Persistence - Version 2.4-SNAPSHOT

Persistence

Akka persistence enables stateful actors to persist their internal state so that it can be recovered when an actor is started, restarted after a JVM crash or by a supervisor, or migrated in a cluster. The key concept behind Akka persistence is that only changes to an actor's internal state are persisted but never its current state directly (except for optional snapshots). These changes are only ever appended to storage, nothing is ever mutated, which allows for very high transaction rates and efficient replication. Stateful actors are recovered by replaying stored changes to these actors from which they can rebuild internal state. This can be either the full history of changes or starting from a snapshot which can dramatically reduce recovery times. Akka persistence also provides point-to-point communication with at-least-once message delivery semantics.

Akka persistence is inspired by and the official replacement of the eventsourced library. It follows the same concepts and architecture of eventsourced but significantly differs on API and implementation level. See also Migration Guide Eventsourced to Akka Persistence 2.3.x

Dependencies

Akka persistence is a separate jar file. Make sure that you have the following dependency in your project:

"com.typesafe.akka" %% "akka-persistence" % "2.4-SNAPSHOT"

The Akka persistence extension comes with few built-in persistence plugins, including in-memory heap based journal, local file-system based snapshot-store and LevelDB based journal.

LevelDB based plugins will require the following additional dependency declaration:

"org.iq80.leveldb"            % "leveldb"          % "0.7"
"org.fusesource.leveldbjni"   % "leveldbjni-all"   % "1.8"

Architecture

  • PersistentActor: 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 internal state from these messages.
  • PersistentView: A view is a persistent, stateful actor that receives journaled messages that have been written by another persistent actor. A view itself does not journal new messages, instead, it updates internal state only from a persistent actor's replicated message stream.
  • AtLeastOnceDelivery: To send messages with at-least-once delivery semantics to destinations, also in case of sender and receiver JVM crashes.
  • AsyncWriteJournal: 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. The persistence extension comes with a "leveldb" journal plugin, which writes to the local filesystem. Replicated journals are available as Community plugins.
  • Snapshot store: A snapshot store persists snapshots of a persistent actor's or a view's internal 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

The basic idea behind Event Sourcing is quite simple. A persistent actor receives a (non-persistent) command which is first validated if it can be applied to the current state. Here validation can mean anything, from simple inspection of a command message's fields up to a conversation with several external services, for example. If validation succeeds, events are generated from the command, representing the effect of the command. These events are then persisted and, after successful persistence, used to change the actor's state. When the persistent actor needs to be recovered, only the persisted events are replayed of which we know that they can be successfully applied. In other words, events cannot fail when being replayed to a persistent actor, in contrast to commands. Event sourced actors may of course also process commands that do not change application state such as query commands for example.

Akka persistence supports event sourcing with the PersistentActor trait. An actor that extends this trait uses the persist method to persist and handle events. The behavior of a PersistentActor is defined by implementing receiveRecover and receiveCommand. This is demonstrated in the following example.

import 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 receiveCommand: Receive = {
    case Cmd(data) =>
      persist(Evt(s"${data}-${numEvents}"))(updateState)
      persist(Evt(s"${data}-${numEvents + 1}")) { event =>
        updateState(event)
        context.system.eventStream.publish(event)
      }
    case "snap"  => saveSnapshot(state)
    case "print" => println(state)
  }

}

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 receiveRecover method defines how state is updated during recovery by handling Evt and SnapshotOffer messages. The persistent actor's receiveCommand method is a command handler. In this example, a command is handled by generating two events which are 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.

The easiest way to run this example yourself is to download Lightbend Activator and open the tutorial named Akka Persistence Samples with Scala. It contains instructions on how to run the PersistentActorExample.

Note

It's also possible to switch between different command handlers during normal processing and recovery with context.become() and context.unbecome(). To get the actor into the same state after recovery you need to take special care to perform the same state transitions with become and unbecome in the receiveRecover method as you would have done in the command handler. Note that when using become from receiveRecover it will still only use the receiveRecover behavior when replaying the events. When replay is completed it will use the new behavior.

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.

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

Note

persistenceId must be unique to a given entity in the journal (database table/keyspace). When replaying messages persisted to the journal, you query messages with a persistenceId. So, if two different entities share the same persistenceId, message-replaying behavior is corrupted.

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 cached and received by a persistent actor after recovery phase completes.

Note

Accessing the sender() for replayed messages will always result in a deadLetters reference, as the original sender is presumed to be long gone. If you indeed have to notify an actor during recovery in the future, store its ActorPath explicitly in your persisted events.

Recovery customization

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

To skip loading snapshots and replay all events you can use SnapshotSelectionCriteria.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.

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

Another example, which can be fun for experiments but probably not in a real application, is setting an upper bound to the replay which allows the actor to be replayed to a certain point "in the past" instead 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.

override def recovery = Recovery(toSequenceNr = 457L)

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

override def recovery = Recovery.none

Recovery status

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

def recoveryRunning: Boolean
def recoveryFinished: Boolean

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 RecoveryCompleted message right after recovery and before any other received messages.

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

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

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 persist\persistAll method persisting events. You can still use/inherit from the Stash interface. The internal stash cooperates with the normal stash by hooking into unstashAll method and 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 don'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 ThrowOverflowExceptionStrategy, which discards the current received message and throws a StashOverflowException, causing actor restart if default supervision strategy is used. you can override the internalStashOverflowStrategy method to return DiscardToDeadLetterStrategy or ReplyToStrategy for any "individual" persistent actor, or define the "default" for all persistent actors by providing FQCN, which must be a subclass of StashOverflowStrategyConfigurator, in the persistence configuration:

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

The DiscardToDeadLetterStrategy strategy also has a pre-packaged companion configurator akka.persistence.DiscardConfigurator.

