9b0062f354
We now allow externalizing application events to a variety of message brokers through the addition of Spring Modulith modules for Kafka, AMQP and JMS to a user project's classpath. Which events shall be externalized and how they're supposed to be routed to the message broker can be configured through either annotations or via a configuration API declared as Spring bean. In case Jackson is on the classpath, we also add auto-configuration to use a Boot-configured ObjectMapper instance with the corresponding message broker client APIs to properly serialize and deserialize messages to JSON.
30 lines
2.6 KiB
Plaintext
30 lines
2.6 KiB
Plaintext
= Spring Modulith -- Kafka event externalization example
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This examples how domain events can automatically be externalized to Kafka.
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The two fundamentally required steps are:
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1. Add the `spring-modulith-events-kafka` dependency to the project (`runtime` scope is sufficient).
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2. Add the `spring-modulith-events-api` dependency to annotate the event types to be externalized automatically with `@Externalized` (see `OrderCompleted`).
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`TestApplication` (in `src/test/java`) declares a `KafkaOperations` instance so that we do not need an actual Kafka instance running for the sample.
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The bean declared simply triggers some log output simulating the actual interaction with Kafka.
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Running the test application using `./mvnw spring-boot:test-run` should show the following output.
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[source]
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----
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22:20:20.398 D - main : Registering domain event externalization to Kafka… <1>
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…
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22:20:21.267 I - main : Triggering order completion… <2>
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22:20:21.277 D - main : Registering publication of example.order.OrderCompleted for org.springframework.modulith.events.support.DelegatingEventExternalizer.externalize(java.lang.Object). <3>
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22:20:21.325 D - task-1 : Externalizing event of type class example.order.OrderCompleted to RoutingTarget[value=order.OrderCompleted]. <4>
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22:20:21.327 I - task-1 : Sending message {"orderId":{"id":"ef3521e8-d498-4539-8745-3a1c74bbe90d"}} to RoutingTarget[value=order.OrderCompleted]. <5>
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22:20:21.376 D - task-1 : Marking publication of event example.order.OrderCompleted to listener org.springframework.modulith.events.support.DelegatingEventExternalizer.externalize(java.lang.Object) completed. <6>
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----
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<1> On application bootstrap, the `spring-modulith-events-kafka` module registers an `ApplicationModuleListener` that will listen to domain events to be externalized.
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<2> Once started, the application's `main` method invokes a business method on the `OrderManagement` that ultimately results in the publication of an `OrderCompleted` event.
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That in turn is annotated with Spring Modulith's `@Externalized` and thus qualifies for externalization.
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<3> The event publication infrastructure detects an `@ApplicationModuleListener` interested in the event, it creates an entry in the Event Publication Registry to track the processing of the event.
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<4> The externalizing `@ApplicationModuleListener` gets triggered (note how it runs asynchronously, indicated by the `task-1` thread).
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<5> Our mock `KafkaOperations` is invoked and triggers the log message simulating the actual sending.
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<6> The Event Publication Registry eventually marks the publication completed as the sending has completed successfully.
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