orders and possibly concurrent updates if multiple waiters use the system. Check Solution
Design a Chess Game – Lay out the classes for a two-player chess game. Likely classes: Game, Board, Piece (with subclasses like King, Queen, etc.), Player, and perhaps a Move or GameRule class. The design needs to handle moves, check/checkmate detection, and turn management. It’s a test of using inheritance (for piece types) and managing game state. Check Solution
Design a Logging Library – This is more of a component design: how would you design a logger utility that different parts of an application can use? Consider a Logger class with methods to log messages at various levels (info, debug, error), and maybe strategies to output logs to console, file, or remote server. This question checks understanding of design patterns (like Singleton for a global logger instance, Strategy for different logging outputs, etc.).

With any LLD question, remember to clarify the requirements first. Are we focusing just on the core classes and their relationships? Do we need to consider concurrency (multiple people using the library or multiple threads logging simultaneously)? Once you have the requirements, list out the main objects and walk through how they interact for the key use-cases. It’s often helpful to discuss a real scenario: e.g., “In the parking lot, when a car enters, how do we find a spot? We might have a method in ParkingLot like findSpot(Vehicle) that iterates through available spots…”. By walking through usage, you can naturally derive the methods and relationships needed.

How to Prepare for System Design Interviews

Preparing for system design interviews requires building a solid understanding of system architecture concepts and lots of practice thinking through design problems. Here are some strategies to get ready for these system design concepts questions:

Brush Up on Basics: Ensure you have a good grasp of fundamental building blocks of large systems. This includes understanding how web applications work (client-server model), basics of networking (HTTP, TCP/IP), databases (SQL vs NoSQL differences), and common system components like caches, message queues, load balancers, etc. Knowing what tools are available (and their pros/cons) is half the battle.
Study Scalable Architecture Patterns: Learn about design patterns at the system level, such as microservices vs monolith, peer-to-peer vs centralized systems, MVC architecture for web apps, etc. Understand concepts like sharding, replication, caching, concurrency, CAP theorem (consistency, availability, partition tolerance), and CDNs. Resources like the System Design Primer (an open-source guide) or high-level design courses can be very helpful.
Learn from Real World Systems: Read case studies or blog posts about how big companies have designed their systems. For example, how Twitter handles the fan-out of tweets to followers, or how Netflix designs for high availability. This gives you concrete examples of trade-offs and solutions used in industry. You don’t need to memorize them, but understanding them will give you ideas and context in an interview.
Practice Common Scenarios: Take the list of common system design interview questions (like the ones above) and practice structuring a design for each. Write out your assumptions and design on paper or a whiteboard. Focus on clearly articulating: what the components would be, how they communicate, where data is stored, and how you’d handle growth or failures. The more problems you practice, the more patterns you’ll recognize (e.g., many systems need a similar approach to handling a burst in traffic or ensuring eventual consistency).
Think in Terms of Trade-offs: There is no one perfect design – every choice has trade-offs (for example, SQL vs NoSQL, or pushing work to background with queues vs doing it synchronously). When preparing, get in the habit of considering the pros and cons of different approaches. This will help you discuss alternatives during the interview, showing that you understand the impact of your design decisions.
Mock Interviews: Simulate system design interviews with a friend or use online platforms. Practicing aloud, explaining your thought process in real-time, is crucial. It helps you get comfortable with the open-ended nature of these questions. You can also get feedback on whether your explanation is clear. Some platforms or courses provide mock design scenarios you can walk through.
Use Guiding Frameworks: During practice, use a structured approach: clarify requirements first (especially constraints like scale), then outline the core components, then dive deeper into each component's design, and finally discuss how the system handles key challenges (scale, failures, security). This framework ensures you cover all important aspects systematically in an interview.

Recommended Resources: Consider enrolling in a proven system design course to boost your preparation. For example, Design Gurus’ Grokking the System Design Interview is a popular course that covers system design fundamentals and example questions in depth. Such courses walk through real-world design problems step by step, which can accelerate your learning. In addition, high-quality tutorials or Q&A articles can fill gaps in your knowledge; for instance, Design Gurus offers an explanation of HLD vs LLD preparation and a catalog of Q&A that can be very insightful. Hands-on practice combined with learning from experts is an effective way to master system design.

Finally, remember that in the actual interview you should communicate constantly. Talk through your thinking, ask for clarification on ambiguous points, and don't be afraid to clarify assumptions (like expected traffic or data size). Interviewers appreciate a collaborative approach. Demonstrating a clear thought process and covering the bases (even if you don't have time to detail every aspect) is often more important than the exact specifics of your design.

