Akka Documentation

Version 2.5.11

Persistence

Akka Persistence is a library for building event sourced actors. For background about how it works see the untyped Akka Persistence section. This documentation shows how the typed API for persistence works and assumes you know what is meant by Command, Event and State.

Warning

This module is currently marked as may change in the sense of being the subject of active research. This means that API or semantics can change without warning or deprecation period and it is not recommended to use this module in production just yet—you have been warned.

Dependency

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

sbt
libraryDependencies += "com.typesafe.akka" %% "akka-persistence-typed" % "2.5.11"
Maven
<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-persistence-typed_2.12</artifactId>
  <version>2.5.11</version>
</dependency>
Gradle
dependencies {
  compile group: 'com.typesafe.akka', name: 'akka-persistence-typed_2.12', version: '2.5.11'
}

Example

Let’s start with a simple example. The minimum required for a PersistentBehavior is:

Scala
sealed trait Command
sealed trait Event
case class State()

val behavior: Behavior[Command] =
  PersistentBehaviors.immutable[Command, Event, State](
    persistenceId = "abc",
    initialState = State(),
    commandHandler = (ctx, state, cmd) ⇒ ???,
    eventHandler = (state, evt) ⇒ ???)
Java
public interface Command {}
public interface Event {}
public static class State {}

public static class MyPersistentBehavior extends PersistentBehavior<Command, Event, State> {

  public MyPersistentBehavior(String persistenceId) {
    super(persistenceId);
  }

  @Override
  public State initialState() {
    return new State();
  }

  @Override
  public CommandHandler<Command, Event, State> commandHandler() {
    return new CommandHandler<Command, Event, State>() {
      @Override
      public Effect<Event, State> apply(ActorContext<Command> ctx, State state, Command command) {
        return Effect().none();
      }
    };
  }

  @Override
  public EventHandler<Event, State> eventHandler() {
    return new EventHandler<Event, State>() {
      @Override
      public State apply(State state, Event event) {
        return state;
      }
    };
  }

  @Override
  public void onRecoveryCompleted(ActorContext<Command> ctx, State state) {
    // called once recovery is completed
  }

  @Override
  public Set<String> tagsFor(Event event) {
    // inspect the event and decide if it should be tagged
    return Collections.emptySet();
  }
}

static Behavior<Command> persistentBehavior = new MyPersistentBehavior("pid");

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 Typed this is now enforced by the type system. The event and state are only used internally.

The components that make up a PersistentBehavior are:

  • persistenceId is the unique identifier for the persistent actor.
  • initialState defines the State when the entity is first created e.g. a Counter would start with 0 as state.
  • commandHandler defines how to handle command, resulting in Effects e.g. persisting events, stopping the persistent actor.
  • eventHandler updates the current state when an event has been persisted.

Next we’ll discuss each of these in detail.

Command handler

The command handler is a function with 3 parameters for the ActorContext, current State, and Command.

A command handler returns an Effect directive that defines what event or events, if any, to persist. Effects are created using a factory that is returned via the Effect() method Effects are created using the Effect factory and can be used to create various effects such as:

  • persist will persist one single event or several events atomically, i.e. all events are stored or none of them are stored if there is an error
  • none no events are to be persisted, for example a read-only command
  • unhandled the command is unhandled (not supported) in current state

External side effects can be performed after successful persist with the andThen function e.g Effect.persist(..).andThenEffect().persist(..).andThen. In the example below a reply is sent to the replyTo ActorRef. Note that the new state after applying the event is passed as parameter to the andThen function.

Event handler

When an event has been persisted successfully the current state is updated by applying the event to the current state with the eventHandler.

The event handler returns the new state, which must be immutable so you return a new instance of the state. The same event handler is also used when the entity is started up to recover its state from the stored events.

It is not recommended to perform side effects in the event handler, as those are also executed during recovery of an persistent actor

Basic example

Command and event:

Scala
sealed trait SimpleCommand
case class Cmd(data: String) extends SimpleCommand

sealed trait SimpleEvent
case class Evt(data: String) extends SimpleEvent
Java
public static class SimpleCommand {
  public final String data;

  public SimpleCommand(String data) {
    this.data = data;
  }
}

State is a List containing all the events:

Scala
case class ExampleState(events: List[String] = Nil)
Java
static class SimpleState {
  private final List<String> events;

  SimpleState(List<String> events) {
    this.events = events;
  }
  SimpleState() {
    this.events = new ArrayList<>();
  }


  SimpleState addEvent(SimpleEvent event) {
    List<String> newEvents = new ArrayList<>(events);
    newEvents.add(event.data);
    return new SimpleState(newEvents);
  }
}

The command handler just persists the Cmd payload in an Evt. In this simple example the command handler is defined using a lambda, for the more complicated example below a CommandHandlerBuilder is used:

