SOLID Principles

Let’s understand the necessity of SOLID principles first.

Your client asked you to create a basic e-commerce website with essential features like searching for products, adding them to a cart, and purchasing them. You set up the system with a single ECommerceSystem class that handles everything—searching for products, managing the cart, processing payments, and even inventory updates.

It works perfectly at first, but as your client starts asking for new features and improvements, chaos begins:

1. Adding a Discount Feature:
   When the client requests discounts for festive seasons, you add discount logic directly into the ECommerceSystem class. Now, every time there’s a new discount type (percentage, fixed, or buy-one-get-one), you have to modify the class, risking breaking the existing functionality.

2. Introducing Multiple Payment Methods:
   The client wants to include credit cards, wallets, and UPI payments. Since all payment logic is embedded in one place, adding new payment options means modifying the same class repeatedly. Bugs keep creeping in with every new update.

3. Expanding Product Categories:
   Adding new categories like electronics and fashion requires you to change the product search logic. As the logic grows more complex, it becomes harder to maintain.

4. Scaling Notification Services:
   Initially, you used email notifications for order confirmations. Now the client wants SMS and push notifications. Since notification logic is hardcoded, it’s nearly impossible to integrate new channels without overhauling the system.

5. Team Collaboration:
   With everything packed into a single class, multiple developers working on different features keep overwriting each other’s changes. Deadlines are missed, and the project becomes unmanageable.

This chaos results in:

• Frequent bugs and downtime.

• Difficulty in adding new features.

• High maintenance costs.

• Poor scalability.

Enter SOLID Principles
By applying SOLID principles, you can update the system as per client needs and avoid such chaos. Each principle addresses a specific aspect of software design, ensuring that your code is maintainable, scalable, and easier to extend.

1. Single Responsibility Principle (SRP)

A class should have only one reason to change, meaning it should have only one job.
Example: A ProductInventory class should handle only inventory-related operations, while a ProductDetails class manages product descriptions and images.

2. Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.
Example: A payment processing system can be extended to support new payment methods (e.g., UPI or cryptocurrency) without modifying the core logic.

3. Liskov Substitution Principle (LSP)

Objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.
Example: If Discount is a parent class, a PercentageDiscount subclass should work seamlessly wherever Discount is used, such as applying discounts at checkout.

4. Interface Segregation Principle (ISP)

A class should not be forced to implement interfaces it does not use.
Example: A PaymentProcessor interface can have smaller interfaces like CreditCardProcessor and WalletProcessor , so a class only implements what it needs.

5. Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules; both should depend on abstractions.
Example: Instead of directly coupling an EmailService class to send order confirmations, use an NotificationService interface that can be implemented for email, SMS, or push notifications.