Classic Persistence

For the full documentation of this feature and for new projects see Event Sourcing.

Module info

The Akka dependencies are available from Akka’s library repository. To access them there, you need to configure the URL for this repository.

sbt

resolvers += "Akka library repository".at("https://repo.akka.io/maven")

Maven

<project>
  ...
  <repositories>
    <repository>
      <id>akka-repository</id>
      <name>Akka library repository</name>
      <url>https://repo.akka.io/maven</url>
    </repository>
  </repositories>
</project>

Gradle

repositories {
    mavenCentral()
    maven {
        url "https://repo.akka.io/maven"
    }
}

To use Akka Persistence, you must add the following dependency in your project:

sbt

val AkkaVersion = "2.9.7"
libraryDependencies ++= Seq(
  "com.typesafe.akka" %% "akka-persistence" % 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_${scala.binary.version}</artifactId>
      <version>2.9.7</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>
<dependencies>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-persistence_${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}:2.9.7")

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

You also have to select journal plugin and optionally snapshot store plugin, see Persistence Plugins.

Project Info: Akka Persistence (classic)
Artifact	com.typesafe.akka akka-persistence 2.9.7 Snapshots are available
JDK versions	Eclipse Temurin JDK 11 Eclipse Temurin JDK 17 Eclipse Temurin JDK 21
Scala versions	2.13.14, 3.3.3
JPMS module name	akka.persistence
License	BUSL-1.1
Readiness level	Supported, support is available from Lightbend Since 2.3.0, 2014-03-05
Home page	https://akka.io/
API documentation	API (Scaladoc) API (Javadoc)
Forums	Lightbend Discuss
Release notes	akka.io blog
Issues	Github issues
Sources	https://github.com/akka/akka

Introduction

See introduction in Persistence

Akka Persistence also provides point-to-point communication with at-least-once message delivery semantics.

Architecture

• PersistentActorAbstractPersistentActor: Is a persistent, stateful actor. It is able to persist events to a journal and can react to them in a thread-safe manner. It can be used to implement both command as well as event sourced actors. When a persistent actor is started or restarted, journaled messages are replayed to that actor so that it can recover its state from these messages.
• AtLeastOnceDeliveryAbstractPersistentActorWithAtLeastOnceDelivery: To send messages with at-least-once delivery semantics to destinations, also in case of sender and receiver JVM crashes.
• AsyncWriteJournalAsyncWriteJournal: A journal stores the sequence of messages sent to a persistent actor. An application can control which messages are journaled and which are received by the persistent actor without being journaled. Journal maintains highestSequenceNr that is increased on each message. The storage backend of a journal is pluggable. Replicated journals are available as Community plugins.
• Snapshot store: A snapshot store persists snapshots of a persistent actor’s state. Snapshots are used for optimizing recovery times. The storage backend of a snapshot store is pluggable. The persistence extension comes with a “local” snapshot storage plugin, which writes to the local filesystem. Replicated snapshot stores are available as Community plugins
• Event Sourcing. Based on the building blocks described above, Akka persistence provides abstractions for the development of event sourced applications (see section Event Sourcing).

Example

Akka persistence supports Event Sourcing with the PersistentActor traitAbstractPersistentActor abstract class. An actor that extends this traitclass uses the persistpersist method to persist and handle events. The behavior of a PersistentActoran AbstractPersistentActor is defined by implementing receiveRecovercreateReceiveRecover and receiveCommandcreateReceive. This is demonstrated in the following example.

Scala

copysourceimport akka.actor._
import akka.persistence._

case class Cmd(data: String)
case class Evt(data: String)

case class ExampleState(events: List[String] = Nil) {
  def updated(evt: Evt): ExampleState = copy(evt.data :: events)
  def size: Int = events.length
  override def toString: String = events.reverse.toString
}

class ExamplePersistentActor extends PersistentActor {
  override def persistenceId = "sample-id-1"

  var state = ExampleState()

  def updateState(event: Evt): Unit =
    state = state.updated(event)

  def numEvents =
    state.size

  val receiveRecover: Receive = {
    case evt: Evt                                 => updateState(evt)
    case SnapshotOffer(_, snapshot: ExampleState) => state = snapshot
  }

  val snapShotInterval = 1000
  val receiveCommand: Receive = {
    case Cmd(data) =>
      persist(Evt(s"${data}-${numEvents}")) { event =>
        updateState(event)
        context.system.eventStream.publish(event)
        if (lastSequenceNr % snapShotInterval == 0 && lastSequenceNr != 0)
          saveSnapshot(state)
      }
    case "print" => println(state)
  }

}

Java

copysource
import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.Props;
import akka.persistence.AbstractPersistentActor;
import akka.persistence.SnapshotOffer;
import java.io.Serializable;
import java.util.ArrayList;

class Cmd implements Serializable {
  private static final long serialVersionUID = 1L;
  private final String data;

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

  public String getData() {
    return data;
  }
}

class Evt implements Serializable {
  private static final long serialVersionUID = 1L;
  private final String data;

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

  public String getData() {
    return data;
  }
}

class ExampleState implements Serializable {
  private static final long serialVersionUID = 1L;
  private final ArrayList<String> events;

  public ExampleState() {
    this(new ArrayList<>());
  }

  public ExampleState(ArrayList<String> events) {
    this.events = events;
  }

  public ExampleState copy() {
    return new ExampleState(new ArrayList<>(events));
  }

  public void update(Evt evt) {
    events.add(evt.getData());
  }

  public int size() {
    return events.size();
  }

  @Override
  public String toString() {
    return events.toString();
  }
}

class ExamplePersistentActor extends AbstractPersistentActor {

  private ExampleState state = new ExampleState();
  private int snapShotInterval = 1000;

  public int getNumEvents() {
    return state.size();
  }

  @Override
  public String persistenceId() {
    return "sample-id-1";
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder()
        .match(Evt.class, state::update)
        .match(SnapshotOffer.class, ss -> state = (ExampleState) ss.snapshot())
        .build();
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            Cmd.class,
            c -> {
              final String data = c.getData();
              final Evt evt = new Evt(data + "-" + getNumEvents());
              persist(
                  evt,
                  (Evt e) -> {
                    state.update(e);
                    getContext().getSystem().getEventStream().publish(e);
                    if (lastSequenceNr() % snapShotInterval == 0 && lastSequenceNr() != 0)
                      // IMPORTANT: create a copy of snapshot because ExampleState is mutable
                      saveSnapshot(state.copy());
                  });
            })
        .matchEquals("print", s -> System.out.println(state))
        .build();
  }
}

The example defines two data types, Cmd and Evt to represent commands and events, respectively. The state of the ExamplePersistentActor is a list of persisted event data contained in ExampleState.

The persistent actor’s receiveRecovercreateReceiveRecover method defines how state is updated during recovery by handling Evt and SnapshotOffer messages. The persistent actor’s receiveCommandcreateReceive method is a command handler. In this example, a command is handled by generating an event which is then persisted and handled. Events are persisted by calling persist with an event (or a sequence of events) as first argument and an event handler as second argument.

The persist method persists events asynchronously and the event handler is executed for successfully persisted events. Successfully persisted events are internally sent back to the persistent actor as individual messages that trigger event handler executions. An event handler may close over persistent actor state and mutate it. The sender of a persisted event is the sender of the corresponding command. This allows event handlers to reply to the sender of a command (not shown).

