yakivy / poppet Edit

Minimal, type-safe RPC Scala library.

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Poppet is a minimal, type-safe RPC Scala library.

Essential differences from autowire:

  • no explicit macro application .call, result of a consumer is an instance of original trait;
  • no hardcoded return kind Future, you can specify any monad (has cats.Monad typeclass);
  • no forced coder dependencies uPickle, you can specify any serialization format;
  • robust error handling mechanism;
  • cleaner macros logic (~50 lines in comparison to ~300).

Table of contents

  1. Quick start
  2. Customizations
    1. Logging
    2. Failure handling
  3. Manual calls
  4. Roadmap
  5. Examples

Quick start

Put cats and poppet dependencies in the build file, as an example I'll take poppet with circe, let's assume you are using SBT:

val version = new {
    cats = "2.0.0"
    poppet = "0.1.2"

libraryDependencies ++= Seq(
    "org.typelevel" %% "cats-core" % version.cats,
    "com.github.yakivy" %% "poppet-circe" % version.poppet, //to use circe
    //"com.github.yakivy" %% "poppet-play-json" % version.poppet, //to use play json
    //"com.github.yakivy" %% "poppet-jackson" % version.poppet, //to use jackson
    //"com.github.yakivy" %% "poppet-core" % version.poppet, //to build custom coder

Define service trait and share it between provider and consumer services:

case class User(email: String, firstName: String)
trait UserService {
    def findById(id: String): Future[User]

Implement service trait with actual logic:

class UserInternalService extends UserService {
    override def findById(id: String): Future[User] = {
        //emulation of business logic
        if (id == "1") Future.successful(User(id, "Antony"))
        else Future.failed(new IllegalArgumentException("User is not found"))

Create service provider (can be created once and shared for all incoming calls), keep in mind that only abstract methods of the service type will be exposed, that's why you need to explicitly specify trait type:

import cats.implicits._
import io.circe._
import io.circe.generic.auto._
import poppet.coder.circe.all._
import poppet.provider.all._

implicit val ec: ExecutionContext = ...

val provider = Provider[Json, Future]()
    .service[UserService](new UserInternalService)

Create service consumer (can be created once and shared everywhere):

import cats.implicits._
import io.circe._
import io.circe.generic.auto._
import poppet.coder.circe.all._
import poppet.consumer.all._
import scala.concurrent.ExecutionContext

//change with serious pool
implicit val ec: ExecutionContext = ExecutionContext.global
//change with transport call
val transport: Transport[Json, Future] = request => provider(request)

val userService = Consumer[Json, Future](transport)

Enjoy 👌



The library is build on following abstractions:

  • [I] - is an intermediate data type what your coding framework is working with, can be any serialization format, but it would be easier to choose from existed coder modules, because they come with a bunch of predefined coders;
  • [F[_]] - is your service data kind, can be any monad (has cats.Monad typeclass);
  • poppet.consumer.Transport - used to transfer the data between consumer and provider, technically it is just the functions from I to I lifted to passed data kind (I => F[I]). So you can use anything as long as it can receive/pass chosen data type;
  • poppet.Coder - used to code I to models and vice versa. It is probably the most complicated technique in the library since it is build on implicits, because of that, poppet comes with a bunch of modules, where you hopefully will find a favourite coder. If it is not there, you can always try to write your own by providing 2 basic implicits like here;
  • poppet.FailureHandler - used to handle internal failures, more info you can find here;
  • poppet.Peek[I, F[_]] - used to decorate request -> response function without changing the types. Good fit for logging, more info you can find here.


Both provider and consumer take Peek[I, F] as an argument, that allows to inject logging logic around the Request[I] => F[Response[I]] function. Let's define simple logging peek:

val peek: Peek[Json, Id] = f => request => {
    println("Request: " + request)
    val response = f(request)
    println("Response: " + response)

Failure handling

All meaningful failures that can appear in the library are being transformed into poppet.Failure, after what, handled with poppet.FailureHandler. Failure handler is a simple function from failure to result:

type FailureHandler[A] = Failure => A

by default, throwing failure handler is being resolved:

implicit def throwingFailureHandler[A]: FailureHandler[A] = throw _

so if your don't want to deal with JVM exceptions, you can provide your own instance of failure handler. Let's assume you want to pack a failure with EitherT[Future, String, A] kind, then failure handler can look like:

type SR[A] = EitherT[Future, String, A]
implicit def fh[A]: FailureHandler[SR[A]] = f => EitherT.leftT(f.getMessage)

For more info you can check Http4s with Circe example project, it is built around EitherT[IO, String, A] kind.

Manual calls

If your coder has a human readable format (JSON for example), you can use a provider without consumer (mostly for debug purposes) by generating requests manually. Here is an example of curl call:

curl --location --request POST '${providerUrl}' \
--data-raw '{
    "service": "poppet.UserService", #full class name of the service
    "method": "findById", #method name
    "arguments": {
        "id": "1" #argument name: encoded value


  • add more details to Can't find processor exception
  • make FailureHandler explicit
  • add Scala 3 support