Akka Persistence Cassandra

Akka is a toolkit for building highly concurrent, distributed, and resilient message-driven applications for Java and Scala.

Akka's approach to handling concurrency is based on the Actor Model. An actor is essentially an object that receives messages and takes actions to handle them. It is a higher level construct that helps to write concurrent code without the trouble of dealing with low-level concurrency primitives when writing multi-threaded high performance applications.

Akka Persistence enables stateful akka-actors to persist their state so that it can be recovered when an actor is either restarted, such as after a JVM crash by a supervisor or a manual stop-start, or migrated in a cluster.

The Akka Persistence Cassandra plugin allows you to use Apache Cassandra as a backend for Akka Persistence and Akka Persistence Query. It uses Alpakka Cassandra for Cassandra access which is based on the Datastax Java Driver. Alpakka Cassandra offers an Akka Streams API on top of a CqlSession from the Datastax Java Driver version 4.0 and later.

The following section illustrates how to run an Akka Persistence Cassandra based application with YugabyteDB.

Prerequisites

To use the Akka Persistence Cassandra plugin, ensure that you have the following:

  • YugabyteDB up and running. Download and install YugabyteDB by following the steps in Quick start.
  • Java Development Kit (JDK) 8, 11 or 17 installed. JDK installers for Linux and macOS can be downloaded from Oracle, Adoptium (OpenJDK), or Azul Systems (OpenJDK). Homebrew users on macOS can install using brew install openjdk.
  • sbt is installed.

The following example is inspired from the akka-cassandra-demo repository.

Create a Scala project

  1. Create a project using the following command:

    sbt new scala/hello-world.g8

    Press enter when you're prompted to name the application.

  2. Replace the existing code in the build.sbt file from your project's home directory with the following:

    cd hello-world-template

    //Delete the existing datastax java driver from the dependencies of the application.

lazy val akkaVersion     = "2.7.0"

libraryDependencies ++= Seq(
 "com.typesafe.akka" %% "akka-actor-typed"           % akkaVersion,
 "com.typesafe.akka" %% "akka-stream"                % akkaVersion,
 "com.typesafe.akka" %% "akka-persistence-typed"     % akkaVersion,
 "com.typesafe.akka" %% "akka-serialization-jackson" % akkaVersion,
 "com.typesafe.akka" %% "akka-cluster-tools"         % akkaVersion,
 "ch.qos.logback"    % "logback-classic"             % "1.2.11",
 "com.typesafe.akka" %% "akka-persistence-cassandra" % "1.1.0"
)

//Add the yugabyte java driver to the dependencies of the application.
libraryDependencies ++= Seq("com.yugabyte" % "java-driver-core" % "4.6.0-yb-11")

//Exclude the datastax java driver from getting pulled due to nested dependency.
excludeDependencies ++= Seq(ExclusionRule("com.datastax.oss", "java-driver-core"))

run / fork := true
Global / cancelable := false // ctrl-c

Use the Akka Persistence Cassandra plugin

To create an application using the plugin, do the following:

Write a sample application

To write a sample application and customize its configuration, do the following:

  1. Replace the existing code in src/main/scala/Main.scala with the following:

    import akka.NotUsed
import akka.actor.typed.scaladsl.{ ActorContext, Behaviors }
import akka.actor.typed.{ ActorRef, ActorSystem, Behavior }
import akka.persistence.typed.PersistenceId
import akka.persistence.typed.RecoveryCompleted
import akka.persistence.typed.scaladsl.{ Effect, EventSourcedBehavior }
import java.util.UUID

// a single bank account
object PersistentBankAccount {

  // commands = messages
  sealed trait Command extends CborSerializable
  final case class CreateBankAccount(
      id: String,
      user: String,
      currency: String,
      initialBalance: Double,
      replyTo: ActorRef[Response])
      extends Command

  // events = to persist to Yugabyte
  trait Event extends CborSerializable
  final case class BankAccountCreated(bankAccount: BankAccount) extends Event

  // state
  final case class BankAccount(
      id: String,
      user: String,
      currency: String,
      balance: Double)
      extends CborSerializable

  // responses
  sealed trait Response
  final case class BankAccountCreatedResponse(id: String)
      extends Response
      with CborSerializable

  val commandHandler: (BankAccount, Command) => Effect[Event, BankAccount] =
    (state, command) =>
      command match {
        case CreateBankAccount(id, user, currency, initialBalance, replyTo) =>
          require(id == state.id, s"Wrong id $id, expected ${state.id}")
          if (state.user == "") {
            // new account
            Effect
              .persist(
                BankAccountCreated(
                  BankAccount(id, user, currency, initialBalance)
                )
              ) // persisted into Yugabyte
              .thenReply(replyTo)(_ => BankAccountCreatedResponse(state.id))
          } else if (state.user == user) {
            // already created, idempotent retry
            Effect.reply(replyTo)(BankAccountCreatedResponse(id))
          } else {
            throw new IllegalArgumentException(
              s"Wrong user $user for existing account $id, expected ${state.user}")
          }
      }