The main responsibility of an event handler is changing persistent actor state using event data and notifying others about successful state changes by publishing events.

When persisting events with persist it is guaranteed that the persistent actor will not receive further commands between the persist call and the execution(s) of the associated event handler. This also holds for multiple persist calls in context of a single command. Incoming messages are stashed until the persist is completed.

If persistence of an event fails, onPersistFailure will be invoked (logging the error by default), and the actor will unconditionally be stopped. If persistence of an event is rejected before it is stored, e.g. due to serialization error, onPersistRejected will be invoked (logging a warning by default) and the actor continues with the next message.

Identifiers

A persistent actor must have an identifier that doesn’t change across different actor incarnations. The identifier must be defined with the persistenceId method.

Scala

copysourceoverride def persistenceId = "my-stable-persistence-id"

Java

copysource@Override
public String persistenceId() {
  return "my-stable-persistence-id";
}

Recovery

By default, a persistent actor is automatically recovered on start and on restart by replaying journaled messages. New messages sent to a persistent actor during recovery do not interfere with replayed messages. They are stashed and received by a persistent actor after recovery phase completes.

The number of concurrent recoveries that can be in progress at the same time is limited to not overload the system and the backend data store. When exceeding the limit the actors will wait until other recoveries have been completed. This is configured by:

akka.persistence.max-concurrent-recoveries = 50

Recovery customization

Applications may also customise how recovery is performed by returning a customised RecoveryRecovery object in the recovery method of a PersistentActorAbstractPersistentActor,

To skip loading snapshots and replay all events you can use SnapshotSelectionCriteria.NoneSnapshotSelectionCriteria.none(). This can be useful if snapshot serialization format has changed in an incompatible way. It should typically not be used when events have been deleted.

Scala

copysourceoverride def recovery =
  Recovery(fromSnapshot = SnapshotSelectionCriteria.None)

Java

copysource@Override
public Recovery recovery() {
  return Recovery.create(SnapshotSelectionCriteria.none());
}

Another possible recovery customization, which can be useful for debugging, is setting an upper bound on the replay, causing the actor to be replayed only up to a certain point “in the past” (instead of being replayed to its most up to date state). Note that after that it is a bad idea to persist new events because a later recovery will probably be confused by the new events that follow the events that were previously skipped.

Scala

copysourceoverride def recovery = Recovery(toSequenceNr = 457L)

Java

copysource@Override
public Recovery recovery() {
  return Recovery.create(457L);
}

Recovery can be disabled by returning Recovery.none() in the recovery method of a PersistentActor:

Scala

copysourceoverride def recovery = Recovery.none

Java

copysource@Override
public Recovery recovery() {
  return Recovery.none();
}

Recovery status

A persistent actor can query its own recovery status via the methods

Scala

copysourcedef recoveryRunning: Boolean
def recoveryFinished: Boolean

Java

copysourcepublic boolean recoveryRunning();

public boolean recoveryFinished();

Sometimes there is a need for performing additional initialization when the recovery has completed before processing any other message sent to the persistent actor. The persistent actor will receive a special RecoveryCompletedRecoveryCompleted message right after recovery and before any other received messages.

Scala

copysource
override def receiveRecover: Receive = {
  case RecoveryCompleted =>
  // perform init after recovery, before any other messages
  //...
  case evt => //...
}

override def receiveCommand: Receive = {
  case msg => //...
}

Java

copysourceclass MyPersistentActor5 extends AbstractPersistentActor {

  @Override
  public String persistenceId() {
    return "my-stable-persistence-id";
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder()
        .match(
            RecoveryCompleted.class,
            r -> {
              // perform init after recovery, before any other messages
              // ...
            })
        .match(String.class, this::handleEvent)
        .build();
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(String.class, s -> s.equals("cmd"), s -> persist("evt", this::handleEvent))
        .build();
  }

  private void handleEvent(String event) {
    // update state
    // ...
  }
}

The actor will always receive a RecoveryCompleted message, even if there are no events in the journal and the snapshot store is empty, or if it’s a new persistent actor with a previously unused persistenceId.

If there is a problem with recovering the state of the actor from the journal, onRecoveryFailure is called (logging the error by default) and the actor will be stopped.

Internal stash

The persistent actor has a private stash for internally caching incoming messages during recovery or the persistpersist\persistAllpersistAll method persisting events. You can still use/inherit from the StashStash interface. The internal stash cooperates with the normal stash by hooking into unstashAllunstashAll making sure messages are unstashed properly to the internal stash to maintain ordering guarantees.

You should be careful to not send more messages to a persistent actor than it can keep up with, otherwise the number of stashed messages will grow without bounds. It can be wise to protect against OutOfMemoryError by defining a maximum stash capacity in the mailbox configuration:

akka.actor.default-mailbox.stash-capacity=10000

Note that the stash capacity is per actor. If you have many persistent actors, e.g. when using cluster sharding, you may need to define a small stash capacity to ensure that the total number of stashed messages in the system doesn’t consume too much memory. Additionally, the persistent actor defines three strategies to handle failure when the internal stash capacity is exceeded. The default overflow strategy is the ThrowOverflowExceptionStrategyThrowOverflowExceptionStrategy, which discards the current received message and throws a StashOverflowExceptionStashOverflowException, causing actor restart if the default supervision strategy is used. You can override the internalStashOverflowStrategyinternalStashOverflowStrategy method to return DiscardToDeadLetterStrategyDiscardToDeadLetterStrategy or ReplyToStrategyReplyToStrategy for any “individual” persistent actor, or define the “default” for all persistent actors by providing FQCN, which must be a subclass of StashOverflowStrategyConfiguratorStashOverflowStrategyConfigurator, in the persistence configuration:

akka.persistence.internal-stash-overflow-strategy=
  "akka.persistence.ThrowExceptionConfigurator"

The DiscardToDeadLetterStrategy strategy also has a pre-packaged companion configurator DiscardConfiguratorDiscardConfigurator.

You can also query the default strategy via the Akka persistence extension singleton:

Scala

Persistence(context.system).defaultInternalStashOverflowStrategy

Java

Persistence.get(getContext().getSystem()).defaultInternalStashOverflowStrategy();

Relaxed local consistency requirements and high throughput use-cases

If faced with relaxed local consistency requirements and high throughput demands sometimes PersistentActorAbstractPersistentActor and its persistpersist may not be enough in terms of consuming incoming Commands at a high rate, because it has to wait until all Events related to a given Command are processed in order to start processing the next Command. While this abstraction is very useful for most cases, sometimes you may be faced with relaxed requirements about consistency – for example you may want to process commands as fast as you can, assuming that the Event will eventually be persisted and handled properly in the background, retroactively reacting to persistence failures if needed.

The persistAsyncpersistAsync method provides a tool for implementing high-throughput persistent actors. It will not stash incoming Commands while the Journal is still working on persisting and/or user code is executing event callbacks.

In the below example, the event callbacks may be called “at any time”, even after the next Command has been processed. The ordering between events is still guaranteed (“evt-b-1” will be sent after “evt-a-2”, which will be sent after “evt-a-1” etc.).

