Core Principles of Software Engineering You Must Know

8 min readMay 31, 2024

In the dynamic world of software development, mastering core software engineering principles is essential for building robust, maintainable, and scalable applications.
These principles form the foundation of high-quality code and are vital for both novice and experienced developers.

This article explores key principles like SOLID, DRY, and KISS, among others. Understanding and applying these concepts will enhance your ability to handle complex projects, reduce bugs, and improve team collaboration. Let’s dive into the core principles of software engineering that every developer must know.

1. SOLID Principles

The SOLID principles are a set of five design principles intended to make software designs more understandable, flexible, and maintainable. These principles were introduced by Robert C. Martin and are widely accepted as essential guidelines for object-oriented programming and design.

Single Responsibility Principle (SRP)

A class should have only one reason to change, meaning it should have only one job or responsibility.

Improves code readability and maintainability
Reduces the risk of bugs
Makes code easier to understand and modify

class User {
    userName() {
        // Fetch user name
    }
}

class UserPrinter {
    printUserName(user: User) {
        console.log(user.userName());
    }
}

In this example, User handles data, while UserPrinter manages display logic, adhering to SRP.

Open/Closed Principle (OCP)

Software entities (classes, modules, functions) should be open for extension but closed for modification.

Promotes code reusability
Minimises the risk of introducing bugs when extending functionality

interface Shape {
    draw(): void;
}

class Circle implements Shape {
    draw() {
        // Draw circle
    }
}
class Square implements Shape {
    draw() {
        // Draw square
    }
}
class ShapePrinter {
    print(shape: Shape) {
        shape.draw();
    }
}

Here, new shapes can be added without modifying existing code, adhering to OCP.

Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types without altering the correctness of the program.

Ensures a robust and predictable codebase
Facilitates polymorphism

class Bird {
    fly(): void {
        // Fly
    }
}

class Sparrow extends Bird {
    // Sparrow can fly
}

class Ostrich extends Bird {
    // Ostrich cannot fly, violating LSP
}

To adhere to LSP, ensure all subclass methods align with expectations set by the base class.

Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

Promotes the creation of lean and focused interfaces
Reduces the impact of changes

interface Printer {
    print(): void;
}

interface Scanner {
    scan(): void;
}

class AllInOnePrinter implements Printer, Scanner {
    print() {
        // Print logic
    }
    
    scan() {
        // Scan logic
    }
}

class SimplePrinter implements Printer {
    print() {
        // Print logic
    }
}

Here, SimplePrinter does not implement Scanner, adhering to ISP.

Dependency Inversion Principle (DIP)

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

Increases the flexibility and reusability of code
Promotes a decoupled architecture

interface Database {
    connect(): void;
}

class MySQLDatabase implements Database {
    connect() {
        // MySQL connection logic
    }
}

class UserRepository {
    private database;

    constructor(database: Database) {
        this.database = $database;
    }

    function getUserData() {
        this.database.connect();
        // Fetch user data
    }
}

In this example, UserRepository depends on the Database interface, not on a specific database implementation, adhering to DIP.

By applying the SOLID principles, developers can create systems that are easier to maintain, extend, and understand, resulting in more robust and scalable software.

2. DRY (Don’t Repeat Yourself)

The DRY principle states that “Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.” In simpler terms, it means that you should avoid duplicating code and functionality.

Maintainability: Changes need to be made in only one place, reducing the risk of errors and inconsistencies.
Readability: Code is easier to understand when it’s not cluttered with duplicate logic.
Reusability: Shared code can be reused across multiple parts of the application, promoting cleaner and more modular design.

Common Pitfalls:

Over-Abstraction: Sometimes in an effort to apply DRY, developers might abstract too early or too much, leading to complex and hard-to-understand code.
False DRY: Duplication can occur in different contexts where it might seem similar but actually represents different concepts. Blindly merging them can lead to confusing code.

Examples:

Before Applying DRY

Consider an example where the logic for connecting to a database is repeated across different classes:

class User {
    connectDatabase() {
        // Database connection logic
    }

    getUserData() {
        this.connectDatabase();
        // Fetch user data
    }
}

class Product {
    connectDatabase() {
        // Database connection logic
    }

    getProductData() {
        this.connectDatabase();
        // Fetch product data
    }
}

After Applying DRY

By extracting the common database connection logic into a separate class, we adhere to the DRY principle:

class DatabaseConnection {
    connect() {
        // Database connection logic
    }
}

class User {
    private database: DatabaseConnection;

    constructor(database: DatabaseConnection) {
        this.database = database;
    }

    getUserData() {
        this.database.connect();
        // Fetch user data
    }
}

class Product {
    private database: DatabaseConnection;

    constructor(database: DatabaseConnection) {
        this.database = database;
    }

    getProductData() {
        this.database.connect();
        // Fetch product data
    }
}

In this refactored example, the DatabaseConnection class encapsulates the database connection logic, eliminating duplication in the User and Product classes.

Techniques to Apply DRY:

Refactoring: Regularly refactor your code to identify and eliminate duplication.
Modular Design: Design your application in a modular way where common functionality is placed in reusable modules or libraries.
Code Reviews: Conduct thorough code reviews to catch duplication and encourage adherence to the DRY principle.
Documentation: Maintain clear documentation to avoid unintentional duplication and to help developers understand existing abstractions and utilities.

