Dependency graph using normal Scala code

dependency_graph.scala

// in test source directory

object LocalSetup { // for running app locally
  private val localConfig = ???
  val instance = new ApplicationSetup(Some(localConfig)).instance
}

// in main source directory
object ProductionSetup extends ApplicationSetup(None)

class ApplicationSetup(
  configReplacementOpt: Option[ApplicationConfig] = None
) {
  private val config = configReplacementOpt.getOrElse {
    ??? // here we read config from .props or .conf files or environment variables etc
  }
  
  private implicit val actorSystem: ActorSystem = ActorSystem("name", config.akkaConfig)
  
  // here we split out part of dependency graph to separate class that will provide databased to other parts of the system
  // NOTE right composition structure depends on a particular case, maybe it would be better to extract modules like
  // UsersModule and BlogsModule instead of DatabasesModule and ServerModule
  private val dbs = new DatabasesModule(config.dbsConfig)
  
  // userDb and blogsDb have the same type (say: app.Db) so with DI container they would need to be differentiated using
  // e.g @Named annotation
  private val usersService = new UsersService(dbs.userDb)
  private val blogsService = new BlogsService(dbs.blogsDb)
  
  // in DI container case we would need to have some sort of multibinding
  private val endpoints = Seq(
      new UsersEndpoint(usersService),
      new BlogsEndpoint(blogsService)
    )
  private val httpServer =
    new HttpServer(endpoints) // uses implicit ActorSystem, so no need to pass it explicitly
  
  val instance: Application =
    new Application(httpServer)
}

// there are no shutdown hooks, but it is relatively easy to add them