Scala

copysourceclass MyPersistentActor extends PersistentActor {

  override def persistenceId = "my-stable-persistence-id"

  override def receiveRecover: Receive = {
    case _ => // handle recovery here
  }

  override def receiveCommand: Receive = {
    case c: String => {
      sender() ! c
      persistAsync(s"evt-$c-1") { e =>
        sender() ! e
      }
      persistAsync(s"evt-$c-2") { e =>
        sender() ! e
      }
    }
  }
}

// usage
persistentActor ! "a"
persistentActor ! "b"

// possible order of received messages:
// a
// b
// evt-a-1
// evt-a-2
// evt-b-1
// evt-b-2

Java

copysourceclass MyPersistentActor extends AbstractPersistentActor {

  @Override
  public String persistenceId() {
    return "my-stable-persistence-id";
  }

  private void handleCommand(String c) {
    getSender().tell(c, getSelf());

    persistAsync(
        String.format("evt-%s-1", c),
        e -> {
          getSender().tell(e, getSelf());
        });
    persistAsync(
        String.format("evt-%s-2", c),
        e -> {
          getSender().tell(e, getSelf());
        });
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder().match(String.class, this::handleCommand).build();
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder().match(String.class, this::handleCommand).build();
  }
}

Deferring actions until preceding persist handlers have executed

Sometimes when working with persistAsyncpersistAsync or persistpersist you may find that it would be nice to define some actions in terms of ‘‘happens-after the previous persistAsync/persist handlers have been invoked’’. PersistentActorAbstractPersistentActor provides utility methods called deferdefer and deferAsyncdeferAsync, which work similarly to persist and persistAsync respectively yet do not persist the passed in event. It is recommended to use them for read operations, and actions which do not have corresponding events in your domain model.

Using those methods is very similar to the persist family of methods, yet they do not persist the passed in event. It will be kept in memory and used when invoking the handler.

Scala

copysourceclass MyPersistentActor extends PersistentActor {

  override def persistenceId = "my-stable-persistence-id"

  override def receiveRecover: Receive = {
    case _ => // handle recovery here
  }

  override def receiveCommand: Receive = {
    case c: String => {
      sender() ! c
      persistAsync(s"evt-$c-1") { e =>
        sender() ! e
      }
      persistAsync(s"evt-$c-2") { e =>
        sender() ! e
      }
      deferAsync(s"evt-$c-3") { e =>
        sender() ! e
      }
    }
  }
}

Java

copysourceclass MyPersistentActor extends AbstractPersistentActor {

  @Override
  public String persistenceId() {
    return "my-stable-persistence-id";
  }

  private void handleCommand(String c) {
    persistAsync(
        String.format("evt-%s-1", c),
        e -> {
          getSender().tell(e, getSelf());
        });
    persistAsync(
        String.format("evt-%s-2", c),
        e -> {
          getSender().tell(e, getSelf());
        });

    deferAsync(
        String.format("evt-%s-3", c),
        e -> {
          getSender().tell(e, getSelf());
        });
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder().match(String.class, this::handleCommand).build();
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder().match(String.class, this::handleCommand).build();
  }
}

Notice that the sender() is safe to access in the handler callback, and will be pointing to the original sender of the command for which this defer or deferAsync handler was called.

The calling side will get the responses in this (guaranteed) order:

Scala

copysourcepersistentActor ! "a"
persistentActor ! "b"

// order of received messages:
// a
// b
// evt-a-1
// evt-a-2
// evt-a-3
// evt-b-1
// evt-b-2
// evt-b-3

Java

copysourcefinal ActorRef persistentActor = system.actorOf(Props.create(MyPersistentActor.class));
persistentActor.tell("a", sender);
persistentActor.tell("b", sender);

// order of received messages:
// a
// b
// evt-a-1
// evt-a-2
// evt-a-3
// evt-b-1
// evt-b-2
// evt-b-3

You can also call defer or deferAsync with persist.

Scala

copysourceclass MyPersistentActor extends PersistentActor {

  override def persistenceId = "my-stable-persistence-id"

  override def receiveRecover: Receive = {
    case _ => // handle recovery here
  }

  override def receiveCommand: Receive = {
    case c: String => {
      sender() ! c
      persist(s"evt-$c-1") { e =>
        sender() ! e
      }
      persist(s"evt-$c-2") { e =>
        sender() ! e
      }
      defer(s"evt-$c-3") { e =>
        sender() ! e
      }
    }
  }
}

Java

copysourceclass MyPersistentActor extends AbstractPersistentActor {

  @Override
  public String persistenceId() {
    return "my-stable-persistence-id";
  }

  private void handleCommand(String c) {
    persist(
        String.format("evt-%s-1", c),
        e -> {
          sender().tell(e, self());
        });
    persist(
        String.format("evt-%s-2", c),
        e -> {
          sender().tell(e, self());
        });

    defer(
        String.format("evt-%s-3", c),
        e -> {
          sender().tell(e, self());
        });
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder().match(String.class, this::handleCommand).build();
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder().match(String.class, this::handleCommand).build();
  }
}

Nested persist calls

It is possible to call persistpersist and persistAsyncpersistAsync inside their respective callback blocks and they will properly retain both the thread safety (including the right value of sender()getSender()) as well as stashing guarantees.

In general it is encouraged to create command handlers which do not need to resort to nested event persisting, however there are situations where it may be useful. It is important to understand the ordering of callback execution in those situations, as well as their implication on the stashing behavior (that persist() enforces). In the following example two persist calls are issued, and each of them issues another persist inside its callback:

Scala

copysourceoverride def receiveCommand: Receive = {
  case c: String =>
    sender() ! c

    persist(s"$c-1-outer") { outer1 =>
      sender() ! outer1
      persist(s"$c-1-inner") { inner1 =>
        sender() ! inner1
      }
    }

    persist(s"$c-2-outer") { outer2 =>
      sender() ! outer2
      persist(s"$c-2-inner") { inner2 =>
        sender() ! inner2
      }
    }
}

Java

copysource@Override
public Receive createReceiveRecover() {
  final Procedure<String> replyToSender = event -> getSender().tell(event, getSelf());

  return receiveBuilder()
      .match(
          String.class,
          msg -> {
            persist(
                String.format("%s-outer-1", msg),
                event -> {
                  getSender().tell(event, getSelf());
                  persist(String.format("%s-inner-1", event), replyToSender);
                });

            persist(
                String.format("%s-outer-2", msg),
                event -> {
                  getSender().tell(event, getSelf());
                  persist(String.format("%s-inner-2", event), replyToSender);
                });
          })
      .build();
}

When sending two commands to this PersistentActorAbstractPersistentActor, the persist handlers will be executed in the following order:

Scala

copysourcepersistentActor ! "a"
persistentActor ! "b"

// order of received messages:
// a
// a-outer-1
// a-outer-2
// a-inner-1
// a-inner-2
// and only then process "b"
// b
// b-outer-1
// b-outer-2
// b-inner-1
// b-inner-2

Java

copysourcepersistentActor.tell("a", ActorRef.noSender());
persistentActor.tell("b", ActorRef.noSender());

// order of received messages:
// a
// a-outer-1
// a-outer-2
// a-inner-1
// a-inner-2
// and only then process "b"
// b
// b-outer-1
// b-outer-2
// b-inner-1
// b-inner-2

First the “outer layer” of persist calls is issued and their callbacks are applied. After these have successfully completed, the inner callbacks will be invoked (once the events they are persisting have been confirmed to be persisted by the journal). Only after all these handlers have been successfully invoked will the next command be delivered to the persistent Actor. In other words, the stashing of incoming commands that is guaranteed by initially calling persist() on the outer layer is extended until all nested persist callbacks have been handled.

