Durable State

Module info

To use Akka Persistence, add the module to your project:

sbt

val AkkaVersion = "2.6.21"
libraryDependencies ++= Seq(
  "com.typesafe.akka" %% "akka-persistence-typed" % AkkaVersion,
  "com.typesafe.akka" %% "akka-persistence-testkit" % AkkaVersion % Test
)

Maven

<properties>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>com.typesafe.akka</groupId>
      <artifactId>akka-bom_${scala.binary.version}</artifactId>
      <version>2.6.21</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>
<dependencies>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-persistence-typed_${scala.binary.version}</artifactId>
  </dependency>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-persistence-testkit_${scala.binary.version}</artifactId>
    <scope>test</scope>
  </dependency>
</dependencies>

Gradle

def versions = [
  ScalaBinary: "2.13"
]
dependencies {
  implementation platform("com.typesafe.akka:akka-bom_${versions.ScalaBinary}:2.6.21")

  implementation "com.typesafe.akka:akka-persistence-typed_${versions.ScalaBinary}"
  testImplementation "com.typesafe.akka:akka-persistence-testkit_${versions.ScalaBinary}"
}

You also have to select durable state store plugin, see Persistence Plugins.

Introduction

This model of Akka Persistence enables a stateful actor / entity to store the full state after processing each command instead of using event sourcing. This reduces the conceptual complexity and can be a handy tool for simple use cases. Very much like a CRUD based operation, the API is conceptually simple - a function from current state and incoming command to the next state which replaces the current state in the database.

(State, Command) => State

The current state is always stored in the database. Since only the latest state is stored, we don’t have access to any of the history of changes, unlike event sourced storage. Akka Persistence would read that state and store it in memory. After processing of the command is finished, the new state will be stored in the database. The processing of the next command will not start until the state has been successfully stored in the database.

Akka Persistence also supports Event Sourcing based implementation, where only the events that are persisted by the actor are stored, but not the actual state of the actor. By storing all events, using this model, a stateful actor can be recovered by replaying the stored events to the actor, which allows it to rebuild its state.

Since each entity lives on one node, consistency is guaranteed and reads can be served directly from memory. For details on how this guarantee is ensured, have a look at the Cluster Sharding and DurableStateBehavior section below.

Example and core API

Let’s start with a simple example that models a counter using an Akka persistent actor. The minimum required for a DurableStateBehaviorDurableStateBehavior is:

Scala

copysourceimport akka.persistence.typed.state.scaladsl.DurableStateBehavior
import akka.persistence.typed.PersistenceId

object MyPersistentCounter {
  sealed trait Command[ReplyMessage] extends CborSerializable

  final case class State(value: Int) extends CborSerializable

  def counter(persistenceId: PersistenceId): DurableStateBehavior[Command[_], State] = {
    DurableStateBehavior.apply[Command[_], State](
      persistenceId,
      emptyState = State(0),
      commandHandler =
        (state, command) => throw new NotImplementedError("TODO: process the command & return an Effect"))
  }
}

Java

copysourcepublic class MyPersistentCounter
    extends DurableStateBehavior<MyPersistentCounter.Command<?>, MyPersistentCounter.State> {

  interface Command<ReplyMessage> {}

  public static class State {
    private final int value;

    public State(int value) {
      this.value = value;
    }

    public int get() {
      return value;
    }
  }

  public static Behavior<Command<?>> create(PersistenceId persistenceId) {
    return new MyPersistentCounter(persistenceId);
  }

  private MyPersistentCounter(PersistenceId persistenceId) {
    super(persistenceId);
  }

  @Override
  public State emptyState() {
    return new State(0);
  }

  @Override
  public CommandHandler<Command<?>, State> commandHandler() {
    return (state, command) -> {
      throw new RuntimeException("TODO: process the command & return an Effect");
    };
  }
}

The first important thing to notice is the Behavior of a persistent actor is typed to the type of the Command because this is the type of message a persistent actor should receive. In Akka this is now enforced by the type system.

The components that make up a DurableStateBehavior are:

• persistenceId is the stable unique identifier for the persistent actor.
• emptyState defines the State when the entity is first created e.g. a Counter would start with 0 as state.
• commandHandler defines how to handle commands and map to appropriate effects e.g. persisting state and replying to actors.

Next we’ll discuss each of these in detail.

