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Domain-Driven Design Deconstructed Domain-Driven Design Deconstructed Andrew Cassell @alc277 andrewcassell.com

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Domain-Driven Design

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Domain-Driven Design

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Domain

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Domain

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Penn State Pattee Library

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book user book_inventory book_inventory_user_link ROLE_USER ROLE_ADMIN

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Source: http://jonclaytonbiz.com/

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People don't want to buy a quarter-inch drill, they want a quarter-inch hole. Theodore Levitt

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Common Sense Software Development

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DDD !=

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DDD CRUD Correctness Testability Usability Maintainability Modifiability

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DDD BALL OF MUD / CRUD Correctness Testability Usability Maintainability Modifiability

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Ubiquitous Language

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Ubiquitous Language Developers Domain Experts

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Ubiquitous Language

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User Customer Client Member Employee

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1. Naming Things 2. Cache Invalidation 3. Off By One Errors Top 10 Reasons Programming is Hard

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CRUD/MUD DDD Users Members BookInventoryRepository LibraryCatalog String ISBN Float(7,3) DeweyDecimal

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$user = User::findOrFail($uuid);
 $book = Books::where('isbn', '=', $isbn)->take(1)->get();
 
 
 if (!$user->userBookLinks()->contains($book->id) && $user->borrowLinks->count() < 5) {
 
 $user->userBookLinks()->attach($book->id, ['date_due' => new DateTime('+2 Weeks')];
 
 }
 
 
 $user->save();

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$catalog = new LibraryCatalog(…); $book = $catalog->findOneBookByISBN($isbn); $members = new Members(…); $borrower = $members->findOneByAccountNumber($acctNumber); $borrower->borrow($book);

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$borrower->borrow($book); $borrower->loan($book); $borrower->obtain($book); $borrower->rent($book);

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@library @loan Feature: A member should be able to borrow a book Scenario: Member borrows a book Given I am authenticated as a librarian And I am on “/member/behat-123" And I press "Borrow Book" And I fill out form with: | ISBN | | 978-032-112521-7 | Then I see "Domain-driven Design"

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@library @loan Feature: A member should be able to borrow a book Scenario: Member borrows a book Given I am authenticated as a librarian And I am on “/member/behat-123" And I press "Borrow Book" And I fill out form with: | ISBN | | 978-032-112521-7 | Then I see "Domain-driven Design"

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[…], we want to establish the idea that a computer language is not just a way of getting a computer to perform operations but rather that it is a novel formal medium for expressing ideas about methodology. Thus, programs must be written for people to read, and only incidentally for machines to execute.

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DDD BALL OF MUD/CRUD Correctness ✓ Testability Usability Maintainability Modifiability

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Sales Marketing Finance Production

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Customer Lead Payee ShipTo

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Slide: Vaughn Vernon

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Billing Loan Catalog

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Billing Loan Catalog

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Billing Loan Catalog Message

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Billing Loan Catalog Domain Event

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7 Dirty Words When Meeting With a Domain Expert 1.Session 2.Repository 3.Abstract 4.Interface 5.Class 6.Database 7.Foreign Key

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User Research

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Photo: Mathias Verraes

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DDD BALL OF MUD/CRUD Correctness ✓✓ Testability Usability Maintainability Modifiability

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Domain Objects Value Objects Entities Aggregates Domain Events

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Value Object Immutable No Identity (Only Values) Book Title Amount Paid ISBN Number Entity Identifiable Mutable Lifecycle Contains Value Objects Book Librarian Invoice Aggregate Entity Responsible For Child Entities Transaction Boundary Member Library

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Book Title Dewey Decimal Number Date Borrowed ISBN Number Email Address Value Objects

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BookTitle DeweyDecimalNumber DateBorrowed ISBNNumber EmailAddress Value Objects

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Not Just Static Typing https://github.com/Fiedzia/type-system-research/blob/master/README.md Value Objects

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Immutable Avoid Spooky Action at a Distance

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ALWAYS IN A VALID STATE Value Objects