It is also possible to nest persistAsync calls, using the same pattern:

Scala

copysourceoverride def receiveCommand: Receive = {
  case c: String =>
    sender() ! c
    persistAsync(c + "-outer-1") { outer =>
      sender() ! outer
      persistAsync(c + "-inner-1") { inner =>
        sender() ! inner
      }
    }
    persistAsync(c + "-outer-2") { outer =>
      sender() ! outer
      persistAsync(c + "-inner-2") { inner =>
        sender() ! inner
      }
    }
}

Java

copysource@Override
public Receive createReceive() {
  final Procedure<String> replyToSender = event -> getSender().tell(event, getSelf());

  return receiveBuilder()
      .match(
          String.class,
          msg -> {
            persistAsync(
                String.format("%s-outer-1", msg),
                event -> {
                  getSender().tell(event, getSelf());
                  persistAsync(String.format("%s-inner-1", event), replyToSender);
                });

            persistAsync(
                String.format("%s-outer-2", msg),
                event -> {
                  getSender().tell(event, getSelf());
                  persistAsync(String.format("%s-inner-1", event), replyToSender);
                });
          })
      .build();
}

In this case no stashing is happening, yet events are still persisted and callbacks are executed in the expected order:

Scala

copysourcepersistentActor ! "a"
persistentActor ! "b"

// order of received messages:
// a
// b
// a-outer-1
// a-outer-2
// b-outer-1
// b-outer-2
// a-inner-1
// a-inner-2
// b-inner-1
// b-inner-2

// which can be seen as the following causal relationship:
// a -> a-outer-1 -> a-outer-2 -> a-inner-1 -> a-inner-2
// b -> b-outer-1 -> b-outer-2 -> b-inner-1 -> b-inner-2

Java

copysourcepersistentActor.tell("a", getSelf());
persistentActor.tell("b", getSelf());

// order of received messages:
// a
// b
// a-outer-1
// a-outer-2
// b-outer-1
// b-outer-2
// a-inner-1
// a-inner-2
// b-inner-1
// b-inner-2

// which can be seen as the following causal relationship:
// a -> a-outer-1 -> a-outer-2 -> a-inner-1 -> a-inner-2
// b -> b-outer-1 -> b-outer-2 -> b-inner-1 -> b-inner-2

While it is possible to nest mixed persist and persistAsync with keeping their respective semantics it is not a recommended practice, as it may lead to overly complex nesting.

Failures

If persistence of an event fails, onPersistFailure will be invoked (logging the error by default), and the actor will unconditionally be stopped.

The reason that it cannot resume when persist fails is that it is unknown if the event was actually persisted or not, and therefore it is in an inconsistent state. Restarting on persistent failures will most likely fail anyway since the journal is probably unavailable. It is better to stop the actor and after a back-off timeout start it again. The BackoffSupervisorBackoffSupervisor actor is provided to support such restarts.

Scala

copysourceval childProps = Props[MyPersistentActor]()
val props = BackoffSupervisor.props(BackoffOpts
  .onStop(childProps, childName = "myActor", minBackoff = 3.seconds, maxBackoff = 30.seconds, randomFactor = 0.2))
context.actorOf(props, name = "mySupervisor")

Java

copysource@Override
public void preStart() throws Exception {
  final Props childProps = Props.create(MyPersistentActor1.class);
  final Props props =
      BackoffSupervisor.props(
          BackoffOpts.onStop(
              childProps, "myActor", Duration.ofSeconds(3), Duration.ofSeconds(30), 0.2));
  getContext().actorOf(props, "mySupervisor");
  super.preStart();
}

See Backoff Supervision strategies for more details about actor supervision.

If persistence of an event is rejected before it is stored, e.g. due to serialization error, onPersistRejected will be invoked (logging a warning by default), and the actor continues with next message.

If there is a problem with recovering the state of the actor from the journal when the actor is started, onRecoveryFailure is called (logging the error by default), and the actor will be stopped. Note that failure to load snapshot is also treated like this, but you can disable loading of snapshots if you for example know that serialization format has changed in an incompatible way, see Recovery customization.

Atomic writes

Each event is stored atomically, but it is also possible to store several events atomically by using the persistAllpersistAll or persistAllAsyncpersistAllAsync method. That means that all events passed to that method are stored or none of them are stored if there is an error.

The recovery of a persistent actor will therefore never be done partially with only a subset of events persisted by persistAll.

Some journals may not support atomic writes of several events and they will then reject the persistAll command, i.e. onPersistRejected is called with an exception (typically UnsupportedOperationException).

Batch writes

In order to optimize throughput when using persistAsyncpersistAsync, a persistent actor internally batches events to be stored under high load before writing them to the journal (as a single batch). The batch size is dynamically determined by how many events are emitted during the time of a journal round-trip: after sending a batch to the journal no further batch can be sent before confirmation has been received that the previous batch has been written. Batch writes are never timer-based which keeps latencies at a minimum.

Message deletion

It is possible to delete all messages (journaled by a single persistent actor) up to a specified sequence number; Persistent actors may call the deleteMessagesdeleteMessages method to this end.

Deleting messages in Event Sourcing based applications is typically either not used at all, or used in conjunction with snapshotting, i.e. after a snapshot has been successfully stored, a deleteMessages(toSequenceNr) up until the sequence number of the data held by that snapshot can be issued to safely delete the previous events while still having access to the accumulated state during replays - by loading the snapshot.

The result of the deleteMessages request is signaled to the persistent actor with a DeleteMessagesSuccessDeleteMessagesSuccess message if the delete was successful or a DeleteMessagesFailureDeleteMessagesFailure message if it failed.

Message deletion doesn’t affect the highest sequence number of the journal, even if all messages were deleted from it after deleteMessages invocation.

Persistence status handling

Persisting, deleting, and replaying messages can either succeed or fail.

Method	Success
persist / persistAsync	persist handler invoked
onPersistRejected	No automatic actions.
recovery	RecoveryCompleted
deleteMessages	DeleteMessagesSuccess

The most important operations (persist and recovery) have failure handlers modelled as explicit callbacks which the user can override in the PersistentActor. The default implementations of these handlers emit a log message (error for persist/recovery failures, and warning for others), logging the failure cause and information about which message caused the failure.

For critical failures, such as recovery or persisting events failing, the persistent actor will be stopped after the failure handler is invoked. This is because if the underlying journal implementation is signalling persistence failures it is most likely either failing completely or overloaded and restarting right-away and trying to persist the event again will most likely not help the journal recover – as it would likely cause a Thundering herd problem, as many persistent actors would restart and try to persist their events again. Instead, using a BackoffSupervisorBackoffSupervisor (as described in Failures) which implements an exponential-backoff strategy which allows for more breathing room for the journal to recover between restarts of the persistent actor.