PersistenceId

The PersistenceIdPersistenceId is the stable unique identifier for the persistent actor in the backend durabe state store.

Cluster Sharding is typically used together with DurableStateBehavior to ensure that there is only one active entity for each PersistenceId (entityId). There are techniques to ensure this uniqueness, an example of which can be found in the Persistence example in the Cluster Sharding documentation. This illustrates how to construct the PersistenceId from the entityTypeKey and entityId provided by the EntityContext.

The entityId in Cluster Sharding is the business domain identifier which uniquely identifies the instance of that specific EntityType. This means that across the cluster we have a unique combination of (EntityType, EntityId). Hence the entityId might not be unique enough to be used as the PersistenceId by itself. For example two different types of entities may have the same entityId. To create a unique PersistenceId the entityId should be prefixed with a stable name of the entity type, which typically is the same as the EntityTypeKey.name that is used in Cluster Sharding. There are PersistenceId.applyPersistenceId.of factory methods to help with constructing such PersistenceId from an entityTypeHint and entityId.

The default separator when concatenating the entityTypeHint and entityId is |, but a custom separator is supported.

A custom identifier can be created with PersistenceId.ofUniqueId.

Command handler

The command handler is a function with 2 parameters, the current State and the incoming Command.

A command handler returns an Effect directive that defines what state, if any, to persist. Effects are created using a factory that is returned via the Effect() method the Effect factory.

The two most commonly used effects are:

• persist will persist the latest value of the state. No history of state changes will be stored
• none no state to be persisted, for example a read-only command

More effects are explained in Effects and Side Effects.

In addition to returning the primary Effect for the command, DurableStateBehaviors can also chain side effects that are to be performed after successful persist which is achieved with the thenRun function e.g. Effect.persist(..).thenRunEffect().persist(..).thenRun.

Completing the example

Let’s fill in the details of the example.

Commands:

Scala

copysourcesealed trait Command[ReplyMessage] extends CborSerializable
final case object Increment extends Command[Nothing]
final case class IncrementBy(value: Int) extends Command[Nothing]
final case class GetValue(replyTo: ActorRef[State]) extends Command[State]
final case object Delete extends Command[Nothing]

Java

copysourceinterface Command<ReplyMessage> {}

public enum Increment implements Command<Void> {
  INSTANCE
}

public static class IncrementBy implements Command<Void> {
  public final int value;

  public IncrementBy(int value) {
    this.value = value;
  }
}

public static class GetValue implements Command<State> {
  private final ActorRef<Integer> replyTo;

  public GetValue(ActorRef<Integer> replyTo) {
    this.replyTo = replyTo;
  }
}

State is a storage for the latest value of the counter.

Scala

copysourcefinal case class State(value: Int) extends CborSerializable

Java

copysourcepublic static class State {
  private final int value;

  public State(int value) {
    this.value = value;
  }

  public int get() {
    return value;
  }
}

The command handler handles the commands Increment, IncrementBy and GetValue.

• Increment increments the counter by 1 and persists the updated value as an effect in the State
• IncrementBy increments the counter by the value passed to it and persists the updated value as an effect in the State
• GetValue retrieves the value of the counter from the State and replies with it to the actor passed in

Scala

copysourceimport akka.persistence.typed.state.scaladsl.Effect

val commandHandler: (State, Command[_]) => Effect[State] = (state, command) =>
  command match {
    case Increment         => Effect.persist(state.copy(value = state.value + 1))
    case IncrementBy(by)   => Effect.persist(state.copy(value = state.value + by))
    case GetValue(replyTo) => Effect.reply(replyTo)(state)
    case Delete            => Effect.delete[State]()
  }

Java

copysource@Override
public CommandHandler<Command<?>, State> commandHandler() {
  return newCommandHandlerBuilder()
      .forAnyState()
      .onCommand(
          Increment.class, (state, command) -> Effect().persist(new State(state.get() + 1)))
      .onCommand(
          IncrementBy.class,
          (state, command) -> Effect().persist(new State(state.get() + command.value)))
      .onCommand(
          GetValue.class, (state, command) -> Effect().reply(command.replyTo, state.get()))
      .build();
}

These are used to create a DurableStateBehavior: These are defined in an DurableStateBehavior:

