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 context transformations.

We define decoration:

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

class LoggingTransform extends ZTransform[Any, RequestContext] {

  def logCause(rc: RequestContext, cause: Cause[StatusException]): UIO[Unit] = ???

  def accessLog(rc: RequestContext): UIO[Unit] = ???

  override def effect[A](
      io: Any => ZIO[Any, StatusException, A]): RequestContext => ZIO[Any, StatusException, A] = {
    rc => io(rc).zipLeft(accessLog(rc)).tapErrorCause(logCause(rc, _))
  }

  override def stream[A](
      io: Any => ZStream[Any, StatusException, A]): RequestContext => ZStream[Any, StatusException, A] = {
    rc => (io(rc) ++ ZStream.fromZIO(accessLog(rc)).drain).onError(logCause(rc, _))
  }
}

and then we apply it to our service:

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

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

// Note we now have a service with a RequestContext as context.
val decoratedService: ZSimpleService[RequestContext] =
  MyService.transform(new LoggingTransform)
// decoratedService: ZSimpleService[RequestContext] = myexample.testservice.ZioTestservice$GSimpleService$$anon$5@14de2eea