Why SOLID principles guide maintainable design in LLD - Design It to Understand It

Design: Maintainable Software Design Using SOLID Principles

Focus on explaining how SOLID principles help create maintainable software design. Out of scope are specific programming language implementations or detailed code examples.

Functional Requirements

FR1: Design software components that are easy to understand and modify

FR2: Ensure components can be extended without changing existing code

FR3: Promote code reuse and reduce duplication

FR4: Allow independent development and testing of components

FR5: Support clear separation of concerns

Non-Functional Requirements

NFR1: Code changes should minimize impact on unrelated parts

NFR2: Design should support scaling team size and codebase

NFR3: Maintain low complexity to reduce bugs and improve readability

NFR4: Design should enable easy debugging and troubleshooting

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Single Responsibility Principle (SRP)

Open/Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Interface Segregation Principle (ISP)

Dependency Inversion Principle (DIP)

Design Patterns

Modular design

Abstraction and interfaces

Loose coupling

Encapsulation

Code reuse

Reference Architecture

 +---------------------+       +---------------------+       +---------------------+
 |  Single Responsibility|       |  Open/Closed        |       |  Liskov Substitution |
 |  Principle (SRP)      |       |  Principle (OCP)    |       |  Principle (LSP)     |
 +----------+----------+       +----------+----------+       +----------+----------+
            |                             |                             |
            v                             v                             v
 +---------------------+       +---------------------+       +---------------------+
 | Interface Segregation|       | Dependency Inversion |       |  Maintainable Design |
 | Principle (ISP)      |       | Principle (DIP)      |       +---------------------+
 +---------------------+       +---------------------+

Components

Single Responsibility Principle (SRP)

Conceptual principle

Ensure each module/class has one reason to change, making code easier to understand and modify.

Open/Closed Principle (OCP)

Conceptual principle

Design modules to be open for extension but closed for modification, enabling new features without breaking existing code.

Liskov Substitution Principle (LSP)

Conceptual principle

Guarantee that subclasses can replace base classes without altering correctness, supporting reliable polymorphism.

Interface Segregation Principle (ISP)

Conceptual principle

Create specific interfaces so clients only depend on what they use, reducing unnecessary dependencies.

Dependency Inversion Principle (DIP)

Conceptual principle

Depend on abstractions, not concrete implementations, to reduce coupling and increase flexibility.

Request Flow

1. 1. Identify responsibilities in the system and assign each to a separate module (SRP).

2. 2. Design modules with abstract interfaces that allow adding new features without changing existing code (OCP).

3. 3. Ensure subclasses can be used wherever their base classes are expected without errors (LSP).

4. 4. Split large interfaces into smaller, client-specific ones so modules depend only on needed methods (ISP).

5. 5. Make high-level modules depend on abstractions rather than concrete classes to allow easy swapping of implementations (DIP).

6. 6. Together, these steps create a system where changes affect minimal parts, enabling easier maintenance and extension.

Database Schema

Not applicable as this is a design principle explanation without specific data storage requirements.

Scaling Discussion

Bottlenecks

Tightly coupled code making changes risky and error-prone

Large classes or modules with multiple responsibilities causing confusion

Rigid code that breaks when adding new features

Interfaces forcing clients to depend on unused methods

High-level modules depending on low-level concrete implementations reducing flexibility

Solutions

Apply SRP to split responsibilities into focused modules

Use OCP to extend behavior via new modules rather than modifying existing ones

Follow LSP to ensure safe inheritance and polymorphism

Implement ISP to create fine-grained interfaces tailored to clients

Adopt DIP to invert dependencies and rely on abstractions

Interview Tips

Time: Spend 10 minutes explaining each SOLID principle with simple examples, 10 minutes discussing how they improve maintainability, and 5 minutes summarizing benefits and answering questions.

Explain each SOLID principle clearly and simply

Show how principles reduce complexity and bugs

Describe how principles support team collaboration and scaling

Give relatable examples of maintainability problems solved by SOLID

Highlight the importance of abstraction and loose coupling