Scala

copysourceimport akka.persistence.typed.state.scaladsl.DurableStateBehavior
import akka.persistence.typed.PersistenceId

def counter(id: String): DurableStateBehavior[Command[_], State] = {
  DurableStateBehavior.apply[Command[_], State](
    persistenceId = PersistenceId.ofUniqueId(id),
    emptyState = State(0),
    commandHandler = commandHandler)
}

Java

copysourceimport akka.persistence.typed.state.javadsl.DurableStateBehavior;
import akka.persistence.typed.PersistenceId;

public class MyPersistentCounter
    extends DurableStateBehavior<MyPersistentCounter.Command<?>, MyPersistentCounter.State> {

  // commands, events and state defined here

  public static Behavior<Command<?>> create(PersistenceId persistenceId) {
    return new MyPersistentCounter(persistenceId);
  }

  private MyPersistentCounter(PersistenceId persistenceId) {
    super(persistenceId);
  }

  @Override
  public State emptyState() {
    return new State(0);
  }

  @Override
  public CommandHandler<Command<?>, State> commandHandler() {
    return newCommandHandlerBuilder()
        .forAnyState()
        .onCommand(
            Increment.class, (state, command) -> Effect().persist(new State(state.get() + 1)))
        .onCommand(
            IncrementBy.class,
            (state, command) -> Effect().persist(new State(state.get() + command.value)))
        .onCommand(
            GetValue.class, (state, command) -> Effect().reply(command.replyTo, state.get()))
        .build();
  }
}

Effects and Side Effects

A command handler returns an Effect directive that defines what state, if any, to persist. Effects are created using a factory that is returned via the Effect() method the Effect factory and can be one of:

• persist will persist the latest state. If it’s a new persistence id, the record will be inserted. In case of an existing persistence id, the record will be updated only if the revision number of the incoming record is 1 more than the already existing record. Otherwise persist will fail.
• delete will delete the state by setting it to the empty state and the revision number will be incremented by 1.
• none no state to be persisted, for example a read-only command
• unhandled the command is unhandled (not supported) in current state
• stop stop this actor
• stash the current command is stashed
• unstashAll process the commands that were stashed with Effect.stashEffect().stash
• reply send a reply message to the given ActorRef

Note that only one of those can be chosen per incoming command. It is not possible to both persist and say none/unhandled.

In addition to returning the primary Effect for the command DurableStateBehaviors can also chain side effects that are to be performed after successful persist which is achieved with the thenRun function that runs the callback passed to it e.g. Effect.persist(..).thenRunEffect().persist(..).thenRun.

All thenRun registered callbacks are executed sequentially after successful execution of the persist statement (or immediately, in case of none and unhandled).

In addition to thenRun the following actions can also be performed after successful persist:

• thenStop the actor will be stopped
• thenUnstashAll process the commands that were stashed with Effect.stashEffect().stash
• thenReply send a reply message to the given ActorRef

In the example below, we use a different constructor of DurableStateBehavior.withEnforcedReplies, which creates a Behavior for a persistent actor that ensures that every command sends a reply back. Hence it will be a compilation error if the returned effect from a CommandHandler isn’t a ReplyEffect.

Instead of Increment we will have a new command IncrementWithConfirmation that, along with persistence will also send an acknowledgement as a reply to the ActorRef passed in the command.

Example of effects and side-effects:

Scala

copysourcesealed trait Command[ReplyMessage] extends CborSerializable
final case class IncrementWithConfirmation(replyTo: ActorRef[Done]) extends Command[Done]
final case class GetValue(replyTo: ActorRef[State]) extends Command[State]

final case class State(value: Int) extends CborSerializable

def counter(persistenceId: PersistenceId): DurableStateBehavior[Command[_], State] = {
  DurableStateBehavior.withEnforcedReplies[Command[_], State](
    persistenceId,
    emptyState = State(0),
    commandHandler = (state, command) =>
      command match {

        case IncrementWithConfirmation(replyTo) =>
          Effect.persist(state.copy(value = state.value + 1)).thenReply(replyTo)(_ => Done)

        case GetValue(replyTo) =>
          Effect.reply(replyTo)(state)
      })
}

Java

copysourceimport akka.Done;
interface Command<ReplyMessage> {}

public static class IncrementWithConfirmation implements Command<Void> {
  public final ActorRef<Done> replyTo;

  public IncrementWithConfirmation(ActorRef<Done> replyTo) {
    this.replyTo = replyTo;
  }
}

public static class GetValue implements Command<State> {
  private final ActorRef<Integer> replyTo;

  public GetValue(ActorRef<Integer> replyTo) {
    this.replyTo = replyTo;
  }
}

public static class State {
  private final int value;

  public State(int value) {
    this.value = value;
  }

  public int get() {
    return value;
  }
}

public static Behavior<Command<?>> create(PersistenceId persistenceId) {
  return new MyPersistentCounterWithReplies(persistenceId);
}

private MyPersistentCounterWithReplies(PersistenceId persistenceId) {
  super(persistenceId);
}

