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Principled N-Tier Design

by Steven Smith on Oct 07, 2012

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Separating applications into separate layers or tiers has long been touted as an architectural best practice, but the typical naïve approach can result in tight coupling and a heavy dependence on the database. Alternative approaches to layered architectural design do not suffer from this limitation, allowing for loosely coupled, testable solutions that can swap alternative infrastructure implementations in and out of the application.

In this session, we’ll examine the traditional N-Tier design and its deficiencies, and introduce an alternative based on the Ports and Adapters or Onion architectural pattern, and we’ll show how to set up a solution from scratch to take advantage of this approach.

Learn more here:
http://pluralsight.com/training/Courses/TableOfContents/n-tier-apps-part1

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Principled N-Tier Design Presentation Transcript

Principled N-Tier Design or,a Solution to the Solution Problem Steve Smith @ardalis | ardalis.com Telerik, Inc.

The Problem

Solution Level Pain• Too Many Projects• Too Few Projects• Incorrect Abstraction• The “Goldilocks” Solution – “Just right”

Relevant Principles• Common Closure Principle• Common Reuse Principle• Stable Dependencies Principle• Stable Abstractions Principle• Separation of Concerns• Dependency Inversion Principle

Common Closure PrincipleClasses that change together are packagedtogether.

Common Reuse PrincipleClasses that are used together are packagedtogether.

Stable Dependencies PrincipleDepend in the direction of stability.

Stable Abstractions PrincipleAbstractness increases with stability. Abstractness Stability

More Principles• Separation of ConcernsEstablish boundaries to separate behaviors andresponsibilities within a system.• Dependency InversionDepend on abstractions, not concreteimplementations.

A Simple Guestbook ApplicationDEMO

Testability• Testability correlates with: – Loose coupling – Separation of Concerns – Maintainability• You don’t have to have unit tests to have testable code! – Unit tests prove (or disprove) testability

Adding “Unit” TestsDEMO

http://smtp4dev.codeplex.com/SMTP4DEV

Incremental Improvement• From a Single Project – Separate responsibilities using folders – Add Project(s)• From Many Projects – Create a new Solution with only what you need – Spin off separate applications into own solutions

Separation via Folders (still 1 project)DEMO

Problems• Tests are still not unit tests – Tight coupling – Static Cling – Dependence on Data and Email Infrastructure concerns• Nothing to prevent improper referencing of classes – e.g. UI code can call DAL code directly and bypass business logic• UI Content Folders mixed with Application Code Folders

Onion Architecture• aka Ports and Adapters,• aka Hexagonal Architecture• Core models and interfaces at center• Infrastructure and Implementation depends on Core• UI depends on Core and has access to Infrastructure

Onion Architecture

Introduce Abstractions and Invert Dependencies (in folders)DEMO

Results• Yay! Unit Tests that work!• Boo! Still nothing to prevent improper referencing of classes – e.g. UI code can call DAL code directly and bypass business logic• Boo! Still mixing content and code folders

Refactor into ProjectsDEMO

Results• Testable Code• Separation of Concerns• Loose Coupling• Minimal logic in UI layer A solid foundation for a maintainable application!

Automate• One Click Build• One Click Test

Solution Automation – Build ScriptsDEMO

More TestsDEMO

References• Separation of Concerns - http://bit.ly/zWujRe• Hexagonal Architecture - http://bit.ly/8Uxl5• Onion Architecture - http://bit.ly/4tWMT3• More Onion Architecture - http://bit.ly/NzF2Sz• New is Glue - http://bit.ly/Ijn98ePluralsight Resources• N-Tier Application Design in C# http://bit.ly/Msrwig• Design Patterns Library http://bit.ly/vyPEgK• SOLID Principles of OO Design http://bit.ly/OWF4la

Thanks!• Find me at http://ardalis.com• We’re hiring developers and trainers at Telerik!• http://telerik.com/company/careers.aspx