Low-Level Design Interview – How to Approach It And Crack It
If you are in the software industry, you have likely heard of Low-Level Design (LLD). While it may not be as popular or well-known as High-Level Design (HLD), it is still an important aspect of software development. And if you want to read about high-level design, please read this article High-Level Design Interview – How to Approach It And Ace It. If you are a fresher, the first question that might come to your mind is, what exactly is Low-Level Design? On the other hand, if you are an experienced professional and already know what it is, you might be wondering how to crack or ace the interview. If you are asking yourself these questions, you’ve come to the right place.
Table of Contents
What is a Low-Level Design?
Low-level design (LLD) is a type of component-level system design where you define the components (classes) and the relationships between them, among other aspects. In addition, concepts such as design patterns, OOPS, SOLID, and others are often relevant in a typical low-level design interview. So when you are creating a low-level design, you need to keep some key things into considerations:
- What design patterns will I be using?
- What are the classes that will be a part of my design?
- What will be the attributes that will be a part of each class?
- How will the different components/ classes interact with each other?
- Is my design scalable?
- Can I incorporate new requirements without disturbing everything?
Among other things. But you get the idea, right?
Also check: How to Prepare for and Crack the Coding Interview
How To Crack Low-Level Design(LLD) Interview?
Let’s explore what needs to be done to crack the low-level design interview. Since there is no one-size-fits-all approach, you must be thorough in your preparation.
Start With the Basics of Low-Level Design

To ace the low-level design interview, you need to have strong fundamentals. If your fundamentals are already solid, you should still brush them up as much as possible. But what do we mean by ‘basics’ in the context of a low-level design interview? There are certain concepts and terminologies you need to understand. Let’s explore what these are.
OOPS Principles
OOPS stands for Object Oriented Programming. Objects are the foundation of OOPS principles. An object can be considered an instance of a class, which can contain attributes and methods that help extend its functionality. For example, objects can include an animal or a house. An animal may have attributes such as name, height, and family. Now that we have some context on what objects are, let’s take a deeper dive into the OOP concepts.
Abstraction
Abstraction is the concept of providing only the necessary details while hiding internal details that are not essential to reveal. In OOP, it can be achieved through two methods: Encapsulation and Inheritance. An example of abstraction is exposing an API that takes an input and provides an output. The user of the API does not need to know how the output is derived; they simply provide an input and receive an output, and that’s all.
Encapsulation
Encapsulation refers to bundling the attributes and methods into a single unit called a Class. It can be achieved using access modifiers such as public, protected, and private. In this way, it ensures that the data is accessible only to the classes that are supposed to access it.
Polymorphism
Polymorphism is the ability to take on multiple forms, meaning that something can behave differently in different situations. There are two types of polymorphism:
- Compile-time polymorphism: Compile-time polymorphism is achieved by method overloading.
- Runtime polymorphism: Runtime polymorphism is achieved by method overriding.
Inheritance
Inheritance is the mechanism that increases code reusability by utilizing the concept of a superclass and a subclass. The superclass is the parent, and the subclass is the child. Instead of repeating code in the subclass, we can simply reuse the code from the superclass.
Design Patterns
Design patterns provide proven solutions to common problems in the software development lifecycle. They are abstract solutions, not code templates, and can be adapted to meet specific needs. Furthermore, they offer proven ways of structuring code and solving specific issues, thereby making it easier for developers to build systems that are reliable, maintainable, and scalable.
Some commonly used design patterns are Singleton, Builder, Decorator, Strategy, Prototype, Factory, and AbstractFactory.
Advantages
There are a lot of advantages of design patterns which is why they are used extensively.
- Reusability
- Consistency
- Improved Code Maintainability
- Scalability
- Flexibility and Extensibility
- Reduced Complexity
- Improved Code Quality
- Encourages Best Practices
- Reduced Development Time
- Improved Testability
SOLID Principles
The SOLID principles are a set of five design principles that help software developers create more maintainable, flexible, and understandable object-oriented systems. And they aim to improve the readability, scalability, and robustness of code, especially in large and complex systems. You should apply these principles in day-to-day software development and coding. SOLID principles are very important in the context of Low-Level Design.
The principles are as follows:
- Single Responsibility Principle
A class should have only one job or responsibility. And it should have only one reason to change, i.e, when its responsibility changes. - Open/Closed Principle
Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. - Liskov Substitution Principle
Objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program. - Interface Segregation Principle
Do not force clients to depend on interfaces they do not use. - Dependency Inversion Principle
High-level modules should not depend on low-level modules; both should depend on abstractions. Additionally, abstractions should not depend on details, but rather, details should depend on abstractions.
Also check: Machine Coding Round – What is it & how to crack it
Approach to be Used in Low-Level Design(LLD) Interview
Since Low-Level Design rounds usually last about 1 hour, I have divided it into a few steps along with time estimates for every step as this will help you in planning your time accordingly.
Requirement Gathering(3-5 mins)
The very first step is to review the functional and non-functional requirements carefully. Then break down the system’s high-level design (HLD) and ensure you understand the components, data flow, and interactions outlined in the HLD. Then ask questions to the interviewer and get it clarified. The sooner the better.
Identify the Components/Modules(2-4 mins)
Divide the system into smaller modules, classes, or components that align with functionality. Then define what each module will be responsible for. Finally list the dependencies between the modules and their interactions. It can be one-to-one, one-to-many, many-to-one, etc.
Design the Data Models(5-7 mins)
You should define the data structures (objects, arrays, linked lists, etc.) that will be used within modules. If applicable, design the database schema or object-relational mapping (ORM) for persistence. Also map out how data moves between modules, components, and external systems.
Define Classes and Methods(4-6 mins)
First define the classes, their attributes, and methods. Then define inputs, outputs, and return types for each method. The next step would be to describe how classes communicate with each other (using parameters, return values, etc.). Finally identify which methods and attributes you should expose and which you should hide (i.e., make private).
Specify Interfaces(4-6 mins)
Define clear interfaces for each module or component and structure the interaction between modules effectively. Then you need to define method signatures, expected behaviors, and error handling for each interface.
Define Error Handling(4-6 mins)
Determine which exceptions and errors each component will throw and how they will be handled. In addition, you should decide if there will be retries, fallbacks, or logging for errors.
Design Sequence Diagrams for the Low-Level Design(8-10 mins)
Draw sequence diagrams to represent the interaction between objects or modules in response to specific requests or events. Also specify the order of method calls and data exchanges.
Identify Performance Considerations(3-5 mins)
You should plan for performance optimization within components (e.g., caching, multi-threading, or load balancing). In addition, you should design the modules with scalability in mind, whether vertical or horizontal.
Define Security Measures(4-6 mins)
Design security measures for authentication (who you are) and authorization (what you can do). Also make sure to incorporate encryption mechanisms, data privacy, and secure communication channels. In addition, you can also define how you will handle input validation and sanitization to prevent security vulnerabilities like SQL injection, XSS, etc.
Review and Iterate the Low-Level Design(4-6 mins)
Review your design with peers or stakeholders to ensure that it meets the requirements, is scalable, maintainable, and free of redundancies. Additionally, iterate the design based on feedback and refinements.
By now, you might have got a fair understanding of what low-level design(LLD) is and what you need to do to crack it.
All the best!!!
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Further Reading
- How to crack technical interview – Google, Amazon & more
- Machine Coding Round – What is it & how to crack it
- Digital Wallet Design – Machine Coding Round Solution
- Best Coding Platform To Become A Coding Expert
- Backtracking Algorithm Explained With Examples
- The ultimate guide to interview preparation
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