@Override
public State emptyState() {
  return new State(0);
}

@Override
public CommandHandler<Command<?>, State> commandHandler() {
  return newCommandHandlerBuilder()
      .forAnyState()
      .onCommand(
          IncrementWithConfirmation.class,
          (state, command) ->
              Effect()
                  .persist(new State(state.get() + 1))
                  .thenReply(command.replyTo, (st) -> Done.getInstance()))
      .onCommand(
          GetValue.class, (state, command) -> Effect().reply(command.replyTo, state.get()))
      .build();
}

The most common way to have a side-effect is to use the thenRun method on Effect. In case you have multiple side-effects that needs to be run for several commands, you can factor them out into functions and reuse for all the commands. For example:

Scala

copysource// Example factoring out a chained effect to use in several places with `thenRun`
val commonChainedEffects: Mood => Unit = _ => println("Command processed")
// Then in a command handler:
Effect
  .persist(Remembered("Yep")) // persist event
  .thenRun(commonChainedEffects) // add on common chained effect

Java

copysource// Example factoring out a chained effect to use in several places with `thenRun`
static final Procedure<ExampleState> commonChainedEffect =
    state -> System.out.println("Command handled!");

      @Override
      public CommandHandler<MyCommand, MyEvent, ExampleState> commandHandler() {
        return newCommandHandlerBuilder()
            .forStateType(ExampleState.class)
            .onCommand(
                Cmd.class,
                (state, cmd) ->
                    Effect()
                        .persist(new Evt(cmd.data))
                        .thenRun(() -> cmd.replyTo.tell(new Ack()))
                        .thenRun(commonChainedEffect))
            .build();
      }

Side effects ordering and guarantees

Any side effects are executed on an at-most-once basis and will not be executed if the persist fails.

Side effects are not run when the actor is restarted or started again after being stopped.

The side effects are executed sequentially, it is not possible to execute side effects in parallel, unless they call out to something that is running concurrently (for example sending a message to another actor).

It’s possible to execute a side effect before persisting the state, but that can result in that the side effect is performed but that the state is not stored if the persist fails.

Cluster Sharding and DurableStateBehavior

Cluster Sharding is an excellent fit to spread persistent actors over a cluster, addressing them by id. It makes it possible to have more persistent actors exist in the cluster than what would fit in the memory of one node. Cluster sharding improves the resilience of the cluster. If a node crashes, the persistent actors are quickly started on a new node and can resume operations.

The DurableStateBehavior can then be run as any plain actor as described in actors documentation, but since Akka Persistence is based on the single-writer principle, the persistent actors are typically used together with Cluster Sharding. For a particular persistenceId only one persistent actor instance should be active at one time. Cluster Sharding ensures that there is only one active entity (or actor instance) for each id.

Accessing the ActorContext

If the DurableStateBehaviorDurableStateBehavior needs to use the ActorContextActorContext, for example to spawn child actors, it can be obtained by wrapping construction with Behaviors.setup:

Scala

copysourceimport akka.persistence.typed.state.scaladsl.Effect
import akka.persistence.typed.state.scaladsl.DurableStateBehavior.CommandHandler

def apply(): Behavior[String] =
  Behaviors.setup { context =>
    DurableStateBehavior[String, State](
      persistenceId = PersistenceId.ofUniqueId("myPersistenceId"),
      emptyState = State(0),
      commandHandler = CommandHandler.command { cmd =>
        context.log.info("Got command {}", cmd)
        Effect.none
      })
  }

Java

copysourcepublic class MyPersistentBehavior
    extends DurableStateBehavior<MyPersistentBehavior.Command, MyPersistentBehavior.State> {

  public static Behavior<Command> create(PersistenceId persistenceId) {
    return Behaviors.setup(ctx -> new MyPersistentBehavior(persistenceId, ctx));
  }

  // this makes the context available to the command handler etc.
  private final ActorContext<Command> context;

  // optionally if you only need `ActorContext.getSelf()`
  private final ActorRef<Command> self;

  public MyPersistentBehavior(PersistenceId persistenceId, ActorContext<Command> ctx) {
    super(persistenceId);
    this.context = ctx;
    this.self = ctx.getSelf();
  }

}

Changing Behavior

After processing a message, actors are able to return the Behavior that is used for the next message.

