Building the Service

Since we are focused on wiring together the subcomponents of the application rather than on implementing the functionality, the service is minimalist, equivalent in functionality to "Hello World".

Defining the API

The API class of the service will be in the moodapi project. At this stage, the service will simply greet a user. To define an API,

1. The trait should extend the Service trait of the Lagom API

2. Define one or more functions that return a ServiceCall[A, B]. The first type parameter of the ServiceCall is the type of request object, and the second is type of the response object

3. Impelment the descriptor method, which defines metadata that is used if an external (i.e. non Lagom) service is consuming your service. At this stage, this is essentially some boiletplate code that needs to be written.

The code for the service is:

package simianquant.mood

import com.lightbend.lagom.scaladsl.api._

trait MoodService extends Service {

  def greet: ServiceCall[String, String]

  override def descriptor = {
    import Service._
    named("moodservice")
      .withCalls(
        pathCall("/api/greet", greet)
      )
      .withAutoAcl(true)
  }
}

Implementing the Service

The implementation of the service will be in the moodimpl project. The simplest way to implement a ServiceCall is to call the apply method on its companion object with a function of type Request => Future[Response]. Concretely, for the simple service described above, the implementation of the service would be:

package simianquant.mood

import com.lightbend.lagom.scaladsl.api._
import concurrent.Future

final class MoodServiceImpl extends MoodService {

  override final def greet: ServiceCall[String, String] = ServiceCall {name =>
    Future.successful(s"hello $name")
  }
}

Implementing the Application

The final step to building a microservice is to define an application that wires all of the components of the microservice together, and an application loader that defines how to start the service.