Low-Level Design (LLD) plays a critical role in software engineering. It transforms high-level requirements into detailed class structures, object interactions, and method signatures. Whether you’re preparing for system design interviews or working on real-world applications, understanding how to approach LLD effectively can set you apart.
Table of Contents
Topics to Master
To tackle LLD problems confidently, make sure you’re familiar with the following foundational topics:
Object-Oriented Principles
Design Patterns
- Creational: Singleton, Factory, Builder
- Structural: Adapter, Decorator, Composite
- Behavioral: Strategy, Observer, Command
UML Diagrams
- Class Diagrams
- Sequence Diagrams
- Object Diagrams
Class Design Best Practices
- High cohesion, low coupling
- Clean code
- Interface segregation
Concurrency
- Thread safety
- Synchronization
- Locks and Semaphores
Tackling Low-Level Design
Understand the Requirements
Start by getting a crystal-clear understanding of the problem statement. Ask clarifying questions to explore:
- Functional requirements: What exactly should the system do?
- Non-functional requirements: Performance, scalability, concurrency, etc.
- Constraints: Time limits, tech stack preferences, etc.
- Example: If designing a parking lot system, ask—what kinds of vehicles are allowed? Is real-time tracking needed? Is payment integration required?
Identify Core Entities (Classes)
Break the problem into key objects. These are typically the nouns in the problem statement.
- In a movie booking system: User, Theatre, Movie, Show, Booking.
This step lays the foundation for your class diagram and object interactions.
Define Relationships Between Classes
Once entities are identified, define how they interact:
- One-to-one, one-to-many, or many-to-many
- Composition vs Inheritance
- Use simple UML Class Diagrams to visualize
- A Theatre has multiple Screens; each Screen has multiple Shows.
List Core Functionalities
List out features or operations your system must perform. These will map to method signatures within the classes.
- For a Parking Lot:
- allocateSlot(vehicle)
- releaseSlot(ticket)
- calculateCharges(duration)
Assign Class Responsibilities
Keep classes clean and focused. Follow the Single Responsibility Principle (SRP).
- ParkingLotManager handles allocation logic
- PaymentService handles billing
- Avoid “God classes” that do everything
Use SOLID principles to improve modularity and maintainability.
Handle Data Flow and State Transitions
Think about how data moves through the system and what states objects can be in.
- A ParkingSlot might transition: Available → Occupied → Available.
Model these changes explicitly in your classes.
Apply Design Patterns (When Needed)
Use design patterns to simplify or decouple your design:
- Factory Pattern: For object creation
- Strategy Pattern: For dynamic behavior (e.g., different payment strategies)
- Singleton Pattern: For global managers
- Observer Pattern: For event-driven flows
Don’t force patterns—use them when they add clarity or flexibility.
Consider Extensibility and Edge Cases
Ask yourself:
- Can this design be extended easily?
- What if we need new vehicle types? New payment options?
- What happens if a user cancels a booking?
Thinking ahead helps you build a scalable and maintainable design.
Think About Concurrency, Caching, and Persistence
Consider:
- Concurrency: How do you handle race conditions or multi-user access?
- Caching: What data can be cached? (e.g., seat availability)
- Persistence: What goes into the database vs stays in-memory?
Use thread-safe classes or locks where necessary.
Walk Through a Real Use-Case
Validate your design by simulating a real use-case.
- Example: A user logs in → selects a movie → picks seats → makes payment → receives a ticket.
This helps catch gaps and ensures end-to-end flow is covered.
Write Class Skeletons
Once your design is clear, write class structures with:
- Proper method names
- Parameters and return types
- Basic comments or docstrings
No need for full implementation unless required.
Explain Trade-offs
Always be ready to explain:
- Why you chose a certain approach
- Why a certain design pattern was used (or not)
- What are the trade-offs of your decisions?
Interviewers love when you can back your design with solid reasoning.
Common LLD Problems to Practice
- Parking Lot System
- Elevator System
- Splitwise
- BookMyShow
- Ride Sharing (Uber/Ola)
- Amazon Cart
- Hotel Booking System
- Library Management System
- Food Delivery App (Swiggy/Zomato)
Conclusion
Mastering Low-Level Design requires more than just coding skills—it’s about thinking like an architect, designing systems that are clean, extensible, and robust.
Next time you face an LLD challenge, follow these 12 steps, and you’ll approach the problem with confidence and structure.