A small eventsourcing scala library that models aggregates as FSMs.
The akka modules provides an implementation based on akka-persistence, akka-persistence-query and
akka-cluster-sharding.
The code has been moved to the main nexus repository as a module and its development will continue there.
Considering the following Event, State, Command and Rejection types:
sealed trait Event extends Product with Serializable {
def rev: Int
}
object Event {
final case class Incremented(rev: Int, step: Int) extends Event
final case class Initialized(rev: Int) extends Event
}
sealed trait State extends Product with Serializable
object State {
final case object Initial extends State
final case class Current(rev: Int, value: Int) extends State
}
import scala.concurrent.duration.FiniteDuration
sealed trait Command extends Product with Serializable {
def rev: Int
}
object Command {
final case class Increment(rev: Int, step: Int) extends Command
final case class IncrementAsync(rev: Int, step: Int, sleep: FiniteDuration) extends Command
final case class Initialize(rev: Int) extends Command
final case class Boom(rev: Int, message: String) extends Command
final case class Never(rev: Int) extends Command
}
sealed trait Rejection extends Product with Serializable
object Rejection {
final case class InvalidRevision(rev: Int) extends Rejection
}
And considering the following functions that describe how recovery and evaluations are executed (notice that the
evaluate is defined for an arbitrary F[_] type that has Timer[F] and Async[F] type class instances from
cats-effect):
val initialState: State = State.Initial
val next: (State, Event) => State = {
case (Initial, Incremented(1, step)) => State.Current(1, step)
case (Initial, Initialized(rev)) => State.Current(rev, 0)
case (Current(_, value), Incremented(rev, step)) => State.Current(rev, value + step)
case (Current(_, _), Initialized(rev)) => State.Current(rev, 0)
case (other, _) => other
}
def evaluate[F[_]](state: State, cmd: Command)(implicit F: Async[F], T: Timer[F]): F[Either[Rejection, Event]] =
(state, cmd) match {
case (Current(revS, _), Boom(revC, message)) if revS == revC => F.raiseError(new RuntimeException(message))
case (Initial, Boom(rev, message)) if rev == 0 => F.raiseError(new RuntimeException(message))
case (_, Boom(rev, _)) => F.pure(Left(InvalidRevision(rev)))
case (Current(revS, _), Never(revC)) if revS == revC => F.never
case (Initial, Never(rev)) if rev == 0 => F.never
case (_, Never(rev)) => F.pure(Left(InvalidRevision(rev)))
case (Initial, Increment(rev, step)) if rev == 0 => F.pure(Right(Incremented(1, step)))
case (Initial, Increment(rev, _)) => F.pure(Left(InvalidRevision(rev)))
case (Initial, IncrementAsync(rev, step, duration)) if rev == 0 =>
T.sleep(duration) *> F.pure(Right(Incremented(1, step)))
case (Initial, IncrementAsync(rev, _, _)) => F.pure(Left(InvalidRevision(rev)))
case (Initial, Initialize(rev)) if rev == 0 => F.pure(Right(Initialized(1)))
case (Initial, Initialize(rev)) => F.pure(Left(InvalidRevision(rev)))
case (Current(revS, _), Increment(revC, step)) if revS == revC => F.pure(Right(Incremented(revS + 1, step)))
case (Current(_, _), Increment(revC, _)) => F.pure(Left(InvalidRevision(revC)))
case (Current(revS, _), IncrementAsync(revC, step, duration)) if revS == revC =>
T.sleep(duration) *> F.pure(Right(Incremented(revS + 1, step)))
case (Current(_, _), IncrementAsync(revC, _, duration)) =>
T.sleep(duration) *> F.pure(Left(InvalidRevision(revC)))
case (Current(revS, _), Initialize(revC)) if revS == revC => F.pure(Right(Initialized(revS + 1)))
case (Current(_, _), Initialize(rev)) => F.pure(Left(InvalidRevision(rev)))
}
Here's an example of using an in memory aggregate (the unsafeRunSync is invocation is not necessary, it is just
to show how it works):
implicit val ctx = IO.contextShift(ExecutionContext.global)
implicit val timer = IO.timer(ExecutionContext.global)
val agg = Aggregate.inMemory[IO, Int]("global", initialState, next, evaluate[IO]).unsafeRunSync()
agg.name // "global"
agg.evaluate(1, Increment(0, 2)).unsafeRunSync() // Right(Incremented(1, 2))
agg.evaluate(1, IncrementAsync(1, 5, 200 millis)).unsafeRunSync() // Right(Incremented(2, 5))
agg.lastSequenceNr(1).unsafeRunSync() // 2L
agg.test(1, Initialize(0)).unsafeRunSync() // Left(InvalidRevision(0))
agg.test(1, Initialize(2)).unsafeRunSync() // Right(Initialized(3))
agg.currentState(1).unsafeRunSync() // Current(2, 7)
agg.exists(1).unsafeRunSync() // true
agg.snapshot(1).unsafeRunSync() // 2L
agg.evaluate(1, Initialize(2)).unsafeRunSync() // Right(Initialized(3))
agg.foldLeft(1, 0) {
case (initializedCount, Initialized(_)) => initializedCount + 1
case (initializedCount, _) => initializedCount
}.unsafeRunSync() // 1
Constructing an akka based aggregate using a deployment of actors in the same actor system:
implicit val system: ActorSystem = ActorSystem()
implicit val mat: ActorMaterializer = ActorMaterializer()
implicit val ctx: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
implicit val timer: Timer[IO] = IO.timer(ExecutionContext.global)
val config = AkkaSourcingConfig(
Timeout(1.second), // maximum duration to wait for a reply from an aggregate actor
"inmemory-read-journal", // the persistence query journal (from "com.github.dnvriend" %% "akka-persistence-inmemory")
200 milliseconds, // command evaluation timeout
ExecutionContext.global // the execution context where commands are evaluated
)
val passivation = PassivationStrategy.immediately[State, Command]
val retry = RetryStrategy.once[IO]
val name = "my-entity-type"
val agg = AkkaAggregate
.tree[IO](name, initialState, next, evaluate[IO], passivation, retry, config, poolSize = 10)
.unsafeRunSync()
Constructing an akka based aggregate using a deployment of actors spread across the cluster:
implicit val system: ActorSystem = ActorSystem()
implicit val mat: ActorMaterializer = ActorMaterializer()
implicit val ctx: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
implicit val timer: Timer[IO] = IO.timer(ExecutionContext.global)
val config = AkkaSourcingConfig(
Timeout(1.second), // maximum duration to wait for a reply from an aggregate actor
"inmemory-read-journal", // the persistence query journal (from "com.github.dnvriend" %% "akka-persistence-inmemory")
200 milliseconds, // command evaluation timeout
ExecutionContext.global // the execution context where commands are evaluated
)
val passivation = PassivationStrategy.immediately[State, Command]
val retry = RetryStrategy.once[IO]
val name = "my-entity-type"
val agg = AkkaAggregate
.sharded[IO](name, initialState, next, evaluate[IO], passivation, retry, config, shards = 10)
.unsafeRunSync()
The development of this software was supported by funding to the Blue Brain Project, a research center of the École polytechnique fédérale de Lausanne (EPFL), from the Swiss government's ETH Board of the Swiss Federal Institutes of Technology.
Copyright © 2015-2021 Blue Brain Project/EPFL
