Scala Protocol Buffer
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Typeful runtime for eventsourced behaviors

Aecor is an opinionated library to help building scalable, distributed eventsourced services written in Scala. It uses Akka for distribution and fault tolerance. With the help of Cats and Shapeless to reach type safety.

Aecor works on Scala 2.11 and 2.12 with Java 8.

The name Aecor (lat. ocean) is inspired by a vision of modern distributed applications, as an ocean of messages with pure behaviors floating in it.

Installing Aecor

To start using Aecor Runtime add the following to your build.sbt file:

scalaOrganization := "org.typelevel"
libraryDependencies += "io.aecor" %% "aecor-core" % "0.15.0"
scalacOptions += "-Ypartial-unification"

Defining and running behavior

Let's start with defining domain events:

sealed trait SubscriptionEvent
object SubscriptionEvent {
  case class SubscriptionCreated(subscriptionId: String, userId: String, productId: String, planId: String) extends SubscriptionEvent
  case class SubscriptionPaused(subscriptionId: String) extends SubscriptionEvent
  case class SubscriptionResumed(subscriptionId: String) extends SubscriptionEvent
  case class SubscriptionCancelled(subscriptionId: String) extends SubscriptionEvent

  implicit val persistentEncoder: PersistentEncoder[SubscriptionEvent] = `define it as you wish`
  implicit val persistentDecoder: PersistentDecoder[SubscriptionEvent] = `and this one too`

Then we define how this events affect aggregate state:

Folder[F, E, S] instance represents the ability to fold Es into S, with effect F on each step. Aecor runtime uses Folded[A] data type, with two possible states: Next(a) - says that a should be used as a state for next folding step Impossible - says that folding should be aborted (e.g. underlying runtime actor throws IllegalStateException)

sealed trait SubscriptionStatus
object SubscriptionStatus {
  case object Active extends SubscriptionStatus
  case object Paused extends SubscriptionStatus
  case object Cancelled extends SubscriptionStatus

case class Subscription(status: SubscriptionStatus) {
  def applyEvent(e: SubscriptionEvent): Folded[Subscription] = e match {
    case e: SubscriptionCreated =>
    case e: SubscriptionPaused =>
      subscription.copy(status = Paused).next
    case e: SubscriptionResumed =>
      subscription.copy(status = Active).next
    case e: SubscriptionCancelled =>
      subscription.copy(status = Cancelled).next
object Subscription {
  import SubscriptionStatus._
  def init(e: SubscriptionEvent): Folded[Subscription] = e match {
    case SubscriptionCreated(subscriptionId, userId, productId, planId) =>
    case _ => impossible
  def folder: Folder[Folded, SubscriptionEvent, Option[Subscription]] =

Now let's define a behavior that converts operation to its handler. A Handler[F, State, Event, Reply] is just a wrapper around State => (Seq[Event], Reply), you can think of it as Kleisli[(Seq[Event], ?), State, Reply] i.e. a side-effecting function with a side effect being a sequence of events representing state change caused by operation.

sealed trait SubscriptionOp[A] {
  def subscriptionId: String
object SubscriptionOp {
  case class CreateSubscription(subscriptionId: String, userId: String, productId: String, planId: String) extends SubscriptionOp[Unit]
  case class PauseSubscription(subscriptionId: String) extends SubscriptionOp[Unit]
  case class ResumeSubscription(subscriptionId: String) extends SubscriptionOp[Unit]
  case class CancelSubscription(subscriptionId: String) extends SubscriptionOp[Unit]

def handler[F[_]](implicit F: Applicative[F]) = Lambda[SubscriptionOp ~> Handler[F, Option[Subscription], SubscriptionEvent, ?]] {
  case CreateSubscription(subscriptionId, userId, productId, planId) => {
    case Some(subscription) =>
      // Do nothing reply with ()
      F.pure(Seq.empty -> ())
    case None =>
      // Produce event and reply with ()
      F.pure(Seq(SubscriptionCreated(subscriptionId, userId, productId, planId)) -> ())
  case PauseSubscription(subscriptionId) => {
    case Some(subscription) if subscription.status == Active =>
      F.pure(Seq(SubscriptionPaused(subscriptionId)) -> ())
    case _ =>
      F.pure(Seq.empty -> ())
  case ResumeSubscription(subscriptionId) => {
    case Some(subscription) if subscription.status == Paused =>
      F.pure(Seq(SubscriptionResumed(subscriptionId)) -> ())
    case _ =>
      F.pure(Seq.empty -> ())
  case CancelSubscription(subscriptionId) => {
    case Some(subscription) =>
      F.pure(Seq(SubscriptionCancelled(subscriptionId)) -> ())
    case _ =>
      F.pure(Seq.empty -> ())

Then you define a correlation function, entity name and a value provided by correlation function form unique primary key for aggregate. It should not be changed between releases, at least without prior event migration.

def correlation: Correlation[SubscriptionOp] = _.subscriptionId

After that we are ready to launch.

import monix.cats._
import monix.eval.Task
import aecor.effect.monix._
import aecor.runtime.akkapersistence.AkkaPersistenceRuntime

val system = ActorSystem("foo")

val subscriptions: SubscriptionOp ~> Task =