As you can see in the above examples this is not supported by persistent actors. Instead, the state is persisted as an Effect by the commandHandler.

The reason a new behavior can’t be returned is that behavior is part of the actor’s state and must also carefully be reconstructed during recovery from the persisted state. This would imply that the state needs to be encoded such that the behavior can also be restored from it. That would be very prone to mistakes which is why it is not allowed in Akka Persistence.

For basic actors you can use the same set of command handlers independent of what state the entity is in. For more complex actors it’s useful to be able to change the behavior in the sense that different functions for processing commands may be defined depending on what state the actor is in. This is useful when implementing finite state machine (FSM) like entities.

The next example demonstrates how to define different behavior based on the current State. It shows an actor that represents the state of a blog post. Before a post is started the only command it can process is to AddPost. Once it is started then one can look it up with GetPost, modify it with ChangeBody or publish it with Publish.

The state is captured by:

Scala

copysourcesealed trait State

case object BlankState extends State

final case class DraftState(content: PostContent) extends State {
  def withBody(newBody: String): DraftState =
    copy(content = content.copy(body = newBody))

  def postId: String = content.postId
}

final case class PublishedState(content: PostContent) extends State {
  def postId: String = content.postId
}

Java

copysourceinterface State {}

enum BlankState implements State {
  INSTANCE
}

static class DraftState implements State {
  final PostContent content;

  DraftState(PostContent content) {
    this.content = content;
  }

  DraftState withContent(PostContent newContent) {
    return new DraftState(newContent);
  }

  DraftState withBody(String newBody) {
    return withContent(new PostContent(postId(), content.title, newBody));
  }

  String postId() {
    return content.postId;
  }
}

static class PublishedState implements State {
  final PostContent content;

  PublishedState(PostContent content) {
    this.content = content;
  }

  PublishedState withContent(PostContent newContent) {
    return new PublishedState(newContent);
  }

  PublishedState withBody(String newBody) {
    return withContent(new PostContent(postId(), content.title, newBody));
  }

  String postId() {
    return content.postId;
  }
}

The commands, of which only a subset are valid depending on the state:

Scala

copysourcesealed trait Command
final case class AddPost(content: PostContent, replyTo: ActorRef[StatusReply[AddPostDone]]) extends Command
final case class AddPostDone(postId: String)
final case class GetPost(replyTo: ActorRef[PostContent]) extends Command
final case class ChangeBody(newBody: String, replyTo: ActorRef[Done]) extends Command
final case class Publish(replyTo: ActorRef[Done]) extends Command
final case class PostContent(postId: String, title: String, body: String)

Java

copysourcepublic interface Command {}
public static class AddPost implements Command {
  final PostContent content;
  final ActorRef<AddPostDone> replyTo;

  public AddPost(PostContent content, ActorRef<AddPostDone> replyTo) {
    this.content = content;
    this.replyTo = replyTo;
  }
}

public static class AddPostDone implements Command {
  final String postId;

  public AddPostDone(String postId) {
    this.postId = postId;
  }
}
public static class GetPost implements Command {
  final ActorRef<PostContent> replyTo;

  public GetPost(ActorRef<PostContent> replyTo) {
    this.replyTo = replyTo;
  }
}

public static class ChangeBody implements Command {
  final String newBody;
  final ActorRef<Done> replyTo;

  public ChangeBody(String newBody, ActorRef<Done> replyTo) {
    this.newBody = newBody;
    this.replyTo = replyTo;
  }
}

public static class Publish implements Command {
  final ActorRef<Done> replyTo;

  public Publish(ActorRef<Done> replyTo) {
    this.replyTo = replyTo;
  }
}

public static class PostContent implements Command {
  final String postId;
  final String title;
  final String body;

  public PostContent(String postId, String title, String body) {
    this.postId = postId;
    this.title = title;
    this.body = body;
  }
}

The command handler to process each command is decided by the state class (or state predicate) that is given to the forStateType of the CommandHandlerBuilder and the match cases in the builders. The command handler to process each command is decided by first looking at the state and then the command. It typically becomes two levels of pattern matching, first on the state and then on the command. Delegating to methods like addPost, changeBody, publish etc. is a good practice because the one-line cases give a nice overview of the message dispatch.

