What is LLD?
Understanding Low-Level Design
Overview
Low-Level Design (LLD) is the process of designing the internal structure and implementation details of a software system. It focuses on how individual components, classes, and modules are designed, how they interact with each other, and how they implement the desired functionality.
LLD bridges the gap between high-level architecture and actual code implementation. While High-Level Design (HLD) focuses on system architecture, databases, and external interfaces, LLD dives deep into class design, data structures, algorithms, and object relationships.
Key Concepts
Class design and object modeling
Defining interfaces and contracts
Choosing appropriate data structures
Implementing design patterns
Writing clean, maintainable code
Creating UML diagrams (class, sequence)
Real-World Example
Imagine building a house:
- HLD is like the architectural blueprint showing rooms, floors, and overall layout
- LLD is like the detailed electrical wiring, plumbing diagrams, and material specifications
In software:
- HLD: "We'll use microservices with a SQL database and REST APIs"
- LLD: "The UserService class will have methods like createUser(), findById(), and will use the Repository pattern for data access"
💡 Interview Tips
LLD interviews typically last 45-60 minutes
You'll be asked to design a system like Parking Lot, BookMyShow, etc.
Focus on identifying entities, relationships, and design patterns
Write actual code, not just diagrams
Explain your thought process clearly