From CRUD to DDD: How did we end up here? - Dutch PHP Conference 2024

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From CRUD to DDD How did we end up here? Arnout Boks - Dutch PHP Conference - 16 March 2024

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CRUD Operations on an entity: ● Create ● Read ● Update ● Delete

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CRUD The database has a central role. The interface (both backend API and UI) are just a thin wrapper.

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CRUD User Interface edit_recipe.php view_recipe.php Database SQL SQL id: 123 id: 123 title: “foo” ingredients: [...] published: true

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CRUD with a modern twist User Interface Recipe:edit controller Recipe:view controller Database SQL GET /recipes/123 POST /recipes/123 title: “foo” ingredients: [...] published: true app.php front controller ORM

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Domain Driven Design (DDD) The domain (the subject area where the software is used) and the business logic therein gets a central role in the application. The rest of the software (the infrastructure) is built around it.

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Domain Driven Design: ‘the full monty’ User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Command bus PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe write model repository RecipePublished event Event store Event bus SQL Recipe event projector ORM Recipe read model repository SQL load & save publish

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How did we end up here?

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Before we start ● Steps may be taken in different order ● Steps may be executed differently ○ Interpretation of principles ○ Implementation choices ○ Chosen technology (language, framework, ORM)

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Before we start New! New! New! Generic, system-wide component Component speciﬁc to this feature/use case New possibility, optional addition

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DDD as an umbrella term ● Ubiquitous language ● Event Sourcing ● Command pattern ● Entity ● CQRS ● Repository ● Aggregate ● Value Object ● Event-driven architecture ● Bounded context ● Command bus ● Anti-corruption layer

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Recap: our CRUD starting point User Interface Recipe:edit controller Database app.php front controller Recipe:view controller ORM SQL

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Aggregates An aggregate is a cluster of domain objects that can be treated as a single unit. An aggregate is responsible for maintaining the integrity and business rules within the cluster.

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Aggregates User Interface Recipe:edit controller Database app.php front controller Recipe:view controller Recipe aggregate Recipe repository ORM SQL load & save edit title: “foo” ingredients: [...] published: true

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Aggregates User Interface Recipe:edit controller Database app.php front controller Recipe:view controller Recipe aggregate Recipe repository ORM SQL Unit tests load & save edit title: “foo” ingredients: [...] published: true

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Command pattern Models the possible actions/changes on a domain aggregate, and separates the intent from its execution.

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Command pattern (1) User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Recipe aggregate Recipe repository ORM SQL load & save publish

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Command pattern (1) User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Recipe aggregate Recipe repository ORM SQL Recipe:addIngredient controller load & save publish

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Command pattern (2) User Interface Recipe:publish controller Database app.php front controller Recipe:view controller PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe repository ORM SQL publish load & save

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Command bus A ‘one stop shop’ to execute commands. The command bus will ﬁnd the correct handler for a command and deliver the command to it.

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Command bus User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Command bus PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe repository ORM SQL

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Command bus decoration Command bus command handler 2 command handler 1 command handler 3 controller 2 controller 1 console command 1

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Command bus decoration Transactional command bus Command bus command handler 2 command handler 1 command handler 3 controller 2 controller 1 console command 1

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Queueing command bus Logging command bus Command bus decoration Transactional command bus Command bus command handler 2 command handler 1 command handler 3 controller 2 controller 1 console command 1

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Command-Query Responsibility Separation CQRS distinguishes commands (which change the state of the system) from queries (which retrieve information, but make no changes) and treats these separately. In particular, this means that both responsibilities can have separate (optimized/specialized) models.

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Command-Query Responsibility Separation User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Command bus PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe write model repository ORM Recipe read model repository SQL ?

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Event-Driven Architecture An Event-Driven Architecture uses events (models of signiﬁcant things that happened in the past) for the communication between, and decoupling of, components in the system.

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Event-Driven Architecture User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Command bus PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe write model repository RecipePublished event Event bus Recipe event projector ORM Recipe read model repository SQL

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Event Sourcing Events are stored, and are the single source of truth in the system. All other information is derived from the events.

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Event Sourcing: role of the aggregate PublishRecipe command Recipe aggregate RecipePublished event RecipeIngredientAdded event ingredient: Onion quantity: 1 RecipeIngredientAdded event ingredient: Garlic quantity: 2 cloves

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Event Sourcing: role of the aggregate PublishRecipe command Recipe aggregate HasNoIngredients exception RecipeIngredientAdded event ingredient: Onion quantity: 1 RecipeIngredientRemoved event ingredient: Onion

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Event Sourcing: role of the aggregate PublishRecipe command Recipe aggregate AlreadyPublished exception RecipeIngredientAdded event ingredient: Onion quantity: 1 RecipeIngredientAdded event ingredient: Garlic quantity: 2 cloves RecipePublished event

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Event Sourcing User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Command bus PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe write model repository RecipePublished event Event store Event bus SQL Recipe event projector ORM Recipe read model repository SQL load & save events

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Reprojection User Interface Recipe:publish controller Database app.php front controller Recipe:view controller Command bus PublishRecipe command PublishRecipe command handler Recipe aggregate Recipe write model repository RecipePublished event Event store Event bus SQL Recipe event projector ORM Recipe read model repository SQL Reprojection system Audit dashboard load & save events

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What did we gain? ● Business domain logic at the core of our code ● Decoupling ● Testability ● Commands conveying clear intention ● Command transactionality ● Command logging ● Command queueing ● Specialized read models ● Specialized data stores ● Managers for distributed business processes ● Notiﬁcations ● Auditability ● Reprojections for data migrations and new read models

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Make informed choices for your architecture, based on your needs.

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Thank you! - Any questions? Arnout Boks @aboks@phpc.social @arnoutboks https://aboks.github.io https://www.moxio.com Feedback https://joind.in/talk/565f8 We’re hiring!