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Value Objects

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“Gateway Drug to Test Driven Development” Value Objects

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“Gateway Drug to Test Driven Development” Value Objects #MYTESTSDONTPASS

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Plain PHP Objects (POPOs) Value Objects

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Plain PHP Objects (POPOs) • Declare Class Properties as Private • No Setters • No References to Mutable Objects • Throw Exceptions in Constructor Value Objects

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BookTitle Example

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DateBorrowed Example

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RenewalsRemaining Example

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Example LoanPeriod

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DDD CRUD/MUD BookTitle String BookCondition String ISBN String DeweyDecimal Float(7,3) DateBorrowed DateTime

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DDD BALL OF MUD/CRUD Correctness ✓✓ Testability ✓ Usability Maintainability Modifiability

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Librarian Library Book Entities

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Identifiable Have State and are Mutable Never Invalid State Operate Using Value Objects No Security or Permission Checks Storage Agnostic Entities

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Doctrine

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SETTERS ARE BAD

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• Special Kind of Entity • Manages Child Entities • Transaction Boundary Aggregates

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Aggregates Member FinesInvoice

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Task Based User Interface

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DDD BALL OF MUD/CRUD Correctness ✓✓ Testability ✓ Usability ✓ Maintainability Modifiability

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Security

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Domain (Business Logic) Application Services Controller Persistence Event/Command Bus Security Templates

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Controller Domain Framework

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Controller Domain Framework

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Mathias Verraes - Decoupling the Model from the Framework at Laracon EU 2014 https://www.youtube.com/watch?v=QaIGN_cTcc8

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Ruby Midwest 2011 - Keynote: Architecture the Lost Years by Robert Martin https://www.youtube.com/watch?v=WpkDN78P884

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Domain (Business Logic) Application Services Controller Persistence Event/Command Bus Security Templates

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DDD BALL OF MUD/CRUD Correctness ✓✓ Testability ✓ Usability ✓ Maintainability ✓ Modifiability

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Domain Events

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Domain Events • Part of the Core Domain • Happened In The Past • Important Enough To Record (Persist) • Important Enough To Concern Other Bounded Contexts • Immutable Value Objects • Do Not Contain Entities or Other Mutable Objects • Can Be Created in an Entity

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DamagedBookWasRemovedFromCirculation

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DamagedBookWasRemovedFromCirculation

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DDD BALL OF MUD/CRUD Correctness ✓✓ Testability ✓ Usability ✓ Maintainability ✓ Modifiability ✓

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Command Query Responsibility Segregation

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CQRS

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Write Read

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Write Model Read Model(s)

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Write Domain Model Read Model(s)

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Controller Command Bus Handler Handler Handler

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Controller Command Bus Handler Handler Handler Request

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Controller Command Bus Handler Handler Handler Command

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Controller Command Bus Handler Handler Handler Command

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Controller Command Bus Handler Handler Handler Command

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Controller Command Bus Handler Handler Handler Command

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Controller Command Bus Handler Handler Handler Response

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Controller Command Bus Handler Handler Handler Response

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Controller Command Bus Handler Handler Handler Response

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Controller Domain Framework

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https://gnugat.github.io/ 2016/05/11/towards-cqrs- command-bus.html

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Event Sourcing

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• Avoids Data Mapping • Avoids Object-relational Impedance Mismatch • Reduces Database Table Counts (Related Tables) • Potentially Reduces Model Counts Event Sourcing

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Event Sourcing • Object Properties are Not Persisted • Events Are Persisted to Append Only Event Storage

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Libraries • https://github.com/broadway • https://github.com/prooph • https://github.com/szjani/predaddy

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Learning More

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DEV BOOK CLUB https://www.youtube.com/user/devbookclub

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DDDinPHP Google Group http://DDDinPHP.org

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https://joind.in/talk/e8752 DDD Topics Covered •Ubiquitous Language •Event Storming •Modelling •Value Objects •Entities •Aggregates •Hexagonal Architecture •CQRS •Event Sourcing