Whether you're aiming to crack system design interviews at FAANG or building real-world scalable applications, understanding High-Level Design (HLD) and Low-Level Design (LLD) is essential. This blog will walk you through the complete roadmap, key concepts, interview expectations, and practical tips for mastering HLD and LLD.
🔹 What is High-Level Design (HLD)?
HLD focuses on the architecture of the system — how different components interact, their responsibilities, and how the system scales.
Key Focus Areas:
- System boundaries
- Major components/modules
- Communication protocols (REST, gRPC, etc.)
- Databases and their types
- Load balancers, caching, CDNs
- High availability & fault tolerance
- API gateway, rate limiting
Deliverables:
- Component Diagram
- Deployment Diagram
- Tech Stack Justification
- Scaling Strategy (horizontal vs vertical)
- Data Flow Diagrams
HLD Interview Questions:
- Design YouTube, Instagram, WhatsApp, etc.
- How would you handle millions of users?
- What DB and architecture would you choose and why?
🔹 What is Low-Level Design (LLD)?
LLD dives into class-level design, data models, and internal working of each module or component. It ensures your architecture can actually be built and maintained.
Key Focus Areas:
- Class diagrams
- Object-oriented principles (SOLID, DRY, YAGNI)
- Design patterns (Factory, Singleton, Strategy, etc.)
- Interfaces and abstractions
- Clean Code & modularization
- Thread safety and concurrency
Deliverables:
- Class Diagrams
- Sequence Diagrams
- Pseudocode for major flows
- Relationships (composition, inheritance)
LLD Interview Questions:
- Design elevator system / parking lot / ATM
- Implement rate limiter / logger / cache
- Write class diagram for social media post/comment system
🔹 Must-Know Design Patterns for LLD Interviews
Mastering the following design patterns is essential for low-level design interviews:
1. Singleton Pattern
Ensures a class has only one instance and provides a global point of access to it.
2. Factory Pattern
Creates objects without exposing the instantiation logic to the client.
3. Builder Pattern
Helps in constructing complex objects step by step.
4. Strategy Pattern
Defines a family of algorithms and encapsulates each one, making them interchangeable.
5. Observer Pattern
Used to notify multiple objects when the state of one object changes (e.g., pub-sub systems).
6. Decorator Pattern
Adds new behavior to objects dynamically without altering their structure.
7. Adapter Pattern
Converts one interface to another that a client expects.
8. Command Pattern
Encapsulates a request as an object, allowing for parameterization and queuing of requests.
9. Prototype Pattern
Creates new objects by copying an existing object.
10. Proxy Pattern
Provides a surrogate or placeholder for another object to control access.
These patterns help in writing scalable, testable, and maintainable code. During interviews, be prepared to:
- Identify the right pattern for the problem.
- Explain your choice clearly.
- Write code/pseudocode demonstrating the pattern.
🔹 Differences Between HLD and LLD
| Aspect | HLD | LLD |
|---|---|---|
| Focus | Architecture & components | Internal structure & logic |
| View | Bird’s eye view | Programmer’s view |
| Audience | Architects, stakeholders | Developers |
| Diagrams | Component, deployment, data flow | Class, sequence, ER diagram |
| Concerns | Scalability, availability | Code reusability, clean design |
🔹 How to Prepare for HLD Interviews
- Learn how to analyze requirements
- Know trade-offs between SQL vs NoSQL, monolith vs microservices
- Practice designing large-scale systems
- Learn tools like Draw.io, Lucidchart
- Understand CAP theorem, latency vs throughput
Recommended Practice Systems:
- Design a URL Shortener
- Design Uber backend
- Design Distributed File Storage (like Dropbox)
🔹 How to Prepare for LLD Interviews
- Master Object-Oriented Programming (Java, C++, Python)
- Practice drawing class and sequence diagrams
- Learn at least 10 core design patterns deeply
- Write clean, modular, testable code
- Implement real-world systems in code
Recommended Practice Scenarios:
- Design BookMyShow LLD
- Design Online Library Management
- Implement a Thread Pool, Logger, or Notification Service
🔹 Final Tips
- Start with HLD to structure your thoughts.
- Gradually go deeper into LLD.
- Always clarify requirements and constraints.
- Communicate your assumptions during interviews.
- Practice diagramming and coding in a timed environment.
Mastering HLD and LLD is the difference between being a good developer and becoming a system design expert. Bookmark this guide, practice often, and walk confidently into your next system design interview!
Top comments (0)