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Understanding SOLID Principles in Low-Level Design

· 3 min read
Sirasudeen P
Sirasudeen P
Software Engineer

"Good software is understandable, extensible, and maintainable. That's exactly what SOLID aims to achieve."

As I started diving into Low-Level Design (LLD) through TUF+ and hands-on practice, one of the first core topics I focused on was the SOLID principles, the five fundamental guidelines that help make object-oriented design cleaner, more maintainable, and more scalable.

Here’s a simple breakdown of what I learned:

🔹 1. Single Responsibility Principle (SRP)​

A class should have one and only one reason to change.

Meaning: Each class should do only one job, not multiple.

Example:
Instead of one Employee class doing database and email work, I split it into:

  • Employee – holds data
  • EmployeeRepository – handles DB logic
  • EmailService – sends mails

This makes each class smaller, testable, and focused.

🔹 2. Open/Closed Principle (OCP)​

Classes should be open for extension but closed for modification.

Meaning: You should be able to add new features without changing existing code.

Example:
Instead of a Shape class using if conditions for area logic, I used a Shape interface and let each shape (Circle, Rectangle) implement its own area() method.

When I added a Triangle, no existing code broke. I just added a new class.

🔹 3. Liskov Substitution Principle (LSP)​

Subclasses should be substitutable for their base classes.

Meaning: Derived classes must behave as expected when used as base types.

Anti-Example: Making an Ostrich extend Bird and overriding fly() with an exception violates LSP.

Fix: I split Bird into Bird (for common behavior) and FlyingBird, so Ostrich doesn’t pretend to fly.

🔹 4. Interface Segregation Principle (ISP)​

Clients should not be forced to depend on interfaces they don't use.

Meaning: Split large interfaces into smaller ones.

Anti-Example: A Worker interface with both work() and eat() but robots don’t eat!

Fix: Created separate Workable and Eatable interfaces. Now robots implement only what they need.

🔹 5. Dependency Inversion Principle (DIP)​

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Meaning: Code to interfaces, not concrete classes.

Bad: App directly depends on MySQLDatabase.

Good: Created a Database interface and passed MySQLDatabase via constructor — now it’s easy to switch to MongoDB or mock in tests.

Final Thoughts​

Learning and applying SOLID has transformed how I write code. I used to just make things “work” and now I try to make them clean, modular, and extendable.

Next, I’m going to explore:

  • Design Patterns (Factory, Strategy, Observer, etc.)
  • Real-world LLD case studies like Splitwise & Parking Lot
  • Class & Sequence diagrams for better system modeling

You can check out my GitHub repo where I’m storing all my code examples 👉 lld-patterns GitHub

Thanks for reading! If you’re also learning LLD, feel free to reach out or suggest improvements.