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Domain-Driven Design

Domain-Driven Design

Talk given at PHP Serbia 2017:
(http://www.conf2017.phpsrbija.rs/)

Andrew Cassell

May 28, 2017
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  1. Domain-Driven
    Design
    Deconstructed
    Domain-Driven
    Design
    Deconstructed
    Andrew Cassell
    @alc277 andrewcassell.com

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  6. Domain-Driven
    Design

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  7. Domain-Driven
    Design

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  12. 12

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  14. Domain

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  15. Domain

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  16. Penn State Pattee Library

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  17. book
    user
    book_inventory
    book_inventory_user_link
    ROLE_USER
    ROLE_ADMIN

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  18. Source: http://jonclaytonbiz.com/

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

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  24. Common Sense
    Software
    Development

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  25. DDD !=

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  32. DDD CRUD
    Correctness
    Testability
    Usability
    Maintainability
    Modifiability

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  33. DDD BALL OF MUD / CRUD
    Correctness
    Testability
    Usability
    Maintainability
    Modifiability

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  38. Ubiquitous
    Language

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  39. Ubiquitous Language
    Developers
    Domain
    Experts

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  40. Ubiquitous
    Language

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  41. User
    Customer
    Client
    Member
    Employee

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

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

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  45. $user = User::findOrFail($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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  47. $catalog = new LibraryCatalog(…);
    $book = $catalog->findOneBookByISBN($isbn);
    $members = new Members(…);
    $borrower = $members->findOneByAccountNumber($acctNumber);
    $borrower->borrow($book);

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

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  49. @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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  50. @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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  51. […], 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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  52. DDD BALL OF MUD/CRUD
    Correctness ✓
    Testability
    Usability
    Maintainability
    Modifiability

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  57. Sales
    Marketing
    Finance
    Production

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  58. Customer
    Lead
    Payee
    ShipTo

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  59. Slide: Vaughn Vernon

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  61. Billing Loan Catalog

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  62. Billing Loan Catalog

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  63. Billing Loan Catalog
    Message

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  64. Billing Loan Catalog
    Domain Event

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  70. 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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  71. User Research

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  72. Photo: Mathias Verraes

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

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  80. Domain Objects
    Value Objects
    Entities
    Aggregates
    Domain Events

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

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  83. BookTitle
    DeweyDecimalNumber
    DateBorrowed
    ISBNNumber
    EmailAddress
    Value Objects

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

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  85. Immutable
    Avoid Spooky
    Action at a
    Distance

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  86. ALWAYS IN A
    VALID STATE
    Value Objects

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  87. Value Objects

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  88. “Gateway Drug to
    Test Driven Development”
    Value Objects

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

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  90. Plain PHP Objects
    (POPOs)
    Value Objects

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  91. 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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  92. BookTitle
    Example

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  100. DateBorrowed
    Example

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  102. RenewalsRemaining
    Example

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  105. Example
    LoanPeriod

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

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

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  111. Librarian
    Library
    Book
    Entities

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  112. 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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  117. Doctrine

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  121. SETTERS
    ARE BAD

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

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  128. Aggregates
    Member
    FinesInvoice

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  134. Task Based
    User Interface

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

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  139. Security

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

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  141. Controller
    Domain
    Framework

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  146. Controller
    Domain
    Framework

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

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

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

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

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  151. Domain
    Events

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  152. 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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  157. DamagedBookWasRemovedFromCirculation

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  158. DamagedBookWasRemovedFromCirculation

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

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  165. Command Query
    Responsibility
    Segregation

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  166. CQRS

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  167. Write Read

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  168. Write
    Model
    Read
    Model(s)

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  169. Write
    Domain
    Model
    Read
    Model(s)

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  170. Controller
    Command Bus
    Handler Handler Handler

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  171. Controller
    Command Bus
    Handler Handler Handler
    Request

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  172. Controller
    Command Bus
    Handler Handler Handler
    Command

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  174. Controller
    Command Bus
    Handler Handler Handler
    Command

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  175. Controller
    Command Bus
    Handler Handler Handler
    Command

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  176. Controller
    Command Bus
    Handler Handler Handler
    Command

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  177. Controller
    Command Bus
    Handler Handler Handler
    Response

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  178. Controller
    Command Bus
    Handler Handler Handler
    Response

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  179. Controller
    Command Bus
    Handler Handler Handler
    Response

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  180. Controller
    Domain
    Framework

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

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  183. Event
    Sourcing

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

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

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

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  193. Learning
    More

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

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  200. DDDinPHP
    Google Group
    http://DDDinPHP.org

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

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