Design Principles for Low-Level Design Interviews

This article serves as a practical guide to design principles that are crucial for success in low-level design (LLD) interviews. It emphasizes that while knowing the acronyms is helpful, the ability to apply these principles to create clean, extensible, and maintainable code is what interviewers value most. The guide outlines two main categories of principles: General Software Design Principles and Object-Oriented Design (OOD) Principles, often known as SOLID.

The General Software Design Principles focus on simplicity and pragmatism. KISS (Keep It Simple, Stupid) advocates for the simplest solution that works, warning against over-engineering with unnecessary patterns. DRY (Don't Repeat Yourself) encourages centralizing logic to ease maintenance, though it cautions against creating artificial coupling. YAGNI (You Aren't Gonna Need It) advises against building for hypothetical future requirements, suggesting that one should only design for what is currently needed. Other key general principles include Separation of Concerns, which ensures different parts of the code handle distinct responsibilities, and the Law of Demeter, which reduces coupling by limiting how objects interact with distinct parts of the system.

The article also details the SOLID principles, which are fundamental to designing robust class hierarchies. SRP (Single Responsibility Principle) states that a class should have only one reason to change. OCP (Open/Closed Principle) suggests classes should be open for extension but closed for modification, often achieved through interfaces. LSP (Liskov Substitution Principle) ensures subclasses can replace parent classes without breaking behavior. ISP (Interface Segregation Principle) promotes small, focused interfaces over large ones. Finally, DIP (Dependency Inversion Principle) argues that high-level modules should depend on abstractions rather than concrete implementations to improve flexibility and testability.

Key Concepts

• KISS (Keep It Simple, Stupid): The simplest solution is usually the right one; avoid over-engineering and adding complexity until it is absolutely necessary.
• DRY (Don't Repeat Yourself): Eliminate redundancy by centralizing logic, which simplifies maintenance, but be careful not to couple unrelated concepts.
• YAGNI (You Aren't Gonna Need It): Implement only what is required for the current problem; do not design for hypothetical future scenarios that may never happen.
• Separation of Concerns: Divide the system so that distinct sections address separate responsibilities (e.g., UI, business logic, data access) and do not depend on each other's internals.
• Law of Demeter: Reduce coupling by ensuring an object only communicates with its immediate neighbors (dependencies), not reaching through them to access other objects.
• SRP (Single Responsibility Principle): A class should have only one responsibility, meaning it should have only one reason to change.
• OCP (Open/Closed Principle): Software entities should be open for extension (adding new behavior) but closed for modification (changing existing code).
• LSP (Liskov Substitution Principle): Objects of a superclass shall be replaceable with objects of its subclasses without breaking the application.
• ISP (Interface Segregation Principle): Clients should not be forced to depend on interfaces they do not use; prefer smaller, specific interfaces.
• DIP (Dependency Inversion Principle): Depend on abstractions (interfaces), not on concrete implementations, to allow for easier testing and flexibility.