In modern technology, the art of system design stands as a pivotal process in the creation of successful, resilient software applications. System design is not merely about choosing the right hardware or software components; it’s about architecting a system that meets current requirements and is also poised for future growth and challenges. This article dives into the 7 core principles system design, providing insights into crafting robust and scalable software architectures.
Understanding System Design
System design is a broad term that encompasses the process of defining the architecture, components, modules, interfaces, and data for a system to satisfy specified requirements. It is a problem-solving discipline that applies both technical knowledge and abstract design principles to create solutions that are scalable, reliable, and maintainable.
7 Key Principles of System Design
System design principles are essential guidelines that help in creating software architectures that are maintainable, scalable, and easy to understand. Among these principles, seven stand out for their foundational role in promoting good design practices, among which 5 come from OOP’s SOLID principles:
1. Single Responsibility Principle (SRP)
A class should have only one reason to change, meaning it should have only one job or responsibility. This principle simplifies maintenance and enhances the cohesiveness of the class. For example, consider a User class that handles user-related properties and a UserPersistence class that manages database operations for the User class. Separating these concerns adheres to SRP, as opposed to a single class handling both user properties and database operations.
2. Open/Closed Principle (OCP)
Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This means you can add new functionality without changing the existing code, which helps in maintaining stability while extending the system’s capabilities. For example, when implementing a reporting system where new types of reports can be added without altering the existing report generation code. You can achieve this by creating a common interface for reports and then extending it for different types of reports.
3. Liskov Substitution Principle (LSP)
Objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program. This principle ensures that a subclass can stand in for its superclass. If you have a Bird class with a method fly(), all subclasses of Bird (like Sparrow and Ostrich) should implement fly(). However, since ostriches don’t fly, this design violates LSP. A solution could be to create a more general class like Animal and separate Bird into FlyingBird and NonFlyingBird.
4. Dependency Inversion Principle (DIP)
High-level modules should not depend on low-level modules. Both should depend on abstractions. Additionally, abstractions should not depend upon details; details should depend upon abstractions. This principle reduces the coupling between modules. For example, in a payment system, a high-level module PaymentProcessor should not directly depend on a low-level module like CreditCardPayment. Instead, both should depend on an IPaymentMethod interface. CreditCardPayment implements IPaymentMethod, and PaymentProcessor interacts with IPaymentMethod, allowing for the easy addition of new payment methods.
5. Interface Segregation Principle (ISP)
Clients should not be forced to depend on interfaces they do not use. This principle encourages creating specific interfaces rather than one general-purpose interface. For example, instead of a single, bulky IWorker interface with methods for working and eating, create two interfaces: IWorkable and IEatable. Workers will implement IWorkable, and if they eat, they also implement IEatable. This way, classes only implement the interfaces that are relevant to them.
In addition to these five SOLID principles, there are two more principles we need to follow:
6. Law of Demeter (LoD)
LoD, or in other words Principle of Least Knowledge, means a module should not know the inner details of the objects it manipulates. This principle suggests that an object should only talk to its immediate friends and not to strangers or objects that are several layers away. For example, if you have an object A that calls a method on object B, which in turn calls a method on object C, to adhere to LoD, A should not directly call C. Instead, B should provide a method that abstracts its interaction with C.
7. Composition Over Inheritance (CARP)
CARP, also known as “Composite Reuse Principle”, favors object composition over class inheritance to achieve code reuse. This principle suggests that combining simple objects into complex ones can create better modular and flexible systems than using rigid inheritance hierarchies. For example, instead of having a Car class inherit from a Vehicle class to reuse code, Car can be composed of objects like Engine, Wheels, and Seats, which are used for defining its features and behaviors. This approach provides more flexibility in how features are combined and reused across different parts of the application.
By adhering to these seven principles, we can create software that is more robust, flexible, and maintainable. These principles encourage good practices that lead to code that is easier to understand, extend, and refactor, ultimately resulting in higher-quality software systems.