How to Prepare for Low-Level Design Interviews

Preparing for low-level design (LLD) or object-oriented design interviews requires a slightly different focus – here you need to sharpen your software design and coding design skills. Here are some tips to help you prepare for LLD questions:

Master OOP Fundamentals: Make sure you have a strong handle on object-oriented programming principles. Review encapsulation, inheritance, polymorphism, and abstraction thoroughly. Understand how these principles help in designing flexible, extensible classes. Also, revisit the SOLID principles (Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion) as they are the guiding ideals for good class design. A clear understanding of these will allow you to argue why your design is good or how you might improve a design.
Learn Common Design Patterns: Familiarize yourself with the classic design patterns (from the “Gang of Four” book or other sources) like Singleton, Factory, Strategy, Observer, Command, etc. Know not just the definitions, but scenarios where each pattern is applicable. In an interview, using a design pattern appropriately (and mentioning why, e.g. “I’ll use a Factory here to create different types of vehicles without exposing the creation logic”) can show maturity in design thinking. It’s equally important to know when not to force a pattern – simplicity is key.
Practice with Real-world Scenarios: Take everyday systems or well-known problems and practice designing them in an object-oriented way. For instance, pick a scenario like a vending machine, a coffee shop ordering system, a file system, or even a simple game. Try to outline the classes, their attributes, and methods, and how objects would interact. Write these down or draw UML class diagrams. Practicing varied scenarios will expose you to different kinds of design challenges. You can follow a structured approach to ensure you cover all details – Design Gurus has a great step-by-step guide on how to write a good LLD that emphasizes understanding requirements, defining classes, and more.
Draw UML Diagrams: While you won’t always be asked to produce UML diagrams in an interview, being able to sketch a quick class diagram or sequence diagram is very helpful. It forces you to be explicit about relationships and interactions. Practice drawing class diagrams for your solutions, showing inheritance hierarchies or associations. Sequence diagrams can help you illustrate the flow of a particular operation (like “User checks out a book” in the library system – what calls what in sequence).
Implement Some Designs in Code: Although in the interview you likely won’t write full code, when preparing it helps to implement a few of your practice designs in your favorite language. This will reveal if your design has any flaws or complexities. For example, try coding the core logic of your parking lot or chess design. This exercise improves your ability to go from diagram to code, and sometimes interviewers do appreciate pseudocode for critical sections. Being comfortable with coding also means you can better judge if a design is easy to implement or overly complicated.
Review Sample LLD Questions and Solutions: Check out resources or discussion forums for common LLD questions. Reading how others approach a design can give you new ideas or highlight alternative class structures. For instance, see how different people design an elevator or a deck of cards for a card game. There are often multiple valid designs – comparing them can teach you the nuances of design choices. You might find Q&A style explanations (like Design Gurus’ post on preparing for low-level system design) that provide structured guidance.
Focus on Communication: In an interview, explaining your design is as important as having a good design. Practice speaking through your thought process: start from requirements, then mention the main classes, why you chose them, and how they interact. It might help to practice with a peer – have them listen and ask questions. This will train you to clearly justify your design decisions and handle any changes on the fly (sometimes an interviewer might tweak a requirement to see how you adapt your design).
Time Management in Design: When practicing, simulate the time pressure of an interview (LLD interviews might be around 30-45 minutes). Get comfortable with not diving too deep into less important details if time is short. Prioritize the core structure first; you can mention that other details exist if you had more time. This is a skill – knowing how much depth to go into for an interview response – and you get better at it with practice.
Additional Resources: There are courses and books focusing on object-oriented design and system design at the low level. While not as numerous as system architecture resources, you might consider sections in Cracking the Coding Interview that discuss OO design or look for online courses that specifically cover design patterns and OOP interview questions. Some platforms (like Design Gurus) also offer mock interview opportunities or a community where you can get feedback on your designs. Leverage these to simulate real interview conditions.

By following these strategies, you’ll build confidence in tackling LLD problems. Always remember, the goal in an LLD interview is to demonstrate clean thinking in code. Even though you’re not writing actual code, your class design should be implementable and make logical sense. Interviewers are checking if you can create a maintainable, extensible design on the fly. With enough practice, you’ll start recognizing patterns (for example, many game designs will have a similar structure, many management systems like library or parking lot have common elements like managing entities, etc.). This pattern recognition will speed up your design process in the interview.

Recommended courses:

Grokking the Object Oriented Design Interview

Conclusion

System design and low-level design interviews each examine a different dimension of your design skills, and understanding the distinction is key to excelling in both. High-level system design questions test your ability to think broadly and architect scalable, reliable systems by considering how all the moving parts come together.

Low-level design questions, on the other hand, test your grasp of software engineering fundamentals – how to craft the building blocks of those systems with clean, logical code structure.

In practice, strong software engineers need both sets of skills. Designing a great system architecture is ineffective if you can’t implement its components cleanly; likewise, crafting beautiful class designs matters little if the overall system can’t meet user demands.

By preparing for both HLD and LLD questions, you ensure you can zoom out to the big picture and zoom in to the details as needed.

Key takeaways: Start by solidifying your fundamentals in system design concepts (like scalability, distributed systems, and integration of services) and in object-oriented design principles (like SOLID and design patterns).

Practice with real interview questions in each category and learn to communicate your ideas clearly. Use resources like courses (for example, Grokking the System Design Interview for system design) and guided examples for LLD to enhance your learning. And importantly, get feedback on your designs – this will help you refine your approach and catch any blind spots.

With diligent preparation, you’ll be well-equipped to handle any design question thrown your way. Whether it’s sketching a high-level architecture for a new system or drafting a class diagram for a specific module, you’ll approach it with confidence and structured thinking.