"The difference between a junior and a senior engineer isn't just code, it’s how they design the code."

In software engineering, Low-Level Design (LLD) is the process of translating abstract ideas into concrete implementation. It’s where you translate High-Level Design (HLD) into detailed class diagrams, interfaces, object relationships, and design patterns.

What LLD Really Means?

Low-Level Design is about answering the “how” of implementation. It moves beyond the "what" and dives into the granular details of the code structure.

High-Level Design (HLD) says: “We need a Notification Service.”
Low-Level Design (LLD) says: “We’ll use an interface NotificationSender with concrete classes like EmailSender, SmsSender, and PushNotificationSender, all managed by a NotificationManager.”

LLD zooms into the actual building blocks of your system. It’s not just about writing code that works, it’s about writing code that’s modular, testable, extensible, and easy to reason about as the system grows.

Core Components of LLD

Let’s break down the core elements that LLD focuses on:

1. Classes and Objects

This is where design truly begins. You identify the main entities and their roles within the system:

What are the key classes?
What are their responsibilities?
What data do they hold (attributes)?
What operations do they perform (methods)?

Example: In a food delivery system, we can have classes like Restaurant, Order, Customer, and DeliveryAgent

Customer places orders
Restaurant prepares food
DeliveryAgent delivers it
Order encapsulates the transaction

An Order class would have attributes like: orderId, customerId, items, totalAmount, status

and methods like: calculateTotal(), addItem(Item item), and updateStatus(newStatus).

2. Interfaces and Abstractions

Interfaces define contracts between components. They are critical to ensure loose coupling, allowing multiple components to interact without depending on each other's internal implementation details.

Ask yourself:

What functionality should a class expose to the outside world?
What details should remain hidden?
Which parts of the system are likely to change or have multiple variations?

Example: A PaymentProcessor interface defines a contract for payment with multiple implementations:

StripePaymentProcessor
RazorpayPaymentProcessor
PayPalPaymentProcessor

Now, the core application logic interacts only with the PaymentProcessor interface. This allows you to switch between Stripe and Razorpay or add a new provider without changing a single line of your business logic.

3. Relationships Between Classes

Classes don't exist in isolation. LLD defines these relationships clearly and precisely.

Key relationships include:

Association: A general "uses-a" relationship. A Doctor uses a Stethoscope.
Aggregation (Weak "has-a"): An object contains other objects, but they can exist independently. A Department has Professors. If the department is closed, the professors still exist.
Composition (Strong "has-a"): An object is composed of other objects, and their lifecycles are tied. A House is composed of Rooms. If you demolish the house, the rooms are destroyed with it.
Inheritance ("is-a"): A class inherits properties and behaviors from a parent. A Car is a Vehicle.

You also define cardinality to specify the number of instances involved in a relationship.

One-to-One: One instance of A is linked to one instance of B.

Example: Each User has one Profile.

One-to-Many: One instance of A is linked to multiple instances of B.

Example: A Customer can have multiple Orders.

Many-to-Many: Multiple instances of A relate to multiple instances of B.

Example: A Student can enroll in multiple Courses, and each Course can have multiple Students.

4. Method Signatures

Once your classes and relationships are defined, the next step is deciding how they behave using methods. A well-designed method signature is self-documenting and intuitive.

You’ll need to decide:

What methods should each class expose?
What are the method's input parameters, and return types?
Should the method be public, private, or protected?
What exceptions might they throw?
Is it synchronous or asynchronous?

Consistency, readability, and clarity in method signatures make your code intuitive and easier to maintain.

Bad: void sendMsg(String str)

Good: void sendNotification(Message message)

By designing expressive methods, you make your system intuitive to use and easy to extend, for example, adding new message types without rewriting existing code.

5. Design Patterns

LLD is also the stage where you apply proven solutions to common design problems using design patterns. These patterns provide reusable templates that bring structure, robustness, and maintainability to your code.

