Both HLD (High-Level Design) and LLD (Low-Level Design) are trending terms in software development interviews, system design discussions, and architecture reviews. New developers or students often feel overwhelmed by these terms. But don’t worry — they are not as scary as they seem.
In this article, we’ll unravel HLD and LLD in a simple and easy way, using analogies and real-world examples so you can confidently understand and apply them.
What are HLD and LLD?
| Term | Stands For | Focus |
|---|---|---|
| HLD | High-Level Design | Architecture and components |
| LLD | Low-Level Design | Internal logic and class-level details |
Think of It Like Building a House
- HLD is the blueprint that shows the rooms, structure, and layout.
- LLD is the detailed plan showing electrical wiring, plumbing, wall materials, and so on.
High-Level Design (HLD) – The Big Picture
What is HLD?
HLD focuses on:
- System architecture
- Major components/modules
- How components talk to each other (APIs, protocols)
- Technology stack (e.g., database, cache, message queues)
- Data flow and high-level diagrams
📘 Example:
Imagine designing Instagram:
- HLD will include:
- User Service
- Feed Service
- Notification Service
- Database and CDN
- How users' posts are stored and fetched
- Communication via REST or gRPC
🎯 Goals of HLD:
- Make the system scalable, reliable, and fault-tolerant.
- Identify performance bottlenecks early.
- Choose the right technologies.
🔧 Low-Level Design (LLD) – The Inner Workings
What is LLD?
LLD focuses on:
- Class diagrams
- Interfaces and methods
- Business logic and rules
- Database schema design
- Relationships between classes (Inheritance, Composition)
📘 Example:
Continuing with Instagram:
- How the
Postclass looks like - How to implement
FollowUser()function - Internal data structures for user feed
- Object relationships (e.g.,
User,Post,Comment)
🎯 Goals of LLD:
- Ensure maintainability and code quality.
- Make sure the design supports HLD architecture.
- Allow developers to write code directly from the design.
Key Characteristics of LLD
- Granular and Code-Level:
- LLD dives deep into the fine details of how each component will function.
- It defines classes, functions, variables, and data structures.
- Example: Instead of just saying “we need user authenticated”, LLD shows how it's built – what classes handle it, what methods validate login, and what happens on failure.
- Implementation-Focused:
- LLD is directly linked to how the actual code will be written.
- It acts as a blueprint for developers, guiding the logic flow, and structure of modules.
- It often includes pseudocode, flow diagrams, and sequences diagrams that show real-time data flow between functions.
- Applies OOP Principles:
- LLD makes heavy use of
Object-Oriented Programming (OOP)concepts like classes, inheritance, abstraction, encapsulation, and polymorphism. - This helps build modular, reusable, and maintainable systems.
- For example, a base Notification class might have subclasses like EmailNotification and SMSNotification using inheritance and polymorphism.
- LLD makes heavy use of
Note that, in Low-Level Design (LLD), the stakeholders are primarily the people directly involved in the actual implementation of the system — Senior Software Developers, Technical Leads, Managers, etc.
Difference Between HLD and LLD
| Aspect | High-Level Design (HLD) | Low-Level Design (LLD) |
|---|---|---|
| Purpose | System overview & modules | Detailed implementation & logic |
| Level of Detail | Abstract | Highly detailed |
| Focus | Architecture, modules, data flow, interfaces | Classes, methods, logic, object relationships |
| Outcome | Architecture diagrams, technology stack, components | Class diagrams, function definitions, DB schema |
| Audience | Architects, senior developers, stakeholders | Developers, engineers |
| Design Elements | Microservices, APIs, databases, load balancers | Classes, objects, functions, variables |
| Tools Used | Draw.io, Lucidchart, Google Slides | UML tools, PlantUML, IDEs, pseudocode |
| Example | Define User Service, Auth Service, Database | Implement User, Post, AuthService classes |
| Timing in Lifecycle | Initial design phase (architecture planning) | After HLD, before or during development |
| Reusability | Focuses on reusable system-level modules | Focuses on reusable classes/methods |
| Scalability Planning | Considered during HLD (e.g., load balancing, sharding) | Not the main focus in LLD |
🔧 Importance of Low-Level Design (LLD)
Other than its vital role in the software development lifecycle and its high demand in senior engineering roles, Low-Level Design (LLD) is crucial for multiple practical and strategic reasons:
✅ 1. Avoids Rework
A well-defined design helps identify logical flaws and architectural mismatches before development begins. This minimizes the chances of:
Late-stage changes
Refactoring large codebases
Costly rework due to overlooked scenarios
✅ 2. Improves Collaboration
LLD acts as a common language for developers, testers, architects, and code reviewers. It:
Clarifies the responsibilities of each component
Defines integration points clearly
Helps teams coordinate parallel development effectively
✅ 3. Promotes Scalability
Modular and thoughtfully designed components can:
Be reused across features
Adapt to increasing user load
Support the addition of new capabilities without breaking the system
🔄 Good LLD enables easy plug-and-play of features.
✅ 4. Encourages Best Practices
LLD enforces:
Use of design patterns
Adherence to object-oriented principles (OOP) like SOLID
Clean code architecture and separation of concerns
This leads to robust, maintainable, and easily testable software.