Domain-driven Design Deconstructed

Presented at php[tek] 2016

Once you try domain-driven design (DDD), you will never design software in the same way again. We will avoid silly buzzwords and break DDD down into easy-to-understand components. We will start by discussing what it means to use a ubiquitous language, encapsulate logic in value objects, and use bounded contexts, entities, and aggregate roots to manage state and protect invariants. We will also cover more-advanced topics in the DDD world, such as event sourcing and command query responsibility segregation.

Transcript

1. Domain-driven
   Design
   Deconstructed
   Domain-driven
   Design
   Deconstructed
   Andrew Cassell
   

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4. Domain-driven
   Design
   

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10. Domain-driven
    Design
    

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11. DDD
    

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

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18. #DDD4ME
    

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23. Common Sense
    Software
    Development
    

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24. Domain
    

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26. http://www.sei.cmu.edu/reports/95tr021.pdf
    

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

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29. Performance
    

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30. Security
    

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31. DDD Common
    Misconceptions
    

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40. CRUD is okay-ish
    

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45. Ubiquitous
    Language
    

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46. Ubiquitous Language
    Developers
    Domain
    Experts
    

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49. Ubiquitous
    Language
    

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50. Customer
    Client
    Member
    Affiliate
    

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

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

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53. $catalog = new LibraryCatalog(…);
    $book = $catalog->findOneBookByISBN($isbn);
    $members = new Members(…);
    $borrower = $members->findOneByAccountNumber($id);
    $borrower->borrow($book);
    $members->updateAccount($borrower);
    

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54. $user = User::findOrFail($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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56. $catalog = new LibraryCatalog(…);
    $book = $catalog->findOneBookByISBN($isbn);
    $members = new Members(…);
    $borrower = $members->findOneByAccountNumber($id);
    $borrower->borrow($book)
    

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57. $borrower->borrow($book);
    $borrower->loan($book);
    $borrower->obtain($book);
    $borrower->take($book);
    

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

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61. Sales
    Marketing
    Finance
    Production
    

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62. Customer
    Lead
    Payee
    ShipTo
    

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63. Slide: Vaughn Vernon
    

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67. Alberto Brandolini
    

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68. http://ziobrando.blogspot.be/
    

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74. 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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77. Photo: Mathias Verraes
    

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91. Domain Objects
    Repositories
    Commands & Command handlers
    Business rules/specifications
    Messages
    Domain events
    

    Aggregates
    Entities
    Value Objects
    Invariants
    Processes
    

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92. Domain Objects
    Repositories
    Commands & Command handlers
    Business rules/specifications
    Messages
    Domain events
    

    Aggregates
    Entities
    Value Objects
    Invariants
    Processes
    

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93. Value Object
    Immutable
    No Identity only Values
    Entity
    Mutable
    Has A Lifecycle
    Always Stable
    Contains Value Objects
    Aggregate
    Entity
    Responsible For Child Entities
    

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94. Value Objects
    Amount Paid
    Email Address
    BookTitle
    Dewey Decimal Number
    ISBN Number
    Domain Event
    

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95. Value Objects
    

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96. Immutability
    Avoid Spooky
    Action at a
    Distance
    

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97. Value Objects
    

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

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

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100. Plain PHP Objects (POPOs) That are Immutable
     •Declare Class Properties as Private
     •No Setters
     •All Parameters Are Injected in the Constructor
     •Do Not Store References to Mutable Objects (Private Scalars Okay)
     •Functions that Appear to Modify Should Return a New Object
     •Declare the Class Final so it can’t be Overridden (probably)
     Value Objects
     

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101. Example
     BookTitle
     

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109. Example
     BookCondition
     

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114. ISBN
     DeweyDecimal
     …
     

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115. Example
     DateBorrowed
     

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117. Example
     LoanPeriod
     

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

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

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121. Identifiable
     Has State and is Mutable
     Never Invalid State
     Required Parameters Passed in Constructor
     Operates Using Value Objects
     No Security or Permission Checks
     Storage Agnostic
     Entities
     

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122. Entities
     Librarian
     Book
     

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126. SETTERS
     ARE BAD
     

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133. Doctrine
     

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135. •Special Kind of Entity
     •Manages Child Entities
     •Transactionally Safe
     Aggregates
     

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136. Aggregates
     Member
     Fines Invoice
     

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142. Task Based UIs
     

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

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146. Security
     

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

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148. Controller
     Domain
     Framework
     

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153. Controller
     Domain
     Framework
     

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

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

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

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

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158. Domain
     Events
     

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159. Domain Events
     •Something That Happened In The Past
     •Immutable
     •Part of the Core Domain
     •Usually Created in Either an Aggregate or as Part of
     Application Service
     •Probably Want To Persist Them
     •Passed to Event Bus (Application Service)
     •Listeners are Application Services
     •Used To Cross Bounded Contexts
     

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162. BookWasOverdue
     

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

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166. Command Query
     Responsibility
     Segregation
     

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167. CQRS
     

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168. Slide: Microsoft
     

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

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177. Controller
     Domain
     Framework
     

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178. Event
     Sourcing
     

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179. Event Sourcing
     •Store the Events That Happen
     •Append Only Event Storage
     •Object Properties are Not Stored
     

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

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184. Slide: Microsoft
     

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189. Libraries
     •https://github.com/szjani/predaddy
     •https://github.com/prooph
     •https://github.com/qandidate-labs/broadway
     

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192. Learning
     More
     

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194. DevBookClub
     https://www.youtube.com/user/devbookclub
     

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

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201. https://joind.in/talk/99c3a
     Topics Covered
     •Ubiquitous Language
     •Event Storming
     •Modelling
     •Value Objects
     •Entities
     •Aggregates
     •Hexagonal Architecture
     •CQRS
     •Event Sourcing
     

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