Scala
val commandHandler: CommandHandler[SimpleCommand, SimpleEvent, ExampleState] =
  CommandHandler.command {
    case Cmd(data) ⇒ Effect.persist(Evt(data))
  }
Java
@Override
public CommandHandler<SimpleCommand, SimpleEvent, SimpleState> commandHandler() {
  return (ctx, state, cmd) -> Effect().persist(new SimpleEvent(cmd.data));
}

The event handler appends the event to the state. This is called after successfully persisting the event in the database . As with the command handler the event handler is defined using a lambda, see below for a more complicated example using the EventHandlerBuilder:

Scala
val eventHandler: (ExampleState, SimpleEvent) ⇒ (ExampleState) = {
  case (state, Evt(data)) ⇒ state.copy(data :: state.events)
}
Java
@Override
public EventHandler<SimpleEvent, SimpleState> eventHandler() {
  return (state, event) -> state.addEvent(event);
}

These are used to create a PersistentBehavior:

Scala
val simpleBehavior: PersistentBehavior[SimpleCommand, SimpleEvent, ExampleState] =
  PersistentBehaviors.immutable[SimpleCommand, SimpleEvent, ExampleState](
    persistenceId = "sample-id-1",
    initialState = ExampleState(Nil),
    commandHandler = commandHandler,
    eventHandler = eventHandler)
Java
public static PersistentBehavior<SimpleCommand, SimpleEvent, SimpleState> pb = new PersistentBehavior<SimpleCommand, SimpleEvent, SimpleState>("p1") {
  @Override
  public SimpleState initialState() {
    return new SimpleState();
  }

  @Override
  public CommandHandler<SimpleCommand, SimpleEvent, SimpleState> commandHandler() {
    return (ctx, state, cmd) -> Effect().persist(new SimpleEvent(cmd.data));
  }

  @Override
  public EventHandler<SimpleEvent, SimpleState> eventHandler() {
    return (state, event) -> state.addEvent(event);
  }
};

The behavior can then be run as with any normal typed actor as described in typed actors documentation.

Larger example

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

As you can see in the above examples this is not supported by typed persistent actors. Instead, the state is returned by eventHandler. 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. If it would have been supported it would mean that the behavior must be restored when replaying events and also encoded in the state anyway when snapshots are used. That would be very prone to mistakes and thus not allowed in Typed Persistence.

For simple actors you can use the same set of command handlers independent of what state the entity is in, as shown in above example. 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 shows how to define different behavior based on the current State. It is 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 it we can look it up with GetPost, modify it with ChangeBody or publish it with Publish.

The state is captured by:

Scala
object BlogState {
  val empty = BlogState(None, published = false)
}

final case class BlogState(content: Option[PostContent], published: Boolean) {
  def withContent(newContent: PostContent): BlogState =
    copy(content = Some(newContent))
  def isEmpty: Boolean = content.isEmpty
  def postId: String = content match {
    case Some(c) ⇒ c.postId
    case None    ⇒ throw new IllegalStateException("postId unknown before post is created")
  }
}
Java
public static class BlogState {
  final Optional<PostContent> postContent;
  final boolean published;

  BlogState(Optional<PostContent> postContent, boolean published) {
    this.postContent = postContent;
    this.published = published;
  }

  public BlogState withContent(PostContent newContent) {
    return new BlogState(Optional.of(newContent), this.published);
  }

  public boolean isEmpty() {
    return postContent.isPresent();
  }

  public String postId() {
    return postContent.orElseGet(() -> {
      throw new IllegalStateException("postId unknown before post is created");
    }).postId;
  }
}

The commands (only a subset are valid depending on state):

Scala
sealed trait BlogCommand extends Serializable
final case class AddPost(content: PostContent, replyTo: ActorRef[AddPostDone]) extends BlogCommand
final case class AddPostDone(postId: String)
final case class GetPost(replyTo: ActorRef[PostContent]) extends BlogCommand
final case class ChangeBody(newBody: String, replyTo: ActorRef[Done]) extends BlogCommand
final case class Publish(replyTo: ActorRef[Done]) extends BlogCommand
final case object PassivatePost extends BlogCommand
final case class PostContent(postId: String, title: String, body: String)
Java
public interface BlogCommand {
}
public static class AddPost implements BlogCommand {
  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 BlogCommand {
  final String postId;

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

  public GetPost(ActorRef<PostContent> replyTo) {
    this.replyTo = replyTo;
  }
}
public static class ChangeBody implements BlogCommand {
  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 BlogCommand {
  final ActorRef<Done> replyTo;

  public Publish(ActorRef<Done> replyTo) {
    this.replyTo = replyTo;
  }
}
public static class PassivatePost implements BlogCommand {

