Software Design Principles

for Evolving Architectures

Firat Atagun

Principles

DRY – Don’t repeat yourself.

Every knowledge in a system must have a single authoritative, unambiguous representation. – Pragmatic Programmers

YAGNI – You aren’t gonna need it.

Embrace principle of last responsible moment

Postpone decisions until you can’t postpone them any further.

Courage is postponing the decisions tomorrow to tomorrow. – Kent Beck.

KISS- Keep it simple and stupid

Simple keeps focus

Inherent & Accidental Complexity

Good software design hides inherent complexity and avoids accidental complexity.

Principle of Cohesion

High cohesion & Low Coupling

Removing and reducing coupling creates better design.

Avoid long methods

• Hard to test• Hard to reason• Hard to debug• Hard to change• Increases coupling, decreases cohesion

SLAP – Single level of abstraction principle and compose method pattern to the rescue.

Principle of least knowledge (Law of Demeter)

Consumer of a method or a class shouldn’t know the inner implementation details.

SOLID Principles

Single Responsibility Principles: A class or method should have only one reason to change.

Open-Closed Principle: A class should be open for extension but closed for modification.

Liskov Substitution Principle: Classes that implements the same interface or base class can be substituted.

Interface Segregation Principle: Interfaces should be specialized but not too special.

Dependency inversion (Inversion of control): Depend on abstractions not implementations.

Architecture

What is architecture?

Architecture is set of design decisions that are hard to change.

Shared understanding of a system between the people who are leading the project.

“Describe me with 4 objects how system works” – Kent Beck.

Avoid big up front design

Time

Understanding

Features based on Value and Architectural impacts

Reversibility

Last Responsible Moment

YAGNI

Minimize Framework and Libraries

Users/ External Quality

Software/ Internal Quality

Design Stamina

ArchitecturalPatterns

Presentation Layer

Business Layer

Service Layer

Persistence Layer

Closed

Closed

Open

Layered Architecture

Monolith

Feature based Slicing

F1 F2 F3 F4 .. FN

Cross cutting concerns

Microkernel Architecture

Core System

Plugin

Plugin

PluginPlugin

Event Driven Architecture

Event Processors

ESB, Queue, Hub

Mediator Topology

Event Driven Architecture

Event processors.

Broker Topology

Microservices Architecture

Layered Event Driven Microkernel MicroService

Agility

Deployment

Testability

Performance

Scalability

Development

Comparison