Relentless Refactoring

Relentless Refactoring Strategic improvement through Domain-Driven Design Mark Rickerby

@maetl maetl.net github.com/maetl

Introducing the most popular software architecture in the world...

The year 2000 in web software ,[object Object],brutalist structure organic miasma procedural PHP, Perl, ASP, etc... spaghetti code jungle J2EE framework standards indirection and bloat

The year 2000 in web software ,[object Object],brutalist structure organic miasma

[object Object],[object Object]

Design patterns ,[object Object]

Convention over configuration ,[object Object],app/controllers/CategoriesController.php app/controllers/ProductsController.php app/models/Category.php app/models/Product.php app/templates/categories/* app/templates/products/*

What is the model? ,[object Object],app/controllers/CategoriesController.php app/controllers/ProductsController.php app/models/Category.php app/models/Product.php app/templates/categories/* app/templates/products/*

Anemic models ,[object Object],— the app/models convention is a sensible default for basic CMS driven websites — Active Record fools developers into thinking that persistence is the core concept of the ‘M’ in MVC

Eventual inconsistency ,[object Object],— when the software model does not capture rich relationships and behavior, controllers and templates expedite the process of adding new functionality

Eventual inconsistency ,[object Object],app/controllers/CategoriesController.php app/controllers/CheckoutController.php app/controllers/ProductsController.php app/models/Category.php app/models/Product.php app/templates/categories/* app/templates/checkout/* app/templates/products/*

Ad-hoc development ,[object Object],— when in doubt, developers add new behavior to existing methods — bug-fixes and knowledge about system behavior accumulates in procedural code

Metaphor shear ,[object Object]

Ad-hoc duplication ,[object Object],$units = $config->defaultWeightUnits; $w = explode(' ', $product->weight); if ($w[1] != 'lbs') { $weight = convertWeight( $w[0], $units, 'lbs' ); } else { $weight = $w[0]; }

Ad-hoc behaviour ,[object Object],class Product extends ActiveRecord { public function create($imported=false) { if ($imported) { // edge case when the // product is created from // a bulk import } } }

Domain Driven Design ,[object Object],really a book about refactoring

Discover missing concepts ,[object Object],— focus on conceptual integrity — be prepared to look at things in a different way — constantly refine the language used to describe the software domain

A unified language ,[object Object],$ mv app/models app/model

A unified language ,[object Object],Entity~ persistent ‘noun’ with identity and state eg: product, category Value~ atomic object with no unique identity eg: price, currency, weight Service~ stateless ‘verb’, action or process Aggregate~ a cluster of related objects

Small values ,[object Object],app/model/Product.php app/model/Weight.php $weight = $product->weight->lbs();

Service layer ,[object Object],class ProductImporter extends BulkImporter { public function importProduct($product) { // no longer an edge case } }

Aggregate roots ,[object Object],app/model/Attribute.php app/model/AttributeValue.php app/model/Product.php app/model/ProductAttribute.php app/model/ProductDetails.php app/model/Sku.php class Product extends Aggregate {}

Benefits of a richer model ,[object Object],— easier to localize changes when behavior and data are close together — negotiate complexity with precise language — objects have a single responsibility; easier to understand and maintain

Design is continuous ,[object Object],— models are dynamic; always changing and growing — models are explanatory; define domain knowledge in code

Ambiguities ,[object Object],— avoid trying to build a monolithic model that captures every aspect of the domain — tackle ambiguity by grouping the model into Bounded Contexts

Model namespace ,[object Object],app/model/ collections /ProductList.php app/model/ entities /Product.php app/model/ services /ShippingProvider.php app/model/ services /TaxRate.php app/model/ values /Weight.php app/model/ values /Price.php

Model namespace ,[object Object],app/model/ Invoicing /TaxRate.php app/model/ Shipping /Provider.php app/model/ Storefront /Price.php app/model/ Storefront /Product.php app/model/ Storefront /ProductList.php app/model/ Storefront /Weight.php

Shared core ,[object Object],app/model/Core/Price.php app/model/Core/Weight.php app/model/Core/Product.php app/model/Invoicing/TaxRate.php app/model/Shipping/Provider.php app/model/Storefront/ProductList.php

Homonyms ,[object Object],— the same word is used to describe many different concepts — the same concept has many different responsibilities

Bounded contexts ,[object Object],app/model/Core/Price.php app/model/Core/Weight.php app/model/Core/Product.php app/model/Invoicing/TaxRate.php app/model/Shipping/Product.php app/model/Shipping/Provider.php app/model/Storefront/Product.php app/model/Storefront/ProductList.php

Anticorruption layer ,[object Object]

Refactoring is design ,[object Object]

Application boundaries ,[object Object],Introduce Strangler~ new abstraction that overtakes the host system like a strangling vine Extract Service~ spawn a separate application by extracting a service from the original system

Summary ,[object Object],Separate infrastructure from the model Use precise language to define domain concepts Model complex behaviour in bounded contexts

Relentless Refactoring

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