}
public static class PostContent implements BlogCommand {
  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 a CommandHandler.byState command handler, which is a function from State => CommandHandler:

Scala
private def commandHandler: CommandHandler[BlogCommand, BlogEvent, BlogState] = CommandHandler.byState {
  case state if state.isEmpty  ⇒ initial
  case state if !state.isEmpty ⇒ postAdded
}
Java
@Override
public CommandHandler<BlogCommand, BlogEvent, BlogState> commandHandler() {
  return byStateCommandHandlerBuilder()
    .matchState(BlogState.class, (state) -> !state.postContent.isPresent(), initialCommandHandler)
    .matchState(BlogState.class, (state) -> state.postContent.isPresent(), postCommandHandler)
    .build();
}

This can refer to many other CommandHandlers e.g one for a post that hasn’t been started:

Scala
private def initial: CommandHandler[BlogCommand, BlogEvent, BlogState] =
  (ctx, state, cmd) ⇒
    cmd match {
      case AddPost(content, replyTo) ⇒
        val evt = PostAdded(content.postId, content)
        Effect.persist(evt).andThen { state2 ⇒
          // After persist is done additional side effects can be performed
          replyTo ! AddPostDone(content.postId)
        }
      case PassivatePost ⇒
        Effect.stop
      case _ ⇒
        Effect.unhandled
    }
Java
private CommandHandler<BlogCommand, BlogEvent, BlogState> initialCommandHandler = commandHandlerBuilder()
  .matchCommand(AddPost.class, (ctx, state, cmd) -> {
    PostAdded event = new PostAdded(cmd.content.postId, cmd.content);
    return Effect().persist(event)
      .andThen(() -> cmd.replyTo.tell(new AddPostDone(cmd.content.postId)));
  })
  .matchCommand(PassivatePost.class, (ctx, state, cmd) -> Effect().stop())
  .build();

And a different CommandHandler for after the post has been added:

Scala
private def postAdded: CommandHandler[BlogCommand, BlogEvent, BlogState] = {
  (ctx, state, cmd) ⇒
    cmd match {
      case ChangeBody(newBody, replyTo) ⇒
        val evt = BodyChanged(state.postId, newBody)
        Effect.persist(evt).andThen { _ ⇒
          replyTo ! Done
        }
      case Publish(replyTo) ⇒
        Effect.persist(Published(state.postId)).andThen { _ ⇒
          println(s"Blog post ${state.postId} was published")
          replyTo ! Done
        }
      case GetPost(replyTo) ⇒
        replyTo ! state.content.get
        Effect.none
      case _: AddPost ⇒
        Effect.unhandled
      case PassivatePost ⇒
        Effect.stop
    }
}
Java
private CommandHandler<BlogCommand, BlogEvent, BlogState> postCommandHandler = commandHandlerBuilder()
  .matchCommand(ChangeBody.class, (ctx, state, cmd) -> {
    BodyChanged event = new BodyChanged(state.postId(), cmd.newBody);
    return Effect().persist(event).andThen(() -> cmd.replyTo.tell(Done.getInstance()));
  })
  .matchCommand(Publish.class, (ctx, state, cmd) -> Effect()
    .persist(new Published(state.postId())).andThen(() -> {
      System.out.println("Blog post published: " + state.postId());
      cmd.replyTo.tell(Done.getInstance());
    }))
  .matchCommand(GetPost.class, (ctx, state, cmd) -> {
    cmd.replyTo.tell(state.postContent.get());
    return Effect().none();
  })
  .matchCommand(AddPost.class, (ctx, state, cmd) -> Effect().unhandled())
  .matchCommand(PassivatePost.class, (ctx, state, cmd) -> Effect().stop())
  .build();

The event handler is always the same independent of state. The main reason for not making the event handler part of the CommandHandler is that all events must be handled and that is typically independent of what the current state is. The event handler can of course still decide what to do based on the state if that is needed.

Scala
private def eventHandler(state: BlogState, event: BlogEvent): BlogState =
  event match {
    case PostAdded(postId, content) ⇒
      state.withContent(content)

    case BodyChanged(_, newBody) ⇒
      state.content match {
        case Some(c) ⇒ state.copy(content = Some(c.copy(body = newBody)))
        case None    ⇒ state
      }

    case Published(_) ⇒
      state.copy(published = true)
  }
Java
@Override
public EventHandler<BlogEvent, BlogState> eventHandler() {
  return eventHandlerBuilder()
    .matchEvent(PostAdded.class, (state, event) -> state.withContent(event.content))
    .matchEvent(BodyChanged.class, (state, newBody) ->
      new BlogState(state.postContent.map(pc -> new PostContent(pc.postId, pc.title, newBody.newBody)), state.published))
    .matchEvent(Published.class, (state, event) -> new BlogState(state.postContent, true))
    .build();
}

