Software Design Principles

Good software makes things work well over time.

Developers benefit from software design principles as follows:-

Developers achieve this while avoiding unnecessary complexity and costly errors.

This guide organizes key software design principles into relevant categories.

Quick Navigation

    1. Architectural Principles
    2. Concurrency & Performance
    3. Security & Reliability
    4. Testing & Maintainability
    5. Emergent & Modern Practices
    6. Philosophical Principles
    7. General Object-Oriented Principles
    8. Human-Centric & Language-Aware Principles
    9. Patterns as Principles
    10. Documentation & Communication

For a quick read, here's the summarized overview.

1. Architectural Design Principles

Architectural principles structure systems at a high level.

Further, they define how components interact and ensure maintainability, flexibility, and scalability.

Separation of Concerns (SoC)

Break a program into distinct features that overlap as little as possible. It improves modularity and clarity.

Single Responsibility Principle (SRP)

Each module or class should be responsible for a single part of the functionality and should encapsulate that part.

Don't Repeat Yourself (DRY)

Eliminate duplication by abstracting repeated logic or data.

Keep It Simple, Stupid (KISS)

Favor simplicity in design to reduce potential bugs and increase readability.

You Aren't Gonna Need It (YAGNI)

Don't implement something until it is necessary. It helps avoid unnecessary complexity.

Design for Testability

Structure code so it can be easily tested, which often leads to better overall design decisions.

Favor Composition Over Inheritance

Use composition to build complex behavior from simpler parts rather than relying heavily on inheritance hierarchies.

Encapsulation

Hide the internal state and require all interaction to be performed through an object's methods.

2. Object-Oriented Design (SOLID Principles)

The SOLID Principles make OOP designs more understandable, flexible, & maintainable.

Single Responsibility Principle (SRP)

A class should have one reason to change. It should only have one job.

Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification. This allows you to add new functionality without changing existing code.

Liskov Substitution Principle (LSP)

Objects of a superclass should be replaceable with objects of its subclasses without altering the correctness of the program.

Interface Segregation Principle (ISP)

No client is forced to depend on methods it doesn't use. Favor small, specific interfaces over large, general-purpose ones.

Dependency Inversion Principle (DIP)

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

3. Functional Programming Principles

Focus on immutability, pure functions, and statelessness to enable safer, more predictable programs.

Immutability

Once data is created, it cannot be changed. It prevents side effects.

Pure Functions

Functions that always return the same result given the same input and have no side effects.

First-Class Functions

Functions are assigned to variables, & passed as arguments. Or sometimes returned from other functions.

Function Composition

Combine smaller functions to build more complex operations.

Referential Transparency

Expressions can be replaced with their corresponding values without changing the program's behavior.

4. Concurrency and Parallelism Principles

Designing software to work efficiently and correctly in concurrent or parallel environments.

Shared Nothing Architecture

Components do not share state, helping to avoid race conditions.

Immutability

Also relevant here, as immutable objects can be safely shared across threads.

Message Passing

Communicate between threads or processes by passing messages rather than sharing state.

Avoid Global State

Global variables can be accessed from anywhere, leading to potential data corruption in concurrent environments.

5. Testing and Reliability Principles

These principles ensure software is verifiable, reliable, and easy to maintain.

Test-Driven Development (TDD)

Write tests before code. It helps define clear objectives and ensures that the code is testable.

Design by Contract

Define formal, precise, and verifiable interface specifications for software components.

Fail Fast

Let systems report errors as early as possible so problems are easier to trace.

Loose Coupling and High Cohesion

Highly cohesive modules do one thing well. Loosely coupled modules depend minimally on each other.

Defensive Programming

Anticipate and handle possible failures gracefully.

6. Maintainability and Readability Principles

Focus on creating code that's easy to read, understand, and extend over time.

Self-Documenting Code

Write code that explains itself through meaningful naming and structure.

Code Comments and Documentation

Use comments to explain why code exists, not what it does (when the code is already readable).

Consistent Naming Conventions

Use clear, predictable naming patterns for variables, functions, and files.

Refactoring

Regularly revise code to improve internal structure without changing behavior.

Avoid Premature Optimization

Optimization is only needed when performance constraints are present and required.

7. Modern Development Practices

They are at the core of today's software development landscape. As a result, they advocate for :~

1) Agility
2) Rapid iteration, &
3) Resilience

Continuous Integration/Continuous Deployment (CI/CD)

Automate testing and deployment to catch issues early and deploy often.

Infrastructure as Code (IaC)

Define infrastructure configurations in code for repeatability and version control.

12-Factor App Methodology

A set of best practices for building scalable, maintainable web apps.

Microservices Architecture

Create small, stand-alone services that interact via well-defined APIs.

Observability

Design systems with monitoring, logging, and tracing built-in for easier debugging and maintenance.

8. Security Principles

Embed security into the foundation of software design to minimize vulnerabilities.

Principle of Least Privilege

Each part of the system should operate using the minimum set of privileges necessary.

Fail Securely

When a system fails, it should do so without compromising security.

Avoid Security Through Obscurity

Do not rely solely on keeping implementation details secret for security.

Input Validation

Always validate input from users or external systems.

Secure Defaults

Systems should be secure by default, not by configuration.

9. API and Interface Design Principles

Ensure that external interfaces to your software are intuitive, stable, and well-structured.

Consistency

Follow consistent patterns across APIs.

Versioning

Don't break existing clients; introduce new versions for breaking changes.

Error Handling

Return informative and structured error messages.

Documentation First

Define and document APIs before implementation.

Least Astonishment

API behavior should match user expectations.

10. UX-Centric Principles for Devs

While not purely technical, these principles help bridge development and user experience.

Progressive Disclosure

Show only what's necessary at any point in time.

Graceful Degradation

Ensure the app remains functional even when some features fail.

Accessibility

Design for users with diverse abilities.

Responsive Design

Ensure applications work across different devices and screen sizes.

When understood and applied thoughtfully, they should help you write software that is robust, extensible, and delightful to maintain.

Don't aim to use them all at once—choose the right tools for the context and constraints of your project.

When to Apply These?

- For APIs: Postel's Law, CQS, Idempotence
- For Microservices: SoC, EDA, 12-Factor
- For Security: Least Privilege, Secure by Default
- For Legacy Code: Boy Scout Rule, KISS

What's Next? Start Applying These Principles Today!

Software design principles aren't just theory—they help you write better code every day.

Start with one principle at a time, refactor your existing projects, and see how your code improves.