Low-Level Design Interview Preparation Guide
Code Crafter
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If you are preparing for Low-Level Design (LLD) interviews, especially for top tech companies like Google, Amazon, Microsoft or product-based startups. you need a structured approach that includes concept mastery, structured practice, system thinking and interview tactics. Here’s your comprehensive guide to preparing for LLD interviews.
What is Low-Level Design (LLD)?
Low-Level Design (LLD) focuses on the internal workings of a system. Low-Level Design involves designing the classes, interfaces, interactions, object relationships , class responsibilities and design patterns that solve a specific feature or functionality of a system. It's about translating business requirements into a working OOP-based structure.
How to Approach an LLD Problem in Interviews
Low-Level Design interviews assess your ability to architect the internals of a system with proper object-oriented modeling, maintainability, and real-world problem solving. Cracking LLD requires structured thinking, design intuition and clarity of communication.
1. Understand the Requirements
- Ask clarifying questions about the system’s scope.
- Understand functional requirements (e.g. “Users can book a cab”).
- Identify non-functional requirements (e.g. performance, scalability, fault-tolerance).
- Identify core entities and their responsibilities.
- Discuss scale & usage patterns: Are we building for 1M users or 100?
- Write down assumptions explicitly and confirm with the interviewer.
- Should the system support real-time updates?
- Are we persisting data or is it in-memory?
- Is authentication in scope?
2. Identify Core Features and Use Cases
- Break the problem down into modules/components (Authentication, Booking Flow, Pricing / Payments, Notifications, Search / Matching Engine etc) .
- Write user stories like: “A user can book a cab” or “Admin can block a user”.
- Prioritize must-have vs optional features.
- Write down each flow you'll design.
3. Identify Entities and Relationships
- List the main classes (e.g., User, Booking, Ride, etc.).
- Identify responsibilities of each class.
- Think of composition/inheritance/associations.
4. Apply OOP and SOLID Principles
- Ensure your design doesn’t violate SRP.
- Ensure that your design is modular, extendable and testable.
- Use interfaces and abstractions properly.
5. Choose Appropriate Design Patterns
- Think which patterns like Strategy, Observer, Factory, etc., help.
- Don’t force patterns. Use them only when they add real value.
Most Asked Design Patterns in LLD Interviews (with Use Cases)
| Design Pattern | Category | Use Case | Why It's Asked |
|---|---|---|---|
| Singleton | Creational | Logger, Configuration Manager, DB connection | Ensures a single instance , often asked in logger and cache-related LLDs |
| Factory | Creational | Notification system (SMS/Email/Push), Shape creator, Vehicle factory | Helps in object creation without exposing logic , shows understanding of extensibility |
| Builder | Creational | Pizza ordering, Complex form builders, HTTP request builders | For constructing complex objects step-by-step , great for immutability & clarity |
| Strategy | Behavioral | Payment method selection, Ride fare calculation, Sorting algorithms | Replaces conditional logic , shows understanding of open/closed principle |
| Observer | Behavioral | Notification system, Event listeners, Stock ticker | Enables publish/subscribe , decouples broadcaster from subscribers |
| Decorator | Structural | Text formatting, Pizza topping system, Logging layers | Adds responsibilities dynamically , tests your layering skills |
| Template Method | Behavioral | Game flow, File parsing, Transaction steps | Common for shared logic with custom steps , shows how to abstract invariants |
| State | Behavioral | ATM operations, Game states, Traffic lights | Manages state transitions cleanly , asked to test command and behavior tracking |
| Command | Behavioral | Undo/redo systems, Remote control, Task queues | Encapsulates actions , shows separation of command from invoker |
| Adapter | Structural | Legacy API integration, Payment gateway wrappers | Helps integrate incompatible interfaces , shows system adaptability |
| Composite | Structural | File-system, UI components, Groups in Splitwise | Tree-like structures , asked in recursive object relationships |
| Chain of Responsibility | Behavioral | Logging framework, Request filters, Event processing | Pass request through chain , shows decoupling and control flow design |
| Prototype | Creational | Cloning objects, Game character templates | Used when object creation is expensive , shows cloning techniques |
6. Use Diagrams to Visualize Your Design
Visuals make your solution easier to explain and understand.
- Class diagrams for structure.
- Sequence diagrams for workflow.
- Component Diagrams for modules and services.
- State Diagrams for systems with changing states (e.g. elevators, orders).
7. Discuss Edge Cases, Constraints and Extensibility
Show that you think ahead not just for now, but for future changes.
- How will you add new payment methods?
- How can you scale this for multiple regions?
- Talk about extensibility, pluggability and separation of concerns.
8. Talk About Persistence and Storage (Optional)
Depending on scope, you may be expected to discuss data layer.
- Will data be stored in DB? If so, relational or NoSQL?
- Should we cache frequently accessed entities like nearby drivers?
- Is data consistency required (e.g., ACID vs eventual)?
- Keep it light unless the interviewer asks.
9. Talk About Testing and Maintainability
Great design is testable and maintainable.
- Use dependency injection for better unit testing.
- Separate pure logic from side effects.
- Write modular classes for easier refactoring.
10. Communicate Throughout the Process
Don’t code silently. Speak your thoughts clearly.
- Talk while drawing diagrams.
- Justify your design decisions.
- Be open to suggestions.
- Adjust your design if interviewer pushes back.
Best Practice Problems (LLD Problems)
| System | Design Patterns Commonly Used |
|---|---|
| Library Management System | Singleton, Factory |
| ATM Machine | State, Command |
| Cab Booking System | Strategy, Observer |
| Logger System | Singleton, Decorator |
| Notification System | Observer, Strategy |
| Splitwise | Composite, Observer |
| Parking Lot System | Strategy, Factory |
| Rate Limiter | Token Bucket, Sliding Window |
| Elevator System | Observer, State |
| Tic-Tac-Toe / Chess | OOP Modeling, Template Pattern |
| E-Commerce Cart System | Strategy, Command, Observer |
| File System | Composite, Iterator |
| Hotel Booking System | Factory, Strategy, Observer |
| Online IDE/Compiler | Interpreter, Command, Observer |
| Cache System (e.g., LRU) | Singleton, Strategy, Proxy |
Summary Checklist
| Step | Key Activity |
|---|---|
| ✅ Understand Requirements | Clarify features, constraints, scale |
| ✅ Identify Entities | Use nouns from problem description |
| ✅ Apply OOP/SOLID | Make your design flexible |
| ✅ Use Patterns | Apply only where needed |
| ✅ Draw Diagrams | Visualize class and flow |
| ✅ Handle Edge Cases | Think like a product engineer |
| ✅ Show Extensibility | Can new features be added easily? |
| ✅ Communicate | Interviewer = Collaborator |