Safely shutting down persistent actors

Special care should be given when shutting down persistent actors from the outside. With normal Actors it is often acceptable to use the special PoisonPill message to signal to an Actor that it should stop itself once it receives this message – in fact this message is handled automatically by Akka, leaving the target actor no way to refuse stopping itself when given a poison pill.

This can be dangerous when used with PersistentActor due to the fact that incoming commands are stashed while the persistent actor is awaiting confirmation from the Journal that events have been written when persist()persist() was used. Since the incoming commands will be drained from the Actor’s mailbox and put into its internal stash while awaiting the confirmation (thus, before calling the persist handlers) the Actor may receive and (auto)handle the PoisonPill before it processes the other messages which have been put into its stash, causing a pre-mature shutdown of the Actor.

The example below highlights how messages arrive in the Actor’s mailbox and how they interact with its internal stashing mechanism when persist() is used. Notice the early stop behavior that occurs when PoisonPill is used:

Scala

copysource/** Explicit shutdown message */
case object Shutdown

class SafePersistentActor extends PersistentActor {
  override def persistenceId = "safe-actor"

  override def receiveCommand: Receive = {
    case c: String =>
      println(c)
      persist(s"handle-$c") { println(_) }
    case Shutdown =>
      context.stop(self)
  }

  override def receiveRecover: Receive = {
    case _ => // handle recovery here
  }
}

Java

copysourcefinal class Shutdown {}

class MyPersistentActor extends AbstractPersistentActor {
  @Override
  public String persistenceId() {
    return "some-persistence-id";
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            Shutdown.class,
            shutdown -> {
              getContext().stop(getSelf());
            })
        .match(
            String.class,
            msg -> {
              System.out.println(msg);
              persist("handle-" + msg, e -> System.out.println(e));
            })
        .build();
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder().matchAny(any -> {}).build();
  }
}

Scala

copysource// UN-SAFE, due to PersistentActor's command stashing:
persistentActor ! "a"
persistentActor ! "b"
persistentActor ! PoisonPill
// order of received messages:
// a
//   # b arrives at mailbox, stashing;        internal-stash = [b]
// PoisonPill is an AutoReceivedMessage, is handled automatically
// !! stop !!
// Actor is stopped without handling `b` nor the `a` handler!

Java

copysource// UN-SAFE, due to PersistentActor's command stashing:
persistentActor.tell("a", ActorRef.noSender());
persistentActor.tell("b", ActorRef.noSender());
persistentActor.tell(PoisonPill.getInstance(), ActorRef.noSender());
// order of received messages:
// a
//   # b arrives at mailbox, stashing;        internal-stash = [b]
//   # PoisonPill arrives at mailbox, stashing; internal-stash = [b, Shutdown]
// PoisonPill is an AutoReceivedMessage, is handled automatically
// !! stop !!
// Actor is stopped without handling `b` nor the `a` handler!

Scala

copysource// SAFE:
persistentActor ! "a"
persistentActor ! "b"
persistentActor ! Shutdown
// order of received messages:
// a
//   # b arrives at mailbox, stashing;        internal-stash = [b]
//   # Shutdown arrives at mailbox, stashing; internal-stash = [b, Shutdown]
// handle-a
//   # unstashing;                            internal-stash = [Shutdown]
// b
// handle-b
//   # unstashing;                            internal-stash = []
// Shutdown
// -- stop --

Java

copysource// SAFE:
persistentActor.tell("a", ActorRef.noSender());
persistentActor.tell("b", ActorRef.noSender());
persistentActor.tell(new Shutdown(), ActorRef.noSender());
// order of received messages:
// a
//   # b arrives at mailbox, stashing;        internal-stash = [b]
//   # Shutdown arrives at mailbox, stashing; internal-stash = [b, Shutdown]
// handle-a
//   # unstashing;                            internal-stash = [Shutdown]
// b
// handle-b
//   # unstashing;                            internal-stash = []
// Shutdown
// -- stop --

Replay Filter

See Replay filter in the documentation of the new API.

Snapshots

As you model your domain using actors, you may notice that some actors may be prone to accumulating extremely long event logs and experiencing long recovery times. Sometimes, the right approach may be to split out into a set of shorter lived actors. However, when this is not an option, you can use snapshots to reduce recovery times drastically.

Persistent actors can save snapshots of internal state by calling the saveSnapshotsaveSnapshot method. If saving of a snapshot succeeds, the persistent actor receives a SaveSnapshotSuccessSaveSnapshotSuccess message, otherwise a SaveSnapshotFailureSaveSnapshotFailure message

Scala

copysourcevar state: Any = _

val snapShotInterval = 1000
override def receiveCommand: Receive = {
  case SaveSnapshotSuccess(metadata)         => // ...
  case SaveSnapshotFailure(metadata, reason) => // ...
  case cmd: String =>
    persist(s"evt-$cmd") { e =>
      updateState(e)
      if (lastSequenceNr % snapShotInterval == 0 && lastSequenceNr != 0)
        saveSnapshot(state)
    }
}

Java

copysourceprivate Object state;
private int snapShotInterval = 1000;

@Override
public Receive createReceive() {
  return receiveBuilder()
      .match(
          SaveSnapshotSuccess.class,
          ss -> {
            SnapshotMetadata metadata = ss.metadata();
            // ...
          })
      .match(
          SaveSnapshotFailure.class,
          sf -> {
            SnapshotMetadata metadata = sf.metadata();
            // ...
          })
      .match(
          String.class,
          cmd -> {
            persist(
                "evt-" + cmd,
                e -> {
                  updateState(e);
                  if (lastSequenceNr() % snapShotInterval == 0 && lastSequenceNr() != 0)
                    saveSnapshot(state);
                });
          })
      .build();
}

where metadata is of type SnapshotMetadataSnapshotMetadata and contains:

• persistenceId
• sequenceNr
• timestamp

During recovery, the persistent actor is offered the latest saved snapshot via a SnapshotOfferSnapshotOffer message from which it can initialize internal state.

Scala

copysourcevar state: Any = _

override def receiveRecover: Receive = {
  case SnapshotOffer(metadata, offeredSnapshot) => state = offeredSnapshot
  case RecoveryCompleted                        =>
  case event                                    => // ...
}

Java

copysourceprivate Object state;

@Override
public Receive createReceiveRecover() {
  return receiveBuilder()
      .match(
          SnapshotOffer.class,
          s -> {
            state = s.snapshot();
            // ...
          })
      .match(
          String.class,
          s -> {
            /* ...*/
          })
      .build();
}

The replayed messages that follow the SnapshotOffer message, if any, are younger than the offered snapshot. They finally recover the persistent actor to its current (i.e. latest) state.

In general, a persistent actor is only offered a snapshot if that persistent actor has previously saved one or more snapshots and at least one of these snapshots matches the SnapshotSelectionCriteriaSnapshotSelectionCriteria that can be specified for recovery.

Scala

copysourceoverride def recovery =
  Recovery(
    fromSnapshot = SnapshotSelectionCriteria(maxSequenceNr = 457L, maxTimestamp = System.currentTimeMillis))

Java

copysource@Override
public Recovery recovery() {
  return Recovery.create(
      SnapshotSelectionCriteria.create(457L, System.currentTimeMillis()));
}

If not specified, they default to SnapshotSelectionCriteria.LatestSnapshotSelectionCriteria.latest() which selects the latest (= youngest) snapshot. To disable snapshot-based recovery, applications should use SnapshotSelectionCriteria.NoneSnapshotSelectionCriteria.none(). A recovery where no saved snapshot matches the specified SnapshotSelectionCriteria will replay all journaled messages.

