tc-vs-inheritance.scala

//simplified logger from scala-logging
trait LoggerTakingImplicit[T] {
  def log(msg: String)(implicit ev: CanLog[T])
}

trait CanLog[T] {
  def logMessage(originalMsg: String, a: T): String
}

// simplified logback MDC
object MDC {
  def put(key: String, value: String): Unit = ()
}

//carrier of traceid ... because microservices
trait RequestCtx {
  def traceId: String
}

object RequestCtx {
  implicit object CanLogInstance extends CanLog[RequestCtx] {
    def logMessage(originalMsg: String, a: RequestCtx): String = {
      MDC.put("traceId", a.traceId)
      originalMsg
    }
  }
}


// class in a library - http client to external service
trait JiraClient {
  def addComment(comment: String)(implicit ctx: RequestCtx)
}

//previously all our apps were backend-only microservices but now we have a frontend so ..
trait UserRequestCtx {
  def traceId: String
  def username: String
}

// Problem: how do we make logs from JiraClient handle username properly


//Version leveraging inheritance: 
object WithInheritance {
  
  trait RequestCtx {
    def traceId: String
    
    def mdcValues: Map[String, String] = Map("traceId" -> traceId)
  }

  object RequestCtx {
    implicit object CanLogInstance extends CanLog[RequestCtx] {
      def logMessage(originalMsg: String, a: RequestCtx): String = {
        a.mdcValues.foreach(x => MDC.put(x._1, x._2))
        originalMsg
      }
    }
  }

  trait UserRequestCtx extends RequestCtx {
    def username: String

    override def mdcValues: Map[String, String] = super.mdcValues + ("username" -> username)
  }
  
}

//Version leveraging typeclasses only: 
object WithoutInheritance {

  // Every user of `LoggerTakingImplicit` must be now parametrized by ctx type
  class JiraClient[Ctx : CanLog] {

    def addComment(comment: String)(implicit ctx: Ctx) = ???
    
  }
  
  // Imagine 20 microservices, and 10 libraries and only one of the microservices wants to change something in 
  // RequestCtx. Parametrization will leak to all of that projects complicating them without any purpose

}