Scala

copysourceprivate val commandHandler: (State, Command) => Effect[State] = { (state, command) =>
  state match {

    case BlankState =>
      command match {
        case cmd: AddPost => addPost(cmd)
        case _            => Effect.unhandled
      }

    case draftState: DraftState =>
      command match {
        case cmd: ChangeBody  => changeBody(draftState, cmd)
        case Publish(replyTo) => publish(draftState, replyTo)
        case GetPost(replyTo) => getPost(draftState, replyTo)
        case AddPost(_, replyTo) =>
          Effect.unhandled[State].thenRun(_ => replyTo ! StatusReply.Error("Cannot add post while in draft state"))
      }

    case publishedState: PublishedState =>
      command match {
        case GetPost(replyTo) => getPost(publishedState, replyTo)
        case AddPost(_, replyTo) =>
          Effect.unhandled[State].thenRun(_ => replyTo ! StatusReply.Error("Cannot add post, already published"))
        case _ => Effect.unhandled
      }
  }
}

private def addPost(cmd: AddPost): Effect[State] = {
  Effect.persist(DraftState(cmd.content)).thenRun { _ =>
    // After persist is done additional side effects can be performed
    cmd.replyTo ! StatusReply.Success(AddPostDone(cmd.content.postId))
  }
}

private def changeBody(state: DraftState, cmd: ChangeBody): Effect[State] = {
  Effect.persist(state.withBody(cmd.newBody)).thenRun { _ =>
    cmd.replyTo ! Done
  }
}

private def publish(state: DraftState, replyTo: ActorRef[Done]): Effect[State] = {
  Effect.persist(PublishedState(state.content)).thenRun { _ =>
    println(s"Blog post ${state.postId} was published")
    replyTo ! Done
  }
}

private def getPost(state: DraftState, replyTo: ActorRef[PostContent]): Effect[State] = {
  replyTo ! state.content
  Effect.none
}

private def getPost(state: PublishedState, replyTo: ActorRef[PostContent]): Effect[State] = {
  replyTo ! state.content
  Effect.none
}

Java

copysource@Override
public CommandHandler<Command, State> commandHandler() {
  CommandHandlerBuilder<Command, State> builder = newCommandHandlerBuilder();

  builder.forStateType(BlankState.class).onCommand(AddPost.class, this::onAddPost);

  builder
      .forStateType(DraftState.class)
      .onCommand(ChangeBody.class, this::onChangeBody)
      .onCommand(Publish.class, this::onPublish)
      .onCommand(GetPost.class, this::onGetPost);

  builder
      .forStateType(PublishedState.class)
      .onCommand(ChangeBody.class, this::onChangeBody)
      .onCommand(GetPost.class, this::onGetPost);

  builder.forAnyState().onCommand(AddPost.class, (state, cmd) -> Effect().unhandled());

  return builder.build();
}

private Effect<State> onAddPost(AddPost cmd) {
  return Effect()
      .persist(new DraftState(cmd.content))
      .thenRun(() -> cmd.replyTo.tell(new AddPostDone(cmd.content.postId)));
}

private Effect<State> onChangeBody(DraftState state, ChangeBody cmd) {
  return Effect()
      .persist(state.withBody(cmd.newBody))
      .thenRun(() -> cmd.replyTo.tell(Done.getInstance()));
}

private Effect<State> onChangeBody(PublishedState state, ChangeBody cmd) {
  return Effect()
      .persist(state.withBody(cmd.newBody))
      .thenRun(() -> cmd.replyTo.tell(Done.getInstance()));
}

private Effect<State> onPublish(DraftState state, Publish cmd) {
  return Effect()
      .persist(new PublishedState(state.content))
      .thenRun(
          () -> {
            System.out.println("Blog post published: " + state.postId());
            cmd.replyTo.tell(Done.getInstance());
          });
}

private Effect<State> onGetPost(DraftState state, GetPost cmd) {
  cmd.replyTo.tell(state.content);
  return Effect().none();
}

private Effect<State> onGetPost(PublishedState state, GetPost cmd) {
  cmd.replyTo.tell(state.content);
  return Effect().none();
}