And finally the behavior is created from the byState command handler:

Scala
def behavior(entityId: String): Behavior[BlogCommand] =
  PersistentBehaviors.immutable[BlogCommand, BlogEvent, BlogState](
    persistenceId = "Blog-" + entityId,
    initialState = BlogState.empty,
    commandHandler,
    eventHandler)
Java
public static class BlogBehavior extends PersistentBehavior<BlogCommand, BlogEvent, BlogState> {

  private CommandHandler<BlogCommand, BlogEvent, BlogState> initialCommandHandler = commandHandlerBuilder()
    .matchCommand(AddPost.class, (ctx, state, cmd) -> {
      PostAdded event = new PostAdded(cmd.content.postId, cmd.content);
      return Effect().persist(event)
        .andThen(() -> cmd.replyTo.tell(new AddPostDone(cmd.content.postId)));
    })
    .matchCommand(PassivatePost.class, (ctx, state, cmd) -> Effect().stop())
    .build();

  private CommandHandler<BlogCommand, BlogEvent, BlogState> postCommandHandler = commandHandlerBuilder()
    .matchCommand(ChangeBody.class, (ctx, state, cmd) -> {
      BodyChanged event = new BodyChanged(state.postId(), cmd.newBody);
      return Effect().persist(event).andThen(() -> cmd.replyTo.tell(Done.getInstance()));
    })
    .matchCommand(Publish.class, (ctx, state, cmd) -> Effect()
      .persist(new Published(state.postId())).andThen(() -> {
        System.out.println("Blog post published: " + state.postId());
        cmd.replyTo.tell(Done.getInstance());
      }))
    .matchCommand(GetPost.class, (ctx, state, cmd) -> {
      cmd.replyTo.tell(state.postContent.get());
      return Effect().none();
    })
    .matchCommand(AddPost.class, (ctx, state, cmd) -> Effect().unhandled())
    .matchCommand(PassivatePost.class, (ctx, state, cmd) -> Effect().stop())
    .build();


  public BlogBehavior(String persistenceId) {
    super(persistenceId);
  }

  @Override
  public BlogState initialState() {
    return new BlogState(Optional.empty(), false);
  }

  @Override
  public CommandHandler<BlogCommand, BlogEvent, BlogState> commandHandler() {
    return byStateCommandHandlerBuilder()
      .matchState(BlogState.class, (state) -> !state.postContent.isPresent(), initialCommandHandler)
      .matchState(BlogState.class, (state) -> state.postContent.isPresent(), postCommandHandler)
      .build();
  }

  @Override
  public EventHandler<BlogEvent, BlogState> eventHandler() {
    return eventHandlerBuilder()
      .matchEvent(PostAdded.class, (state, event) -> state.withContent(event.content))
      .matchEvent(BodyChanged.class, (state, newBody) ->
        new BlogState(state.postContent.map(pc -> new PostContent(pc.postId, pc.title, newBody.newBody)), state.published))
      .matchEvent(Published.class, (state, event) -> new BlogState(state.postContent, true))
      .build();
  }
}

Serialization

The same serialization mechanism as for untyped actors is also used in Akka Typed, also for persistent actors. When picking serialization solution for the events you should also consider that it must be possible read old events when the application has evolved. Strategies for that can be found in the schema evolution.

Recovery

Since it is strongly discouraged to perform side effects in applyEvent, side effects should be performed once recovery has completed in the onRecoveryCompleted callback. by overriding onRecoveryCompleted

Scala
val recoveryBehavior: Behavior[Command] =
  PersistentBehaviors.immutable[Command, Event, State](
    persistenceId = "abc",
    initialState = State(),
    commandHandler = (ctx, state, cmd) ⇒ ???,
    eventHandler = (state, evt) ⇒ ???)
    .onRecoveryCompleted { (ctx, state) ⇒
      ???
    }
Java
@Override
public void onRecoveryCompleted(ActorContext<Command> ctx, State state) {
  // called once recovery is completed
}

The onRecoveryCompleted takes on an ActorContext and the current State.

Tagging

Persistence typed allows you to use event tags without using EventAdapter:

Scala
val taggingBehavior: Behavior[Command] =
  PersistentBehaviors.immutable[Command, Event, State](
    persistenceId = "abc",
    initialState = State(),
    commandHandler = (ctx, state, cmd) ⇒ ???,
    eventHandler = (state, evt) ⇒ ???
  ).withTagger(_ ⇒ Set("tag1", "tag2"))
Java
@Override
public Set<String> tagsFor(Event event) {
  // inspect the event and decide if it should be tagged
  return Collections.emptySet();
}

Current limitations

  • The PersistentBehavior can’t be wrapped in other behaviors, such as Behaviors.setup. See #23694
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