§Understanding Hello World
After creating and running Hello World from the command line, you no doubt appreciate what Lagom framework did for you. There was no need to determine what infrastructure you might need and then install and configure it. The template removed the necessity to set up a project or build structure. And, as you create services of your own, Lagom detects changes and performs a hot reload! Lagom allows you to concentrate on satisfying your business needs.
The separation of concerns illustrated in Hello World and an introduction to service descriptors and the registry will help you as you start developing your own microservices:
§Service interface
The service interface belongs in the api
project. For instance, the service interface for the hello
service resides in the hello-api
project (look for the HelloService.scala
source file).
import akka.Done
import akka.NotUsed
import com.lightbend.lagom.scaladsl.api._
import play.api.libs.json._
trait HelloService extends Service {
def hello(id: String): ServiceCall[NotUsed, String]
def useGreeting(id: String): ServiceCall[GreetingMessage, Done]
final override def descriptor = {
import Service._
named("hello")
.withCalls(
pathCall("/api/hello/:id", hello _),
pathCall("/api/hello/:id", useGreeting _)
)
.withAutoAcl(true)
}
}
case class GreetingMessage(message: String)
object GreetingMessage {
implicit val format: Format[GreetingMessage] = Json.format[GreetingMessage]
}
Note that:
-
The service interface inherits from
Service
and provides an implementation ofService.descriptor
method. -
The implementation of
Service.descriptor
returns aDescriptor
. TheHelloService
descriptor defines the service name and the REST endpoints it offers. For each endpoint, declare an abstract method in the service interface as illustrated in theHelloService.hello
method. For more information, see Service Descriptors.
§Service implementation
The related impl
project, hello-impl
provides implementation for the service abstract methods. For instance, the HelloServiceImpl.scala
source file contains the service implementation of the HelloService.hello
method for the hello
service. The PersistentEntityRegistry
supports data persistence using Event Sourcing and CQRS.
import com.lightbend.lagom.scaladsl.api.ServiceCall
import com.lightbend.lagom.scaladsl.persistence.PersistentEntityRegistry
class HelloServiceImpl(persistentEntityRegistry: PersistentEntityRegistry) extends HelloService {
override def hello(id: String) = ServiceCall { _ =>
val ref = persistentEntityRegistry.refFor[HelloEntity](id)
ref.ask(Hello(id, None))
}
override def useGreeting(id: String) = ServiceCall { request =>
val ref = persistentEntityRegistry.refFor[HelloEntity](id)
ref.ask(UseGreetingMessage(request.message))
}
}