  val eventHandler: (BankAccount, Event) => BankAccount = (state, event) =>
    event match {
      case BankAccountCreated(bankAccount) =>
        bankAccount
    }

  def apply(id: String): Behavior[Command] =
    EventSourcedBehavior[Command, Event, BankAccount](
      persistenceId = PersistenceId.ofUniqueId(id),
      emptyState = BankAccount(id, "", "", 0.0),
      commandHandler = commandHandler,
      eventHandler = eventHandler)
}

object Bank {

  import PersistentBankAccount.{ Command, CreateBankAccount }

  sealed trait Event extends CborSerializable
  case class BankAccountCreated(id: String) extends Event

  case class State(accounts: Set[String]) extends CborSerializable

  def commandHandler(context: ActorContext[Command])
      : (State, Command) => Effect[Event, State] = (state, command) =>
    command match {
      case createCommand @ CreateBankAccount(id, _, _, _, _) =>
        if (state.accounts.contains(id)) {
          val account = context
            .child(id)
            .getOrElse(context.spawn(PersistentBankAccount(id), id))
            .unsafeUpcast[Command]
          account ! createCommand
          Effect.none
        } else {
          Effect.persist(BankAccountCreated(id)).thenRun { _ =>
            val newBankAccount = context.spawn(PersistentBankAccount(id), id)
            newBankAccount ! createCommand
          }
        }
    }

  def eventHandler(context: ActorContext[Command]): (State, Event) => State =
    (state, event) =>
      event match {
        case BankAccountCreated(id) =>
          state.copy(accounts = state.accounts + id)
      }

  // behavior
  def apply(): Behavior[Command] = Behaviors.setup { context =>
    EventSourcedBehavior[Command, Event, State](
      persistenceId = PersistenceId.ofUniqueId("bank"),
      emptyState = State(Set.empty),
      commandHandler = commandHandler(context),
      eventHandler = eventHandler(context)).receiveSignal {
      case (state, RecoveryCompleted) =>
        context.log.info(s"Accounts [{}]", state.accounts.mkString(", "))
        state.accounts.foreach(id =>
          context.spawn(PersistentBankAccount(id), id))
    }
  }
}

object BankPlayground {
  import PersistentBankAccount.{
    BankAccountCreatedResponse,
    CreateBankAccount,
    Response
  }

  def main(args: Array[String]): Unit = {
    val rootBehavior: Behavior[NotUsed] = Behaviors.setup { context =>
      val bank = context.spawn(Bank(), "bank")

      val responseHandler = context.spawn(ResponseHandler(), "replyHandler")

      val id = UUID.randomUUID().toString
      bank ! CreateBankAccount(id, "Harsh", "INR", 10000, responseHandler)

      Behaviors.empty
    }
    val system = ActorSystem(rootBehavior, "Demo")
  }

  object ResponseHandler {
    def apply(): Behavior[Response] = {
      Behaviors.receive[Response] {
        case (context, BankAccountCreatedResponse(id)) =>
          context.log.info(s"successfully created account $id")
          Behaviors.same
      }
    }
  }
}

/**
 * Marker trait for serialization with Jackson CBOR
 */
trait CborSerializable

  2. Create a resources directory in src/main using mkdir src/main/resources.

  3. Create a file src/main/resources/application.conf and copy the following code:

    # Journal
akka.persistence.journal.plugin = "akka.persistence.cassandra.journal"
akka.persistence.cassandra.journal.keyspace-autocreate = true
akka.persistence.cassandra.journal.tables-autocreate = true
datastax-java-driver.advanced.reconnect-on-init = true

# Snapshot
akka.persistence.snapshot-store.plugin = "akka.persistence.cassandra.snapshot"
akka.persistence.cassandra.snapshot.keyspace-autocreate = true
akka.persistence.cassandra.snapshot.tables-autocreate = true

akka.actor.serialization-bindings {
  "CborSerializable" = jackson-cbor
}

Run the application

Run the application from the project home directory using the command sbt run.

You should see output similar to the following:

successfully created account b2b51f94-b021-4fb8-ba33-219cd0aea3c1

Note that it can take some time to view the output as all the keyspaces and tables that are required by Akka Persistence Cassandra are created for the first time.

Verify the integration using ycqlsh

Run ycqlsh to connect to your database using the YCQL API as follows:

./bin/ycqlsh localhost

Connected to local cluster at 127.0.0.1:9042.
[ycqlsh 5.0.1 | Cassandra 3.9-SNAPSHOT | CQL spec 3.4.2 | Native protocol v4]
Use HELP for help.
ycqlsh>

Run the following query:

select * from akka.messages;

The output should include an event with a persistance_id same as the account_id (b2b51f94-b021-4fb8-ba33-219cd0aea3c1) obtained after running the application.

Limitations

Some older versions of Akka Persistence Cassandra use Cassandra Materialized Views which may not work with YugabyteDB. Use versions of Akka Persistence Cassandra that do not use Cassandra Materialized Views.