Snapshot deletion

A persistent actor can delete individual snapshots by calling the deleteSnapshotdeleteSnapshot method with the sequence number of when the snapshot was taken.

To bulk-delete a range of snapshots matching SnapshotSelectionCriteriaSnapshotSelectionCriteria, persistent actors should use the deleteSnapshotsdeleteSnapshots method. Depending on the journal used this might be inefficient. It is best practice to do specific deletes with deleteSnapshot or to include a minSequenceNr as well as a maxSequenceNr for the SnapshotSelectionCriteria.

Snapshot status handling

Saving or deleting snapshots can either succeed or fail – this information is reported back to the persistent actor via status messages as illustrated in the following table.

Method	Success	Failure message
saveSnapshot(Any)	SaveSnapshotSuccess	SaveSnapshotFailure
deleteSnapshot(Long)	DeleteSnapshotSuccess	DeleteSnapshotFailure
deleteSnapshots(SnapshotSelectionCriteria)	DeleteSnapshotsSuccess	DeleteSnapshotsFailure

If failure messages are left unhandled by the actor, a default warning log message will be logged for each incoming failure message. No default action is performed on the success messages, however you’re free to handle them e.g. in order to delete an in memory representation of the snapshot, or in the case of failure to attempt save the snapshot again.

Optional snapshots

By default, the persistent actor will unconditionally be stopped if the snapshot can’t be loaded in the recovery. It is possible to make snapshot loading optional. This can be useful when it is alright to ignore snapshot in case of for example deserialization errors. When snapshot loading fails it will instead recover by replaying all events.

Enable this feature by setting snapshot-is-optional = true in the snapshot store configuration.

Scaling out

See Scaling out in the documentation of the new API.

At-Least-Once Delivery

To send messages with at-least-once delivery semantics to destinations you can mix-in AtLeastOnceDelivery trait to your PersistentActorextend the AbstractPersistentActorWithAtLeastOnceDelivery class instead of AbstractPersistentActor on the sending side. It takes care of re-sending messages when they have not been confirmed within a configurable timeout.

The state of the sending actor, including which messages have been sent that have not been confirmed by the recipient must be persistent so that it can survive a crash of the sending actor or JVM. The AtLeastOnceDelivery traitAbstractPersistentActorWithAtLeastOnceDelivery class does not persist anything by itself. It is your responsibility to persist the intent that a message is sent and that a confirmation has been received.

Use the deliverdeliver method to send a message to a destination. Call the confirmDeliveryconfirmDelivery method when the destination has replied with a confirmation message.

Relationship between deliver and confirmDelivery

To send messages to the destination path, use the deliver method after you have persisted the intent to send the message.

The destination actor must send back a confirmation message. When the sending actor receives this confirmation message you should persist the fact that the message was delivered successfully and then call the confirmDelivery method.

If the persistent actor is not currently recovering, the deliver method will send the message to the destination actor. When recovering, messages will be buffered until they have been confirmed using confirmDelivery. Once recovery has completed, if there are outstanding messages that have not been confirmed (during the message replay), the persistent actor will resend these before sending any other messages.

Deliver requires a deliveryIdToMessage function to pass the provided deliveryId into the message so that the correlation between deliver and confirmDelivery is possible. The deliveryId must do the round trip. Upon receipt of the message, the destination actor will send the samedeliveryId wrapped in a confirmation message back to the sender. The sender will then use it to call confirmDelivery method to complete the delivery routine.

Scala

copysourceimport akka.actor.{ Actor, ActorSelection }
import akka.persistence.AtLeastOnceDelivery

case class Msg(deliveryId: Long, s: String)
case class Confirm(deliveryId: Long)

sealed trait Evt
case class MsgSent(s: String) extends Evt
case class MsgConfirmed(deliveryId: Long) extends Evt

class MyPersistentActor(destination: ActorSelection) extends PersistentActor with AtLeastOnceDelivery {

  override def persistenceId: String = "persistence-id"

  override def receiveCommand: Receive = {
    case s: String           => persist(MsgSent(s))(updateState)
    case Confirm(deliveryId) => persist(MsgConfirmed(deliveryId))(updateState)
  }

  override def receiveRecover: Receive = {
    case evt: Evt => updateState(evt)
  }

  def updateState(evt: Evt): Unit = evt match {
    case MsgSent(s) =>
      deliver(destination)(deliveryId => Msg(deliveryId, s))

    case MsgConfirmed(deliveryId) => confirmDelivery(deliveryId)
  }
}

class MyDestination extends Actor {
  def receive = {
    case Msg(deliveryId, s) =>
      // ...
      sender() ! Confirm(deliveryId)
  }
}

Java

copysource
class Msg implements Serializable {
  private static final long serialVersionUID = 1L;
  public final long deliveryId;
  public final String s;

  public Msg(long deliveryId, String s) {
    this.deliveryId = deliveryId;
    this.s = s;
  }
}

class Confirm implements Serializable {
  private static final long serialVersionUID = 1L;
  public final long deliveryId;

  public Confirm(long deliveryId) {
    this.deliveryId = deliveryId;
  }
}

class MsgSent implements Serializable {
  private static final long serialVersionUID = 1L;
  public final String s;

  public MsgSent(String s) {
    this.s = s;
  }
}

class MsgConfirmed implements Serializable {
  private static final long serialVersionUID = 1L;
  public final long deliveryId;

  public MsgConfirmed(long deliveryId) {
    this.deliveryId = deliveryId;
  }
}

class MyPersistentActor extends AbstractPersistentActorWithAtLeastOnceDelivery {
  private final ActorSelection destination;

  public MyPersistentActor(ActorSelection destination) {
    this.destination = destination;
  }

  @Override
  public String persistenceId() {
    return "persistence-id";
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            String.class,
            s -> {
              persist(new MsgSent(s), evt -> updateState(evt));
            })
        .match(
            Confirm.class,
            confirm -> {
              persist(new MsgConfirmed(confirm.deliveryId), evt -> updateState(evt));
            })
        .build();
  }

  @Override
  public Receive createReceiveRecover() {
    return receiveBuilder().match(Object.class, evt -> updateState(evt)).build();
  }

  void updateState(Object event) {
    if (event instanceof MsgSent) {
      final MsgSent evt = (MsgSent) event;
      deliver(destination, deliveryId -> new Msg(deliveryId, evt.s));
    } else if (event instanceof MsgConfirmed) {
      final MsgConfirmed evt = (MsgConfirmed) event;
      confirmDelivery(evt.deliveryId);
    }
  }
}

class MyDestination extends AbstractActor {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            Msg.class,
            msg -> {
              // ...
              getSender().tell(new Confirm(msg.deliveryId), getSelf());
            })
        .build();
  }
}

The deliveryId generated by the persistence module is a strictly monotonically increasing sequence number without gaps. The same sequence is used for all destinations of the actor, i.e. when sending to multiple destinations the destinations will see gaps in the sequence. It is not possible to use custom deliveryId. However, you can send a custom correlation identifier in the message to the destination. You must then retain a mapping between the internal deliveryId (passed into the deliveryIdToMessage function) and your custom correlation id (passed into the message). You can do this by storing such mapping in a Map(correlationId -> deliveryId) from which you can retrieve the deliveryId to be passed into the confirmDelivery method once the receiver of your message has replied with your custom correlation id.