By following the DRY principle, developers can create cleaner, more efficient, and maintainable codebases. It helps in reducing redundancy, making code changes easier and more reliable, and fostering better software development practices.

3. KISS (Keep It Simple, Stupid)

The KISS principle asserts that simplicity should be a key goal in design, and unnecessary complexity should be avoided. The idea is that systems work best if they are kept simple rather than made complicated.

Maintainability: Simple code is easier to maintain and extend.
Readability: Code that is straightforward is easier for others to read and understand.
Debugging: Fewer lines of code and simpler logic make it easier to identify and fix bugs.

Common Pitfalls:

Over-Engineering: Adding unnecessary features or abstractions that complicate the design.
Premature Optimization: Trying to optimize code before it’s clear that the optimization is needed.
Ignoring Readability: Writing clever but unreadable code that might seem simple to the author but is confusing to others.

Examples:

Over-Complicated Code

Consider a function that calculates the sum of an array of numbers. An over-complicated approach might involve unnecessary abstractions:

class NumberArray {
    private numbers: number[];

    constructor(numbers: number[]) {
        this.numbers = numbers;
    }

    public sum(): number {
        return this.numbers.reduce((acc, curr) => acc + curr, 0);
    }
}

const numberArray = new NumberArray([1, 2, 3, 4, 5]);
console.log(numberArray.sum());

While this code works, the NumberArray class is unnecessary for a simple summation.

Simplified Code

A simpler approach eliminates the unnecessary class and directly performs the calculation:

function sum(numbers: number[]): number {
    return numbers.reduce((acc, curr) => acc + curr, 0);
}

console.log(sum([1, 2, 3, 4, 5]));

This simplified version is easier to read, understand, and maintain.

Techniques to Apply KISS:

Avoid Over-Engineering: Focus on solving the problem at hand without adding unnecessary complexity.
Use Clear and Concise Naming: Use descriptive names for variables and functions to make the code self-explanatory.
Break Down Complex Problems: Divide complex problems into smaller, more manageable pieces.
Refactor Regularly: Continuously improve your code by refactoring to simplify and remove any unnecessary parts.
Follow Established Conventions: Adhere to best practices and coding standards that promote simplicity.

By following the KISS principle, developers can produce code that is easier to understand, maintain, and extend. Simplicity in design and implementation leads to more reliable and efficient software development processes.

4. YAGNI (You Aren’t Gonna Need It)

The YAGNI principle states that you should not add functionality until it is necessary. This principle encourages developers to focus on the immediate requirements rather than speculating on potential future needs.

Reduced Complexity: Keeping the codebase simple by avoiding unnecessary features.
Improved Focus: Concentrating on current requirements rather than hypothetical scenarios.
Faster Development: Spending time on what’s needed now, rather than implementing and maintaining unused features.

Common Pitfalls:

Speculative Generality: Adding features or abstractions that might be needed in the future, but end up unused.
Over-Engineering: Creating complex systems to accommodate potential future requirements, leading to unnecessary complexity.
Premature Optimization: Optimizing code for scenarios that might never occur, making it harder to understand and maintain.

Examples:

Before Applying YAGNI

Consider a function designed to handle multiple types of user notifications, even though the current requirement is only for email notifications:

class NotificationService {
    sendEmail(email: string, message: string) {
        // Send email logic
    }

    sendSMS(phone: string, message: string) {
        // Send SMS logic (not currently needed)
    }

    sendPushNotification(deviceId: string, message: string) {
        // Send push notification logic (not currently needed)
    }
}

const notificationService = new NotificationService();
notificationService.sendEmail('user@example.com', 'Hello, User!');

In this example, the NotificationService class includes unnecessary methods for SMS and push notifications, which are not currently needed.

After Applying YAGNI

Focus on the current requirement by only implementing email notifications:

class EmailNotificationService {
    sendEmail(email: string, message: string) {
        // Send email logic
    }
}

const emailNotificationService = new EmailNotificationService();
emailNotificationService.sendEmail('user@example.com', 'Hello, User!');

This simplified version is easier to understand and maintain, adhering to the YAGNI principle.

Techniques to Apply YAGNI:

Focus on Immediate Requirements: Implement only what is necessary to meet the current requirements.
Iterative Development: Develop in small, incremental steps, adding functionality as it becomes necessary.
Avoid Premature Optimization: Optimise only when there is a clear and present need for it.
Refactor When Necessary: Be prepared to refactor and extend the code as new requirements emerge, rather than trying to predict them in advance.

By following the YAGNI principle, developers can avoid the pitfalls of over-engineering and speculative generality, leading to more focused, efficient, and maintainable code. It encourages building only what is needed now and adapting as requirements evolve.

Understanding and applying core software engineering principles like SOLID, DRY, KISS, and YAGNI is crucial for any developer aiming to create high-quality, maintainable, and scalable software. These principles help simplify complex problems, reduce redundancy, and promote better coding practices.

By adhering to SOLID principles, you can design robust and flexible systems.
The DRY principle ensures your codebase remains clean and maintainable by eliminating redundancy.
The KISS principle emphasizes simplicity, making your code more readable and easier to manage.
Finally, the YAGNI principle helps you focus on current requirements, avoiding unnecessary complexity and over-engineering.

In the next part of this article, we will explore additional essential software engineering principles, including separation of Concerns, Encapsulation and Modularity, Code Refactoring, TDD, Version Control and CI/CD. There’s a lot to learn, so jump right in below