This recipe demonstrates how to create a service in Lagom that only consumes from other services and does not provide any service calls of its own.
It is similar to the default "hello" application created by the Lagom Java template, but the HelloStreamService interface and implementation have been removed.
Instead, the hello-stream-impl project only subscribes to events in Kafka and logs each one to the console.
The key change is in HelloStreamModule.
Ordinarily, the service module binds a service interface and implementation:
bindService(HelloStreamService.class, HelloStreamServiceImpl.class);In a consumer service, this is changed to call bindServiceInfo with the name of the service:
bindServiceInfo(ServiceInfo.of("hello-stream"));The service name should be unique, and is used in a few ways:
- To construct the Kafka consumer group ID
- To identify the service when running in development mode
- Production tooling can read the service name to generate deployment configuration
You can test this recipe using 2 separate terminals.
On one terminal start the service:
sbt runAll
On a separate terminal, use curl to trigger some events in HelloService:
curl -H "Content-Type: application/json" -X POST -d '{"message": "Hi"}' http://localhost:9000/api/hello/Alice
curl -H "Content-Type: application/json" -X POST -d '{"message": "Good morning"}' http://localhost:9000/api/hello/Bob
curl -H "Content-Type: application/json" -X POST -d '{"message": "Hi"}' http://localhost:9000/api/hello/Carol
curl -H "Content-Type: application/json" -X POST -d '{"message": "Howdy"}' http://localhost:9000/api/hello/David
After a few seconds, HelloStreamSubscriber will output some log messages indicating that the events have been received:
16:30:22.919 [info] com.lightbend.lagom.recipes.consumer.hellostream.impl.HelloStreamSubscriber [] - Received event: [HelloEvent.GreetingMessageChanged(name=Alice, message=Hi)]
16:30:25.915 [info] com.lightbend.lagom.recipes.consumer.hellostream.impl.HelloStreamSubscriber [] - Received event: [HelloEvent.GreetingMessageChanged(name=Bob, message=Good morning)]
16:30:28.876 [info] com.lightbend.lagom.recipes.consumer.hellostream.impl.HelloStreamSubscriber [] - Received event: [HelloEvent.GreetingMessageChanged(name=Carol, message=Hi)]
16:30:31.827 [info] com.lightbend.lagom.recipes.consumer.hellostream.impl.HelloStreamSubscriber [] - Received event: [HelloEvent.GreetingMessageChanged(name=David, message=Howdy)]
This is described in more detail in the Lagom documentation: