In the realm of software architecture, **low-level design** focuses on the detailed implementation aspects of the system. While high-level design provides a broad overview of the architecture, low-level design dives into the specifics, including class definitions, methods, and their interactions.   
  
This chapter will explore how to project classes effectively, using the e-commerce platform as our primary example.

## Key Principles of Class Design
In low-level design each class handles a specific part of the business logic, working independently while integrating seamlessly into the larger system. 

Below are key principles for effective class design, illustrated through the e-commerce domain.

### Single Responsibility Principle (SRP)  
A class should have only one reason to change, meaning it should focus on one specific responsibility. For example, in an e-commerce system, the `Order` class manages orders, including the items purchased and the status of the order. The `Payment` class, on the other hand, handles only the payment process. If the way orders are handled changes, the `Payment` class remains unaffected, ensuring separation of concerns.

### Open/Closed Principle (OCP)  
A class should be open for extension but closed for modification. This means that new functionality can be added to a class without changing its existing code. For instance, if the platform wants to add a new payment gateway, the system can introduce a new class (e.g., `StripePayment`) that extends the `Payment` interface—without modifying the core `Payment` processing code.

### Liskov Substitution Principle (LSP)  
Subtypes should be substitutable for their base types without altering the behavior of the system. For example, if the platform has a `Product` interface, both `PhysicalProduct` and `DigitalProduct` should be interchangeable in any part of the system that relies on `Product`. This ensures that adding new product types does not break existing functionality.

### Interface Segregation Principle (ISP)  
Interfaces should be small and specific to avoid forcing classes to implement unnecessary methods. In the e-commerce example, instead of one large `OrderInterface` covering all possible operations (like `cancelOrder()`, `shipOrder()`, and `returnOrder()`), the system could have smaller interfaces such as `Cancelable`, `Shippable`, and `Returnable`. This ensures that each class only implements what it needs.

### Dependency Inversion Principle (DIP)  
High-level modules should not depend on low-level modules; both should rely on abstractions. In the e-commerce platform, the `OrderService` class should not depend directly on a specific payment provider like `Stripe`. Instead, it should depend on a `PaymentGatewayInterface`. This allows switching payment providers (e.g., to PayPal) without changing the core logic in `OrderService`.

## Example

Below is the design of key classes and interfaces for the e-commerce platform using SOLID principles. This table provides an overview of responsibilities, associations, and behaviors for each class or interface.



### Single Responsibility Principle (SRP)  
Each class or interface focuses on a specific task. For example, the `Order` class only manages order-related data, while the `PaymentGateway` interface deals solely with payment processing, ensuring that responsibilities are clearly separated.

### Open/Closed Principle (OCP)  
Adding new functionality—like integrating PayPal—does not require changes to the existing `OrderService` class. Instead, a new class (e.g., `PayPalPayment`) can implement the `PaymentGateway` interface without altering the core code.

### Liskov Substitution Principle (LSP)  
Both `PhysicalProduct` and `DigitalProduct` implement the `Product` interface, ensuring they can be used interchangeably wherever a `Product` is required. This guarantees that adding new product types won't break the existing system.

### Interface Segregation Principle (ISP)  
The `PaymentGateway` interface only defines essential payment methods, ensuring that unrelated classes are not forced to implement unnecessary methods. This keeps each class focused on what it needs to do.

### Dependency Inversion Principle (DIP)  
High-level components like `OrderService` depend on the abstraction provided by the `PaymentGateway` interface rather than specific payment providers. This design allows the payment provider to be switched easily (e.g., from Stripe to PayPal) without impacting the core business logic.

### Flexibility and Maintainability  
- **Adding new product types**: introducing a new type (like `SubscriptionProduct`) only requires creating a new class that implements the `Product` interface; 
- **Switching payment providers**: implementing a new class (e.g., `PayPalPayment`) allows the system to use a different payment provider without changing core logic.  

This structure ensures a **scalable and maintainable design**, following the principles of modularity and separation of concerns. It allows the platform to evolve over time without introducing unnecessary complexity or coupling between components.

In the revised example, how does the OrderService class interact with the Customer class in compliance with the Law of Demeter?


Software architecture is essential for building scalable, maintainable, and high-performing software systems. This course provides a comprehensive introduction to core design principles and practices, covering both high-level and low-level concepts. We will explore key architectures and modern development techniques, gaining the skills needed to design effective solutions for real-world projects.

Now you will be introduced to the core principles of system design and why it plays a critical role in software development. You will explore what system design entails, including the process of planning and structuring software to meet business and technical requirements. Key components such as architecture, scalability, performance, and security will be covered to provide a solid foundation for evaluating and building effective systems. You will also learn the distinction between high-level design (which focuses on the overall system structure) and low-level design (which deals with detailed components like classes, methods, and data structures).

Here you will explore various architectural patterns that define the overall structure of software systems, including monolithic architecture, service-oriented architecture (SOA), microservices, and onion architecture. You will analyze the strengths and weaknesses of each approach and learn when to apply them in different scenarios. By the end of this section, you'll have a solid understanding of high-level design principles that can guide your software development decisions.

Let's dive into low-level design concepts, focusing on the creation of class diagrams to represent system structure and the design of efficient methods and functions for reusability. You will also explore various data structures and best practices for database design, equipping you with the skills to build detailed and effective software designs that enhance system performance and maintainability.

This section introduces modern design methodologies, including Test-Driven Development (TDD), where you'll learn to write tests before code to ensure reliability. You'll also explore Domain-Driven Design (DDD), focusing on aligning software with business domains, and Behavior-Driven Development (BDD), which emphasizes collaboration and user-centered design through behavior-based testing. These approaches will provide practical techniques to enhance software quality and business alignment.

Software Architecture Fundamentals

Class Design