Transactors (Scala)

Transactors (Scala)

Why Transactors?

Actors are excellent for solving problems where you have many independent processes that can work in isolation and only interact with other Actors through message passing. This model fits many problems. But the actor model is unfortunately a terrible model for implementing truly shared state. E.g. when you need to have consensus and a stable view of state across many components. The classic example is the bank account where clients can deposit and withdraw, in which each operation needs to be atomic. For detailed discussion on the topic see this JavaOne presentation.

STM on the other hand is excellent for problems where you need consensus and a stable view of the state by providing compositional transactional shared state. Some of the really nice traits of STM are that transactions compose, and it raises the abstraction level from lock-based concurrency.

Akka’s Transactors combine Actors and STM to provide the best of the Actor model (concurrency and asynchronous event-based programming) and STM (compositional transactional shared state) by providing transactional, compositional, asynchronous, event-based message flows.

Generally, the STM is not needed very often when working with Akka. Some use-cases (that we can think of) are:

  • When you really need composable message flows across many actors updating their internal local state but need them to do that atomically in one big transaction. Might not be often but when you do need this then you are screwed without it.
  • When you want to share a datastructure across actors.

Actors and STM

You can combine Actors and STM in several ways. An Actor may use STM internally so that particular changes are guaranteed to be atomic. Actors may also share transactional datastructures as the STM provides safe shared state across threads.

It’s also possible to coordinate transactions across Actors or threads so that either the transactions in a set all commit successfully or they all fail. This is the focus of Transactors and the explicit support for coordinated transactions in this section.

Coordinated transactions

Akka provides an explicit mechanism for coordinating transactions across Actors. Under the hood it uses a CommitBarrier, similar to a CountDownLatch.

Here is an example of coordinating two simple counter Actors so that they both increment together in coordinated transactions. If one of them was to fail to increment, the other would also fail.

import akka.actor._
import akka.transactor._
import scala.concurrent.stm._

case class Increment(friend: Option[ActorRef] = None)
case object GetCount

class Counter extends Actor {
  val count = Ref(0)

  def receive = {
    case coordinated @ Coordinated(Increment(friend))  {
      friend foreach (_ ! coordinated(Increment()))
      coordinated atomic { implicit t 
        count transform (_ + 1)
      }
    }
    case GetCount  sender ! count.single.get
  }
}
import akka.dispatch.Await
import akka.util.duration._
import akka.util.Timeout
import akka.pattern.ask

val system = ActorSystem("app")

val counter1 = system.actorOf(Props[Counter], name = "counter1")
val counter2 = system.actorOf(Props[Counter], name = "counter2")

implicit val timeout = Timeout(5 seconds)

counter1 ! Coordinated(Increment(Some(counter2)))

val count = Await.result(counter1 ? GetCount, timeout.duration)

// count == 1

Note that creating a Coordinated object requires a Timeout to be specified for the coordinated transaction. This can be done implicitly, by having an implicit Timeout in scope, or explicitly, by passing the timeout when creating a a Coordinated object. Here’s an example of specifying an implicit timeout:

import akka.util.duration._
import akka.util.Timeout

implicit val timeout = Timeout(5 seconds)

To start a new coordinated transaction that you will also participate in, just create a Coordinated object (this assumes an implicit timeout):

val coordinated = Coordinated()

To start a coordinated transaction that you won’t participate in yourself you can create a Coordinated object with a message and send it directly to an actor. The recipient of the message will be the first member of the coordination set:

actor ! Coordinated(Message)

To receive a coordinated message in an actor simply match it in a case statement:

def receive = {
  case coordinated @ Coordinated(Message)  {
    // coordinated atomic ...
  }
}

To include another actor in the same coordinated transaction that you’ve created or received, use the apply method on that object. This will increment the number of parties involved by one and create a new Coordinated object to be sent.

actor ! coordinated(Message)

To enter the coordinated transaction use the atomic method of the coordinated object:

coordinated atomic { implicit t 
  // do something in the coordinated transaction ...
}

The coordinated transaction will wait for the other transactions before committing. If any of the coordinated transactions fail then they all fail.

Note

The same actor should not be added to a coordinated transaction more than once. The transaction will not be able to complete as an actor only processes a single message at a time. When processing the first message the coordinated transaction will wait for the commit barrier, which in turn needs the second message to be received to proceed.

Transactor

Transactors are actors that provide a general pattern for coordinating transactions, using the explicit coordination described above.

Here’s an example of a simple transactor that will join a coordinated transaction:

import akka.transactor._
import scala.concurrent.stm._

case object Increment

class Counter extends Transactor {
  val count = Ref(0)

  def atomically = implicit txn  {
    case Increment  count transform (_ + 1)
  }
}

You could send this Counter transactor a Coordinated(Increment) message. If you were to send it just an Increment message it will create its own Coordinated (but in this particular case wouldn’t be coordinating transactions with any other transactors).

To coordinate with other transactors override the coordinate method. The coordinate method maps a message to a set of SendTo objects, pairs of ActorRef and a message. You can use the include and sendTo methods to easily coordinate with other transactors. The include method will send on the same message that was received to other transactors. The sendTo method allows you to specify both the actor to send to, and the message to send.

Example of coordinating an increment:

import akka.actor._
import akka.transactor._
import scala.concurrent.stm._

case object Increment

class FriendlyCounter(friend: ActorRef) extends Transactor {
  val count = Ref(0)

  override def coordinate = {
    case Increment  include(friend)
  }

  def atomically = implicit txn  {
    case Increment  count transform (_ + 1)
  }
}

Using include to include more than one transactor:

override def coordinate = {
  case Message  include(actor1, actor2, actor3)
}

Using sendTo to coordinate transactions but pass-on a different message than the one that was received:

override def coordinate = {
  case SomeMessage   sendTo(someActor -> SomeOtherMessage)
  case OtherMessage  sendTo(actor1 -> Message1, actor2 -> Message2)
}

To execute directly before or after the coordinated transaction, override the before and after methods. These methods also expect partial functions like the receive method. They do not execute within the transaction.

To completely bypass coordinated transactions override the normally method. Any message matched by normally will not be matched by the other methods, and will not be involved in coordinated transactions. In this method you can implement normal actor behavior, or use the normal STM atomic for local transactions.

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