Decorating services

When implementing a server, sometimes you might want to decorate all methods (effects or streams) in the service, for example to add access and error logging.

It can be done with the help of ZTransform. Instances of this class can be used to apply a transformation to all methods of a service to generate a new "decorated" service. This can be used for pre- or post-processing of requests/responses and also for environment and context transformations.

We define decoration:

import io.grpc.Status
import scalapb.zio_grpc.{ RequestContext, ZTransform }
import zio._
import zio.stream.ZStream

class LoggingTransform[R] extends ZTransform[R, Status, R with Has[RequestContext]] {

  def logCause(cause: Cause[Status]): URIO[Has[RequestContext], Unit] = ???

  def accessLog: URIO[Has[RequestContext], Unit] = ???

  override def effect[A](io: ZIO[R, Status, A]): ZIO[R with Has[RequestContext], Status, A] =
    io.zipLeft(accessLog).tapCause(logCause)

  override def stream[A](io: ZStream[R, Status, A]): ZStream[R with Has[RequestContext], Status, A] =
    (io ++ ZStream.fromEffect(accessLog).drain).onError(logCause)
}

and then we apply it to our service:

import myexample.testservice.ZioTestservice.ZSimpleService
import myexample.testservice.{Request, Response}

object MyService extends ZSimpleService[Any, Any] {
  def sayHello(req: Request): ZIO[Any, Status, Response] =
    ZIO.succeed(Response(s"Hello user"))
}

val decoratedService =
  MyService.transform(new LoggingTransform[Any])
// decoratedService: ZSimpleService[Has[RequestContext], Any] = myexample.testservice.ZioTestservice$ZSimpleService$$anon$6$$anon$7@67739b99