Some commonly used patterns in LLD include:

Singleton: useful when you need exactly one instance of a class across your system.
Factory: useful when you want to delegate object creation without exposing the instantiation details to the client.
Strategy: useful when you need to switch between multiple algorithms or behaviors at runtime.
Observer: useful for event-driven systems where you need to establish a publisher–subscriber relationship between objects.
Decorator: useful when you want to add new behavior to objects without altering existing code.
Adapter: useful when you need to bridge incompatible interfaces to work together.
Facade: useful when you want to provide a simplified interface to a complex subsystem.

Using the right design patterns isn’t about memorizing their definitions, it’s about recognizing the problems they solve.

Tip: Don’t force patterns. Let the problem shape the pattern, not the other way around.

Importance of LLD in Software Development

A good LLD bridges the gap between idea and execution. It turns broad architectural concepts into practical, working software components that can evolve and scale over time.

Maintainability

A well-designed system is easy to read, debug, and extend. When components have clear responsibilities and clean interfaces, you can make changes without fear of breaking unrelated parts of the system.

Scalability & Performance

While High-Level Design (HLD) focuses on infrastructure-level scalability (like horizontal scaling or database sharding), LLD ensures that individual components can scale gracefully—e.g., a sorting module that works efficiently whether it handles 100 or 10,000 records.

Testability

Clean LLD naturally leads to loosely coupled components, making unit testing straightforward. When each class does one thing well and depends only on abstractions, testing becomes faster, easier, and more reliable.

Collaboration

LLD provides a clear blueprint for developers, enabling multiple engineers to work on the same module concurrently. With well-defined contracts and responsibilities, there's less confusion and fewer merge conflicts.

Reusability

When you design modules with well-thought-out responsibilities and abstractions, those modules can often be reused in different parts of your codebase or even across projects.

Importance of LLD in Interviews

Low-Level Design (LLD) interviews go far beyond checking if you can write code that compiles. They assess how you think, how you structure code, and how you make trade-offs when building real-world systems.

In other words, interviewers aren’t just evaluating whether you can solve a problem, they’re evaluating whether you can build software that’s easy to understand, maintain, and evolve over time.

Let’s break down what exactly interviewers are looking for.

Problem-Solving

Before writing code, great engineers think about design.

Interviewers want to see if you can take an open-ended problem and break it down into smaller, manageable components.

Object-Oriented Principles

Object-Oriented Programming (OOP) is the backbone of LLD interviews. Interviewers want to see if you truly understand and apply OOP principles like encapsulation, abstraction, inheritance, and polymorphism in a meaningful way.

Design Principles

A good engineer doesn’t just write code that works; they write code that works well over time. That’s where principles like SOLID, DRY, and KISS come in.

Interviewers want to know if you can use good design principles like the SOLID principle to build systems that are robust, flexible, and easy to change.

Design Patterns

Interviewers often expect you to recognize when a classic pattern fits naturally into your design.

Common examples include:

Strategy Pattern for interchangeable behaviors (e.g., sorting algorithms, payment methods)
Observer Pattern for event-driven systems (e.g., notifications, subscribers)
Factory Pattern for flexible object creation without exposing instantiation logic

Clean Code

Do you care about clarity? Do your method names make sense? Are your responsibilities well-scoped? Interviewers look for signs that you write code others can read, maintain, and trust.

Communication & Trade-offs

Interviewers evaluate how well you can:

Articulate your design choices.
Justify trade-offs (e.g., flexibility vs. simplicity, performance vs. readability).
Discuss alternative approaches and their pros/cons.

Now that you have a good understanding of what Low-Level Design (LLD) is, it’s time to take a step back and look at where LLD fits within the broader software design process.

This brings us to an important question:

“How is Low-Level Design different from High-Level Design?”

In the next chapter, we will answer this question.

What is Low-Level Design (LLD)?

Ashish Pratap Singh