The AtLeastOnceDelivery traitAbstractPersistentActorWithAtLeastOnceDelivery class has a state consisting of unconfirmed messages and a sequence number. It does not store this state itself. You must persist events corresponding to the deliverdeliver and confirmDeliveryconfirmDelivery invocations from your PersistentActor so that the state can be restored by calling the same methods during the recovery phase of the PersistentActor. Sometimes these events can be derived from other business level events, and sometimes you must create separate events. During recovery, calls to deliver will not send out messages, those will be sent later if no matching confirmDelivery will have been performed.

Support for snapshots is provided by getDeliverySnapshotgetDeliverySnapshot and setDeliverySnapshotsetDeliverySnapshot. The AtLeastOnceDeliverySnapshotAtLeastOnceDeliverySnapshot contains the full delivery state, including unconfirmed messages. If you need a custom snapshot for other parts of the actor state you must also include the AtLeastOnceDeliverySnapshot. It is serialized using protobuf with the ordinary Akka serialization mechanism. It is easiest to include the bytes of the AtLeastOnceDeliverySnapshot as a blob in your custom snapshot.

The interval between redelivery attempts is defined by the redeliverIntervalredeliverInterval method. The default value can be configured with the akka.persistence.at-least-once-delivery.redeliver-interval configuration key. The method can be overridden by implementation classes to return non-default values.

The maximum number of messages that will be sent at each redelivery burst is defined by the redeliverBurstLimitredeliverBurstLimit method (burst frequency is half of the redelivery interval). If there’s a lot of unconfirmed messages (e.g. if the destination is not available for a long time), this helps to prevent an overwhelming amount of messages to be sent at once. The default value can be configured with the akka.persistence.at-least-once-delivery.redelivery-burst-limit configuration key. The method can be overridden by implementation classes to return non-default values.

After a number of delivery attempts a UnconfirmedWarningUnconfirmedWarning message will be sent to self. The re-sending will still continue, but you can choose to call confirmDelivery to cancel the re-sending. The number of delivery attempts before emitting the warning is defined by the warnAfterNumberOfUnconfirmedAttemptswarnAfterNumberOfUnconfirmedAttempts method. The default value can be configured with the akka.persistence.at-least-once-delivery.warn-after-number-of-unconfirmed-attempts configuration key. The method can be overridden by implementation classes to return non-default values.

The AtLeastOnceDelivery traitAbstractPersistentActorWithAtLeastOnceDelivery class holds messages in memory until their successful delivery has been confirmed. The maximum number of unconfirmed messages that the actor is allowed to hold in memory is defined by the maxUnconfirmedMessagesmaxUnconfirmedMessages method. If this limit is exceed the deliver method will not accept more messages and it will throw AtLeastOnceDelivery.MaxUnconfirmedMessagesExceededExceptionAtLeastOnceDelivery.MaxUnconfirmedMessagesExceededException. The default value can be configured with the akka.persistence.at-least-once-delivery.max-unconfirmed-messages configuration key. The method can be overridden by implementation classes to return non-default values.

Event Adapters

In long running projects using Event Sourcing sometimes the need arises to detach the data model from the domain model completely.

Event Adapters help in situations where:

• Version Migrations – existing events stored in Version 1 should be “upcasted” to a new Version 2 representation, and the process of doing so involves actual code, not just changes on the serialization layer. For these scenarios the toJournaltoJournal function is usually an identity function, however the fromJournalfromJournal is implemented as v1.Event=>v2.Event, performing the necessary mapping inside the fromJournal method. This technique is sometimes referred to as “upcasting” in other CQRS libraries.
• Separating Domain and Data models – thanks to EventAdapters it is possible to completely separate the domain model from the model used to persist data in the Journals. For example one may want to use case classes in the domain model, however persist their protocol-buffer (or any other binary serialization format) counter-parts to the Journal. A simple toJournal:MyModel=>MyDataModel and fromJournal:MyDataModel=>MyModel adapter can be used to implement this feature.
• Journal Specialized Data Types – exposing data types understood by the underlying Journal, for example for data stores which understand JSON it is possible to write an EventAdapter toJournal:Any=>JSON such that the Journal can directly store the json instead of serializing the object to its binary representation.

Implementing an EventAdapter is rather straightforward:

Scala

copysourceclass MyEventAdapter(system: ExtendedActorSystem) extends EventAdapter {
  override def manifest(event: Any): String =
    "" // when no manifest needed, return ""

  override def toJournal(event: Any): Any =
    event // identity

  override def fromJournal(event: Any, manifest: String): EventSeq =
    EventSeq.single(event) // identity
}

Java

copysourceclass MyEventAdapter implements EventAdapter {
  @Override
  public String manifest(Object event) {
    return ""; // if no manifest needed, return ""
  }

  @Override
  public Object toJournal(Object event) {
    return event; // identity
  }

  @Override
  public EventSeq fromJournal(Object event, String manifest) {
    return EventSeq.single(event); // identity
  }
}

Then in order for it to be used on events coming to and from the journal you must bind it using the below configuration syntax:

copysourceakka.persistence.journal {
  inmem {
    event-adapters {
      tagging        = "docs.persistence.MyTaggingEventAdapter"
      user-upcasting = "docs.persistence.UserUpcastingEventAdapter"
      item-upcasting = "docs.persistence.ItemUpcastingEventAdapter"
    }

    event-adapter-bindings {
      "docs.persistence.Item"        = tagging
      "docs.persistence.TaggedEvent" = tagging
      "docs.persistence.v1.Event"    = [user-upcasting, item-upcasting]
    }
  }
}

It is possible to bind multiple adapters to one class for recovery, in which case the fromJournal methods of all bound adapters will be applied to a given matching event (in order of definition in the configuration). Since each adapter may return from 0 to n adapted events (called as EventSeq), each adapter can investigate the event and if it should indeed adapt it return the adapted event(s) for it. Other adapters which do not have anything to contribute during this adaptation simply return EventSeq.emptyEventSeq.empty. The adapted events are then delivered in-order to the PersistentActor during replay.

Custom serialization

Serialization of snapshots and payloads of Persistent messages is configurable with Akka’s Serialization infrastructure. For example, if an application wants to serialize

• payloads of type MyPayload with a custom MyPayloadSerializer and
• snapshots of type MySnapshot with a custom MySnapshotSerializer

it must add

copysourceakka.actor {
  serializers {
    my-payload = "docs.persistence.MyPayloadSerializer"
    my-snapshot = "docs.persistence.MySnapshotSerializer"
  }
  serialization-bindings {
    "docs.persistence.MyPayload" = my-payload
    "docs.persistence.MySnapshot" = my-snapshot
  }
}

to the application configuration. If not specified, an exception will be throw when trying to persist events or snapshots.

For more advanced schema evolution techniques refer to the Persistence - Schema Evolution documentation.

