Design Guide
It will help a candidate to translate product requirements into clean, modular, and code-ready designs. It focuses on object-oriented thinking, class design, design patterns, and applying engineering principles like SOLID and DRY.
🧑🏻💻 Low-Level Design (LLD) Interview — At a Glance
A Low-Level Design (LLD) interview (sometimes called an “Object-Oriented Design” or “Component Design” interview) focuses on how you translate high-level product or system requirements into concrete, code-ready designs. It bridges the gap between a broad “what should the system do?” conversation (High-Level Design, HLD) and the actual implementation you would check into a repo.
🎯 What the Interviewer Wants to See
| Aspect | What They Evaluate | Typical Signals |
|---|---|---|
| Object Modeling | Correct identification of entities, relationships, and responsibilities | Well-named classes, clear ownership, proper use of composition/aggregation |
| Design Principles | Understanding and application of SOLID, DRY, YAGNI, etc. | Ability to justify choices (“I’m using Strategy here to support extension without modification…”) |
| Design Patterns | Knowing when (and when not) to apply patterns | Thoughtful use of Factory, Observer, Builder, etc.—not pattern-driven over-engineering |
| Scalability & Extensibility | How your design evolves when requirements change | Discussing future features, versioning, interface segregation |
| Code-Level Details | Clarity on data structures, method signatures, APIs, edge cases | Sketching class diagrams, writing key interfaces on a whiteboard or editor |
| Trade-off Discussion | Awareness of performance, memory, concurrency, and simplicity | Articulating why you chose a hash map over a tree, or async processing over sync |
🔶 Typical Format
| Stage | What Happens |
|---|---|
| Clarify Requirements | Ask questions: scope, constraints, performance goals. |
| Identify Core Entities | List nouns and verbs, decide on classes/modules. |
| Draw Relationships | UML-lite: associations, inheritance, composition. |
| Define Interfaces & Key Methods | Show method names, arguments, return types. |
| Discuss Data Structures & Algorithms | Pick concrete structures, explain complexities. |
| Handle Edge Cases & Extensions | Fault tolerance, version upgrades, new features. |
| Review & Iterate | Invite feedback, adjust design, highlight trade-offs. |
📕 How It Differs from High-Level Design (HLD)
| HLD | LLD |
|---|---|
| Focuses on system architecture—services, databases, communication protocols | Focuses on internal structure—classes, APIs, algorithms |
| Talks about scale, availability, partitions | Talks about methods, data structures, object responsibilities |
| Typically involves diagrams like service boxes, load balancers, queues | Involves class diagrams, sequence diagrams, sometimes pseudo-code |
| Output is a blueprint for multiple teams | Output is a blueprint for actual code in one team/repo |
🧩 A Simple Approach System
Clarify Requirements
- Ask questions to understand the full scope of the problem
- Identify the core features and basic flows
- Clarify edge cases and constraints
- Agree on assumptions if requirements are vague
Define Core Objects/Classes
- Identify the high level objects/classes needed based on requirements
- Consider entities like User, Order, Product etc.
- Outline key attributes for each class
- Keep it high level at this stage
Establish Relationships Between Objects
- Map out how the core classes would interact with each other
- Note the direction of communication between objects
- Define clean interfaces for how objects will collaborate/exchange data
- Consider 1-1, 1-many, inheritance relationships
Define Required Functionality
- Outline main methods/functions needed in each class
- Focus on core functionality at first
- Provide method signatures, inputs and outputs
- Consider class scope, access modifiers
Apply OOP Principles and Patterns
- Use abstraction, encapsulation properly
- Identify where inheritance makes sense
- Apply patterns like MVC, singleton etc. where applicable
- Break code into modules/packages for better organization
Write Clean, Structured Code (If asked)
- Use proper naming conventions for classes, attributes, methods
- Modularize code into files, folders based on functionality
- Format code properly with line spacing for readability
- Add comments to explain complex parts
✅ Conclusion
The LLD interview tests how well you can move from requirements to a clean, extensible, code-level design—demonstrating sound engineering judgment, knowledge of design principles, and clear communication. Nail those, and you’ll show hiring teams you can own real-world features from concept to commit.