PITCHME.md

Domain Driven Design

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https://github.com/idugalic/digital-restaurant

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Sample application

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• Customers use the website applications to place food orders at local restaurants
• Application coordinates a network of couriers who deliver the orders to customers

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Technology

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• It is built using Event Sourcing and CQRS patterns
• It is written in Kotlin, and uses Spring Boot
• It is built using Axonframework, which is focused on making applications based on the DDD principles
• It is driven using Maven

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Clone & Build

$ git clone https://github.com/idugalic/digital-restaurant
$ mvn clean install

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Run monolith 1 (HTTP & Websockets)

$ cd drestaurant-apps/drestaurant-monolith
$ mvn spring-boot:run

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Run monolith 2 (REST)

$ cd drestaurant-apps/drestaurant-monolith-rest
$ mvn spring-boot:run

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Run monolith 3 (Websockets)

$ cd drestaurant-apps/drestaurant-monolith-websockets
$ mvn spring-boot:run

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Run microservices 1 (Monolith 1 decomposed)

https://github.com/idugalic/digital-restaurant/tree/master/drestaurant-apps/drestaurant-microservices

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Run microservices 2 (Monolith 2 decomposed)

https://github.com/idugalic/digital-restaurant/tree/master/drestaurant-apps/drestaurant-microservices-rest

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Run microservices 3 (Monolith 3 decomposed)

https://github.com/idugalic/digital-restaurant/tree/master/drestaurant-apps/drestaurant-microservices-websockets

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Domain

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• This layer contains information about the domain
• This is the heart of the business software
• The state of business objects is held here

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Subdomains

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• getting different parts of an organization to agree on a single unified model is a monumental task.
• from the perspective of a given part of the organization, the model is overly complex for their needs.
• the domain model can be confusing since different parts of the organization might use either the same term for different concepts or different terms for the same concept.

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Subdomains - Analyze

• Analyze the business and identify the different areas of expertise. The end result is very likely to be subdomains
• http://eventstorming.com/

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Subdomains - Context

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Core subdomains

Core subdomains are more important to the business

• Courier subdomain - drestaurant-libs/drestaurant-courier/
• Restaurant subdomain - drestaurant-libs/drestaurant-restaurant/
• Customer subdomain - drestaurant-libs/drestaurant-customer/
• Order subdomain - drestaurant-libs/drestaurant-order/

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Generic subdomains

Generic subdomains facilitate the business, but are not core to the business. In general, these types of pieces can be purchased from a vendor or outsourced.

• Accounting subdomain - drestaurant-libs/drestaurant-accounting/

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Eventsourcing

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• We use event sourcing to persist our event sourced aggregates as a sequence of events
• Each event represents a state change of the aggregate
• An application rebuild the current state of an aggregate by replaying the events

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Eventsourcing benefits

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• It preserves the history of aggregates (100%), which is valuable for auditing and regulatory purposes
• It reliably publishes domain events, which is particularly useful in a microservice architecture
• You can use any database technology to store the state

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Eventsourcing drawbacks

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• There is a learning curve because its a different way to write your business logic
• Events will change shape over time
• Forces you to use the CQRS pattern
• Consider using event sourcing within 'core subdomain' only

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Eventsourcing & snapshotting

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• A Snapshot represents the state when all events to that point in time have been replayed
• They are used as a heuristic to prevent the need to load all events for the entire history of an aggregate

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@Aggregate(snapshotTriggerDefinition = "courierSnapshotTriggerDefinition")

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Saga

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• Sagas are used to manage business transactions
• The saga transaction model is ACD. It lack of Isolation
• They respond on Events and may dispatch Commands, invoke external applications, ...
• Orchestration vs. Choreography
• Saga orchestrators as state machines

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Saga example

@Saga
@ProcessingGroup("restaurantordersaga")
class RestaurantOrderSaga {
    @Autowired
    @Transient
    private lateinit var commandGateway: CommandGateway
    private lateinit var orderId: String

    @StartSaga
    @SagaEventHandler(associationProperty = "aggregateIdentifier")
    internal fun on(event: RestaurantOrderCreationInitiatedInternalEvent) {
        orderId = event.aggregateIdentifier
        associateWith("orderId", orderId)
        commandGateway.send(ValidateOrderByRestaurantInternalCommand(orderId, event.restaurantId, event.orderDetails.lineItems, event.auditEntry), LoggingCallback.INSTANCE)
    }

    @EndSaga
    @SagaEventHandler(associationProperty = "orderId")
    internal fun on(event: OrderValidatedWithSuccessByRestaurantInternalEvent) = commandGateway.send(MarkRestaurantOrderAsCreatedInternalCommand(event.orderId, event.auditEntry), LoggingCallback.INSTANCE)

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Applications

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• This is a thin layer which coordinates the application activity
• It does not contain business logic
• It does not hold the state of the business objects

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• Monolith 1 - HTTP and WebSockets API
• Monolith 2 - HTTP/REST API
• Monolith 3 - WebSockets API
• Microservices 1 - Monolith 1 decomposed
• Microservices 2 - Monolith 2 decomposed
• Microservices 3 - Monolith 3 decomposed

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Monolith 1 (HTTP and WebSockets API)

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• Event handler consumes events, and creates query models
• It can be replied (@AllowReplay(true)) to re-create (@ResetHandler) query model
• It is publishing a WebSocket events to notify on update of a query model
• Query models are exposed via Spring Data Rest
• One-to-many relation between a command resource and query resource

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Monolith 2 (HTTP/REST API)

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• Event handler consumes events, and creates query models
• It can be replied (@AllowReplay(true)) to re-create (@ResetHandler) query model
• We emit 'any change on Query Model' to Axon subscription queries, and we subscribe on them within CommandController
• One-to-one relation between a command resource and query resource (Spring Data Rest)

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Monolith 3 (WebSockets API)

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• Event handler consumes domain events, and creates query models
• It can be replied (@AllowReplay(true)) to re-create (@ResetHandler) query model
• It is publishing a WebSocket messages to topics on every update of a query model
• WebController expose WebSocket message endpoints

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Microservices 1 (Monolith 1 decomposed)

Each microservice:

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• has its own bounded context,
• has its own JPA event store (we are not sharing the JPA Event Store)
• we distribute events between them via Apache Kafka
• we distribute commands via Spring Cloud discovery and registry service (Eureka)

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Microservices 2 (Monolith 2 decomposed)

Each microservice:

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• has its own bounded context,
• has its own JPA event store (we are not sharing the JPA Event Store)
• we distribute events between them via RabbitMQ
• we distribute commands via Spring Cloud discovery and registry service (Eureka)

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Microservices 3 (Monolith 3 decomposed)

Each microservice:

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• has its own bounded context,
• has shared event(sourcing) storage (AxonServer)
• and we distribute messages (commands, events and queries) between them via AxonServer

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Thank you

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@fa[terminal] Ivan Dugalic, software engineer

@fa[globe] http://idugalic.pro

@fa[envelope] idugalic@gmail.com

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@fagithub