Beyond SOLID:The Package Principles

Red Panda Innovation LabsCode Words 2016-04-22

Christian HujerCEO / CTO Nelkinda Software Craft Pvt Ltd

Agenda1.The Four Design Smells2.Origins and Reprise of SOLID3.ACCESS / CASE - The Package Principles

a.Package Cohesion Principlesb.Package Coupling Principlesc.Mnemonics

DefinitionsSoftware Entity

Statement / line / method / class / package / serviceComponent

Logical (model / architecture / design) group of Software EntitiesPackage

Physical (e.g. directory, gem, jar) group of Software EntitiesCollocation

Putting things in the same Component / PackageCohesion

Things that are changed togetherCoupling

Dependency that potentially propagates change

1.The Four Design Smells

The Four Design Smells●Rigidity●Fragility●Immobility●Viscosity

RigidityWhat is Rigidity?

Existing Code that causes Effort (known Effort)What causes Rigidity?

Coupling

Fragility●What is Fragility?

○Existing Code that causes Risk (unknown Effort)●What causes Fragility?

○Coupling○Lack of Cohesion

ImmobilityWhat is Immobility?

The difficulty (extra Effort) to reuse something.Rigidity and Fragility in the context of reuse.

What causes Immobility?CouplingLack of Cohesion

ViscosityWhat is Viscosity?

Continuous extra Effort (Time)What causes Viscosity?

CouplingBad development setup / tools

2.The SOLID Principles

A short reminder

SRP - Single ResponsibilityOCP - Open-ClosedLSP - Liskov SubstitutionISP - Interface SegregationDIP - Dependency Inversion

Reminder of SOLID

Design Principles and Design Patterns (2000)Mentions 10 principles● Principles of Object Oriented Class Design:

OCP, LSP, DIP, ISP● Principles of Package Architecture:

REP, CCP, CRP, ADP, SDP, SAP

Origin of SOLID

3.The Package Principles

The Package PrinciplesPackage Cohesion Principles

REP - Release Reuse EquivalencyCCP - Common ClosureCRP - Common Reuse

Package Coupling PrinciplesADP - Acyclic DependenciesSDP - Stable DependenciesSAP - Stable Abstractions

3.a.Package Cohesion

Principles

Release Reuse Equivalency Principle

“The granule of reuse is the granule of release.”

Reuse === ReleaseBundler / Gems, NPM, Jars / Maven, RPM, APT, Brew...

Common Closure Principle (Cohesion) “Classes that change together belong together.”

⇒ This also is a nice pragmatic definition of cohesion and that cohesive elements should be collocated.I call it: Cohesion Collocation Principle

Common Reuse Principle (Cohesion)

“Classes that aren’t reused together should not be grouped together.”

This is really the inversion of the idea that “Classes that change together belong together.”

3.b.Package Coupling

Principles

Acyclic Dependencies Principle“The dependencies between packages must not form cycles.”

Stable Dependencies Principle“Depend in the direction of stability.”

Stable Abstractions Principle“Stable packages should be abstract packages.”

3.c.Mnemonics

How to remember all this stuff?!

ACCESSYour new mnemonic acronymCombine to SOLID ACCESS

ACCESS Mnemonic AcronymADP - Acyclic DependenciesCCP - Common Closure (Cohesion Collocation)CRP - Common ReuseREP - Release Reuse EquivalencySAP - Stable AbstractionsSDP - Stable Dependencies

CASEA shorter mnemonic acronym

Combine to SOLID CASE

CASE - A new mnemonic acronymCohesion / Common

CCP / CRP Common Closure / ReuseAcyclic

ADP Acyclic DependenciesStability

SAP / SDP Stable Abstractions / DependenciesEquivalence

REP Release Reuse Equivalency

Related TopicsSoftware Architecture and DesignSoftware MetricsClean CodeRefactoringTDD, ATDD and BDDAgile Methods, Scrum, Extreme ProgrammingContinuous Integration / Delivery / Deployment, DevOpsSoftware CraftsmanshipLaw of Demeter / Tell Don’t AskCQS - Command Query SeparationProgramming Paradigms (Functional, Structured, Object-Oriented, Object-

Functional, Logic)

References● SOLID and Package Principles, Design Smells:

Design Principles and Design Patterns (Robert C. Martin, 2000)

● ACCESS mnemonic: Christian Hujer● CASE mnemonic: Christian Hujer and Mark Burns

Thank you!Questions?

Follow me on Twitter:@christianhujer