And finally the behavior is created from the DurableStateBehavior.apply:

Scala

copysourceobject BlogPostEntityDurableState {
  // commands, state defined here

  def apply(entityId: String, persistenceId: PersistenceId): Behavior[Command] = {
    Behaviors.setup { context =>
      context.log.info("Starting BlogPostEntityDurableState {}", entityId)
      DurableStateBehavior[Command, State](persistenceId, emptyState = BlankState, commandHandler)
    }
  }

  // commandHandler defined here
}

Java

copysourcepublic class BlogPostEntityDurableState
    extends DurableStateBehavior<
        BlogPostEntityDurableState.Command, BlogPostEntityDurableState.State> {
  // commands and state as in above snippets

  public static Behavior<Command> create(String entityId, PersistenceId persistenceId) {
    return Behaviors.setup(
        context -> {
          context.getLog().info("Starting BlogPostEntityDurableState {}", entityId);
          return new BlogPostEntityDurableState(persistenceId);
        });
  }

  private BlogPostEntityDurableState(PersistenceId persistenceId) {
    super(persistenceId);
  }

  @Override
  public State emptyState() {
    return BlankState.INSTANCE;
  }

  // commandHandler, eventHandler as in above snippets
}

This can be refactored one or two steps further by defining the command handlers in the state class as illustrated in command handlers in the state.

There is also an example illustrating an optional initial state.

Replies

The Request-Response interaction pattern is very common for persistent actors, because you typically want to know if the command was rejected due to validation errors and when accepted you want a confirmation when the events have been successfully stored.

Therefore you typically include a ActorRef[ReplyMessageType]ActorRef<ReplyMessageType>. If the command can either have a successful response or a validation error returned, the generic response type StatusReply[ReplyType]] StatusReply<ReplyType> can be used. If the successful reply does not contain a value but is more of an acknowledgement a pre defined StatusReply.AckStatusReply.ack() of type StatusReply[Done]StatusReply<Done> can be used.

After validation errors or after persisting events, using a thenRun side effect, the reply message can be sent to the ActorRef.

Scala

copysourcefinal case class AddPost(content: PostContent, replyTo: ActorRef[StatusReply[AddPostDone]]) extends Command
final case class AddPostDone(postId: String)

Java

copysourcepublic static class AddPost implements Command {
  final PostContent content;
  final ActorRef<AddPostDone> replyTo;

  public AddPost(PostContent content, ActorRef<AddPostDone> replyTo) {
    this.content = content;
    this.replyTo = replyTo;
  }
}

public static class AddPostDone implements Command {
  final String postId;

  public AddPostDone(String postId) {
    this.postId = postId;
  }
}

Scala

copysourceEffect.persist(DraftState(cmd.content)).thenRun { _ =>
  // After persist is done additional side effects can be performed
  cmd.replyTo ! StatusReply.Success(AddPostDone(cmd.content.postId))
}

Java

copysourcereturn Effect()
    .persist(new DraftState(cmd.content))
    .thenRun(() -> cmd.replyTo.tell(new AddPostDone(cmd.content.postId)));

Since this is such a common pattern there is a reply effect for this purpose. It has the nice property that it can be used to enforce that you do not forget to specify replies when implementing the DurableStateBehavior. If it’s defined with DurableStateBehavior.withEnforcedRepliesDurableStateBehaviorWithEnforcedReplies there will be compilation errors if the returned effect isn’t a ReplyEffect, which can be created with Effect.replyEffect().reply, Effect.noReplyEffect().noReply, Effect.thenReplyEffect().thenReply, or Effect.thenNoReplyEffect().thenNoReply.

Scala

copysourcedef apply(persistenceId: PersistenceId): Behavior[Command] = {
  DurableStateBehavior
    .withEnforcedReplies[Command, Account](persistenceId, EmptyAccount, (state, cmd) => state.applyCommand(cmd))
}

Java

copysourcepublic class AccountEntity
    extends DurableStateBehaviorWithEnforcedReplies<
        AccountEntity.Command, AccountEntity.Account> {

The commands must have a field of ActorRef[ReplyMessageType]ActorRef<ReplyMessageType> that can then be used to send a reply.

