Solid Principles

Imagine you join a project.

The project has been running for three years.

The codebase contains:

500,000+ lines of code

200+ classes

50+ developers have touched it

Hundreds of features

You receive a simple task:

Add a payment provider

Sounds easy.

You begin coding.

Suddenly:

Payment module breaks.

Invoice module breaks.

Notifications stop working.

Refunds fail.

Tests fail.

You wonder:

Why is everything connected?

This situation is extremely common.

Many systems begin like this:

Simple
Clean
Easy to Understand

A few years later:

Complex
Fragile
Rigid
Difficult to Modify

This phenomenon is known as:

Software Design Rot

or

Architectural Decay

The industry faced this problem for decades.

Object-Oriented Programming helped.

Design Patterns helped.

UML helped.

But developers still created systems that became:

Hard to Maintain
Hard to Extend
Hard to Test

The solution emerged through a collection of design principles known today as SOLID.

The Central Problem of Software Engineering

Most beginners think software engineering is about:

Writing Code

Professionals know:

Maintaining Code

Is the real challenge.

Consider:

Version 1.0

Development 3 Months

Maintenance 5 Years

Most software costs occur after release.

The challenge becomes:

How do we make future changes inexpensive?

The Reality of Software

A critical mindset shift:

Beginners think:

Software Is Built Once

Reality:

Software Constantly Evolves

Example:

Version 1.0
Version 1.1
Version 2.0

Every version introduces:

New Features
Bug Fixes
Business Changes
Regulatory Changes
Technology Changes

A good design survives these changes.

A bad design collapses under them.

What Is Good Software Design?

A common misconception:

Good Design = Clever Design

False.

Good design means:

Easy To Change

That is the ultimate metric.

If adding a feature require:

Changing 50 files
that's bad design

If adding a feature requires:

Changing 1 File

good design.

Characteristics of Good Design

Professional software usually exhibits:

1 Maintainability

Easy to understand.

Easy to modify.

Example:

calculateSalary()

is easier than:

processCompensationWorkflowAndTaxCalculation()

with 1000 lines.

2 Extensibility

New features can be added safely.

Example:

Adding:

New Payment Provider

should not require rewriting existing payment code.

3 Testability

Classes can be tested independently.

4 Reusability

Useful components can be reused elsewhere.

5 Flexibility

Software adapts to change.

The Cost of Poor Design

Poor design creates:

Rigid Systems

Changing one area requires changing many others.

Fragile Systems

Small changes create unexpected bugs.

Immobile Systems

Code cannot be reused. Everything is tightly coupled.

Viscous Systems

The “correct” solution is harder than the hack. Developers start writing shortcuts. Technical debt grows.

What Is SOLID?

SOLID is an acronym. Created through the work of: Rober C. Martin.

SOLID represents five design principles.

Together they provide a framework for creating:

Maintainable

Flexible

Extensible

Testable

software.

What is Solid Principles?

SOLID is an acronym representing five fundamental principles of object-oriented programming and design:

1. Single Responsibility Principle
2. Open/Closed Principle
3. Liskov Substitution Principle
4. Interface Segregation Principle
5. Dependency Inversion Principle