Testing with LevelDB journal

The LevelDB journal is deprecated and will be removed from a future Akka version, it is not advised to build new applications with it. For testing the built in “inmem” journal or the actual journal that will be used in production of the application is recommended. See Persistence Plugins for some journal implementation choices.

When running tests with LevelDB default settings in sbt, make sure to set fork := true in your sbt project. Otherwise, you’ll see an UnsatisfiedLinkError. Alternatively, you can switch to a LevelDB Java port by setting

copysourceakka.persistence.journal.leveldb.native = off

or

copysourceakka.persistence.journal.leveldb-shared.store.native = off

in your Akka configuration. Also note that for the LevelDB Java port, you will need the following dependencies:

sbt

libraryDependencies += "org.iq80.leveldb" % "leveldb" % "0.9"

Maven

<dependencies>
  <dependency>
    <groupId>org.iq80.leveldb</groupId>
    <artifactId>leveldb</artifactId>
    <version>0.9</version>
  </dependency>
</dependencies>

Gradle

dependencies {
  implementation "org.iq80.leveldb:leveldb:0.9"
}

Configuration

There are several configuration properties for the persistence module, please refer to the reference configuration.

The journal and snapshot store plugins have specific configuration, see reference documentation of the chosen plugin.

Multiple persistence plugin configurations

By default, a persistent actor will use the “default” journal and snapshot store plugins configured in the following sections of the reference.conf configuration resource:

copysource# Absolute path to the default journal plugin configuration entry.
akka.persistence.journal.plugin = "akka.persistence.journal.inmem"
# Absolute path to the default snapshot store plugin configuration entry.
akka.persistence.snapshot-store.plugin = "akka.persistence.snapshot-store.local"

Note that in this case the actor overrides only the persistenceIdpersistenceId method:

Scala

copysourcetrait ActorWithDefaultPlugins extends PersistentActor {
  override def persistenceId = "123"
}

Java

copysourceabstract class AbstractPersistentActorWithDefaultPlugins extends AbstractPersistentActor {
  @Override
  public String persistenceId() {
    return "123";
  }
}

When the persistent actor overrides the journalPluginIdjournalPluginId and snapshotPluginIdsnapshotPluginId methods, the actor will be serviced by these specific persistence plugins instead of the defaults:

Scala

copysourcetrait ActorWithOverridePlugins extends PersistentActor {
  override def persistenceId = "123"

  // Absolute path to the journal plugin configuration entry in the `reference.conf`.
  override def journalPluginId = "akka.persistence.chronicle.journal"

  // Absolute path to the snapshot store plugin configuration entry in the `reference.conf`.
  override def snapshotPluginId = "akka.persistence.chronicle.snapshot-store"
}

Java

copysourceabstract class AbstractPersistentActorWithOverridePlugins extends AbstractPersistentActor {
  @Override
  public String persistenceId() {
    return "123";
  }

  // Absolute path to the journal plugin configuration entry in the `reference.conf`
  @Override
  public String journalPluginId() {
    return "akka.persistence.chronicle.journal";
  }

  // Absolute path to the snapshot store plugin configuration entry in the `reference.conf`
  @Override
  public String snapshotPluginId() {
    return "akka.persistence.chronicle.snapshot-store";
  }
}

Note that journalPluginId and snapshotPluginId must refer to properly configured reference.conf plugin entries with a standard class property as well as settings which are specific for those plugins, i.e.:

copysource# Configuration entry for the custom journal plugin, see `journalPluginId`.
akka.persistence.chronicle.journal {
  # Standard persistence extension property: provider FQCN.
  class = "akka.persistence.chronicle.ChronicleSyncJournal"
  # Custom setting specific for the journal `ChronicleSyncJournal`.
  folder = $${user.dir}/store/journal
}
# Configuration entry for the custom snapshot store plugin, see `snapshotPluginId`.
akka.persistence.chronicle.snapshot-store {
  # Standard persistence extension property: provider FQCN.
  class = "akka.persistence.chronicle.ChronicleSnapshotStore"
  # Custom setting specific for the snapshot store `ChronicleSnapshotStore`.
  folder = $${user.dir}/store/snapshot
}

Give persistence plugin configurations at runtime

By default, a persistent actor will use the configuration loaded at ActorSystemActorSystem creation time to create journal and snapshot store plugins.

When the persistent actor overrides the journalPluginConfigjournalPluginConfig and snapshotPluginConfigsnapshotPluginConfig methods, the actor will use the declared Config objects with a fallback on the default configuration. It allows a dynamic configuration of the journal and the snapshot store at runtime:

Scala

copysourcetrait ActorWithRuntimePluginConfig extends PersistentActor with RuntimePluginConfig {
  // Variable that is retrieved at runtime, from an external service for instance.
  val runtimeDistinction = "foo"

  override def persistenceId = "123"

  // Absolute path to the journal plugin configuration entry, not defined in the `reference.conf`.
  override def journalPluginId = s"journal-plugin-$runtimeDistinction"

  // Absolute path to the snapshot store plugin configuration entry, not defined in the `reference.conf`.
  override def snapshotPluginId = s"snapshot-store-plugin-$runtimeDistinction"

  // Configuration which contains the journal plugin id defined above
  override def journalPluginConfig =
    ConfigFactory
      .empty()
      .withValue(
        s"journal-plugin-$runtimeDistinction",
        context.system.settings.config
          .getValue("journal-plugin") // or a very different configuration coming from an external service.
      )

  // Configuration which contains the snapshot store plugin id defined above
  override def snapshotPluginConfig =
    ConfigFactory
      .empty()
      .withValue(
        s"snapshot-plugin-$runtimeDistinction",
        context.system.settings.config
          .getValue("snapshot-store-plugin") // or a very different configuration coming from an external service.
      )

}

Java

copysourceabstract class AbstractPersistentActorWithRuntimePluginConfig extends AbstractPersistentActor
    implements RuntimePluginConfig {
  // Variable that is retrieved at runtime, from an external service for instance.
  String runtimeDistinction = "foo";

  @Override
  public String persistenceId() {
    return "123";
  }

  // Absolute path to the journal plugin configuration entry in the `reference.conf`
  @Override
  public String journalPluginId() {
    return "journal-plugin-" + runtimeDistinction;
  }

  // Absolute path to the snapshot store plugin configuration entry in the `reference.conf`
  @Override
  public String snapshotPluginId() {
    return "snapshot-store-plugin-" + runtimeDistinction;
  }

  // Configuration which contains the journal plugin id defined above
  @Override
  public Config journalPluginConfig() {
    return ConfigFactory.empty()
        .withValue(
            "journal-plugin-" + runtimeDistinction,
            getContext()
                .getSystem()
                .settings()
                .config()
                .getValue(
                    "journal-plugin") // or a very different configuration coming from an external
            // service.
            );
  }

  // Configuration which contains the snapshot store plugin id defined above
  @Override
  public Config snapshotPluginConfig() {
    return ConfigFactory.empty()
        .withValue(
            "snapshot-plugin-" + runtimeDistinction,
            getContext()
                .getSystem()
                .settings()
                .config()
                .getValue(
                    "snapshot-store-plugin") // or a very different configuration coming from an
            // external service.
            );
  }
}

See also

• Persistence plugins
• Building a new storage backend