Scala

copysourcesealed trait Command extends CborSerializable
final case class Withdraw(amount: BigDecimal, replyTo: ActorRef[StatusReply[Done]]) extends Command

Java

copysourceinterface Command extends CborSerializable {}

The ReplyEffect is created with Effect.replyEffect().reply, Effect.noReplyEffect().noReply, Effect.thenReplyEffect().thenReply, or Effect.thenNoReplyEffect().thenNoReply.

Note that command handlers are defined with newCommandHandlerWithReplyBuilder when using EventSourcedBehaviorWithEnforcedReplies, as opposed to newCommandHandlerBuilder when using EventSourcedBehavior.

Scala

copysourceprivate def deposit(cmd: Deposit) = {
  Effect.persist(copy(balance = balance + cmd.amount)).thenReply(cmd.replyTo)(_ => StatusReply.Ack)
}

private def withdraw(cmd: Withdraw) = {
  if (canWithdraw(cmd.amount))
    Effect.persist(copy(balance = balance - cmd.amount)).thenReply(cmd.replyTo)(_ => StatusReply.Ack)
  else
    Effect.reply(cmd.replyTo)(
      StatusReply.Error(s"Insufficient balance ${balance} to be able to withdraw ${cmd.amount}"))
}

Java

copysourceprivate ReplyEffect<Account> withdraw(OpenedAccount account, Withdraw command) {
  if (!account.canWithdraw(command.amount)) {
    return Effect()
        .reply(
            command.replyTo,
            StatusReply.error("not enough funds to withdraw " + command.amount));
  } else {
    return Effect()
        .persist(account.makeWithdraw(command.amount))
        .thenReply(command.replyTo, account2 -> StatusReply.ack());
  }
}

These effects will send the reply message even when DurableStateBehavior.withEnforcedRepliesDurableStateBehaviorWithEnforcedReplies is not used, but then there will be no compilation errors if the reply decision is left out.

Note that the noReply is a way of making a conscious decision that a reply shouldn’t be sent for a specific command or that a reply will be sent later, perhaps after some asynchronous interaction with other actors or services.

Serialization

The same serialization mechanism as for actor messages is also used for persistent actors.

You need to enable serialization for your commands (messages) and state. Serialization with Jackson is a good choice in many cases and our recommendation if you don’t have other preference.

Tagging

Persistence allows you to use tags in persistence query. Tagging allows you to identify a subset of states in the durable store and separately consume them as a stream through the DurableStateStoreQuery interface.

Scala

copysourceDurableStateBehavior[Command[_], State](
  persistenceId = PersistenceId.ofUniqueId("abc"),
  emptyState = State(0),
  commandHandler = (state, cmd) => throw new NotImplementedError("TODO: process the command & return an Effect"))
  .withTag("tag1")

Java

copysourcepublic class MyPersistentBehavior
    extends DurableStateBehavior<MyPersistentBehavior.Command, MyPersistentBehavior.State> {
  @Override
  public String tag() {
    return "tag1";
  }

Wrapping DurableStateBehavior

When creating a DurableStateBehavior, it is possible to wrap DurableStateBehavior in other behaviors such as Behaviors.setup in order to access the ActorContext object. For instance to access the logger from within the ActorContext to log for debugging the commandHandler.

Scala

copysourceBehaviors.setup[Command[_]] { context =>
  DurableStateBehavior[Command[_], State](
    persistenceId = PersistenceId.ofUniqueId("abc"),
    emptyState = State(0),
    commandHandler = CommandHandler.command { cmd =>
      context.log.info("Got command {}", cmd)
      Effect.none
    })
}

Java

copysourcepublic class MyPersistentBehavior
    extends DurableStateBehavior<MyPersistentBehavior.Command, MyPersistentBehavior.State> {


  public static Behavior<Command> create(PersistenceId persistenceId) {
    return Behaviors.setup(context -> new MyPersistentBehavior(persistenceId, context));
  }

  private final ActorContext<Command> context;

  private MyPersistentBehavior(PersistenceId persistenceId, ActorContext<Command> context) {
    super(
        persistenceId,
        SupervisorStrategy.restartWithBackoff(
            Duration.ofSeconds(10), Duration.ofSeconds(30), 0.2));
    this.context = context;
  }

  @Override
  public CommandHandler<Command, State> commandHandler() {
    return (state, command) -> {
      context.getLog().info("In command handler");
      return Effect().none();
    };
  }