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

Persistence(context.system).defaultInternalStashOverflowStrategy

Note

The bounded mailbox should be avoided in the persistent actor, by which the messages come from storage backends may be discarded. You can use bounded stash instead of it.

Relaxed local consistency requirements and high throughput use-cases

If faced with relaxed local consistency requirements and high throughput demands sometimes PersistentActor and its persist 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 persistAsync 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.).

class 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

Note

In order to implement the pattern known as "command sourcing" simply call persistAsync(cmd)(...) right away on all incoming messages and handle them in the callback.

Warning

The callback will not be invoked if the actor is restarted (or stopped) in between the call to persistAsync and the journal has confirmed the write.

Deferring actions until preceding persist handlers have executed

Sometimes when working with persistAsync you may find that it would be nice to define some actions in terms of ''happens-after the previous persistAsync handlers have been invoked''. PersistentActor provides an utility method called deferAsync, which works similarly to persistAsync yet does not persist the passed in event. It is recommended to use it for read operations, and actions which do not have corresponding events in your domain model.

Using this method is very similar to the persist family of methods, yet it does not persist the passed in event. It will be kept in memory and used when invoking the handler.

class 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 }
    }
  }
}

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 deferAsync handler was called.

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

persistentActor ! "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

Warning

The callback will not be invoked if the actor is restarted (or stopped) in between the call to deferAsync and the journal has processed and confirmed all preceding writes.

Nested persist calls

It is possible to call persist and persistAsync inside their respective callback blocks and they will properly retain both the thread safety (including the right value of sender()) 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 behaviour (that persist() enforces). In the following example two persist calls are issued, and each of them issues another persist inside its callback:

override 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
      }
    }
}

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

persistentActor ! "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

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:

override 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 }
    }
}

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

persistentActor ! "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

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.

Warning

While it is possible to nest persist calls within one another, it is not legal call persist from any other Thread than the Actors message processing Thread. For example, it is not legal to call persist from Futures! Doing so will break the guarantees that the persist methods aim to provide. Always call persist and persistAsync from within the Actor's receive block (or methods synchronously invoked from there).

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 akka.pattern.BackoffSupervisor actor is provided to support such restarts.

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

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 of course stored atomically, but it is also possible to store several events atomically by using the persistAll or persistAllAsync 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 persistAsync, 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 deleteMessages 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.

Warning

If you are using Persistence Query, query results may be missing deleted messages in a journal, depending on how deletions are implemented in the journal plugin. Unless you use a plugin which still shows deleted messages in persistence query results, you have to design your application so that it is not affected by missing messages.

The result of the deleteMessages request is signaled to the persistent actor with a DeleteMessagesSuccess message if the delete was successful or a DeleteMessagesFailure 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 Failure / Rejection After failure handler invoked
persist / persistAsync persist handler invoked onPersistFailure Actor is stopped.
onPersistRejected No automatic actions.
recovery RecoveryCompleted onRecoveryFailure Actor is stopped.
deleteMessages DeleteMessagesSuccess DeleteMessagesFailure No automatic actions.

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 BackoffSupervisor (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.

Note

Journal implementations may choose to implement a retry mechanism, e.g. such that only after a write fails N number of times a persistence failure is signalled back to the user. In other words, once a journal returns a failure, it is considered fatal by Akka Persistence, and the persistent actor which caused the failure will be stopped.

Check the documentation of the journal implementation you are using for details if/how it is using this technique.

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() 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.

Warning

Consider using explicit shut-down messages instead of PoisonPill when working with persistent actors.

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 behaviour that occurs when PoisonPill is used:

/** 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
  }
}
// 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!
// 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 --

Replay Filter

There could be cases where event streams are corrupted and multiple writers (i.e. multiple persistent actor instances) journaled different messages with the same sequence number. In such a case, you can configure how you filter replayed messages from multiple writers, upon recovery.

In your configuration, under the akka.persistence.journal.xxx.replay-filter section (where xxx is your journal plugin id), you can select the replay filter mode from one of the following values:

  • repair-by-discard-old
  • fail
  • warn
  • off

For example, if you configure the replay filter for leveldb plugin, it looks like this:

# The replay filter can detect a corrupt event stream by inspecting
# sequence numbers and writerUuid when replaying events.
akka.persistence.journal.leveldb.replay-filter {
  # What the filter should do when detecting invalid events.
  # Supported values:
  # `repair-by-discard-old` : discard events from old writers,
  #                           warning is logged
  # `fail` : fail the replay, error is logged
  # `warn` : log warning but emit events untouched
  # `off` : disable this feature completely
  mode = repair-by-discard-old
}

Persistent Views

Warning

PersistentView is deprecated. Use Persistence Query instead. The corresponding query type is EventsByPersistenceId. There are several alternatives for connecting the Source to an actor corresponding to a previous PersistentView actor:

  • Sink.actorRef is simple, but has the disadvantage that there is no back-pressure signal from the destination actor, i.e. if the actor is not consuming the messages fast enough the mailbox of the actor will grow
  • mapAsync combined with Ask: Send-And-Receive-Future is almost as simple with the advantage of back-pressure being propagated all the way
  • ActorSubscriber in case you need more fine grained control

The consuming actor may be a plain Actor or a PersistentActor if it needs to store its own state (e.g. fromSequenceNr offset).

Persistent views can be implemented by extending the PersistentView trait and implementing the receive and the persistenceId methods.

class MyView extends PersistentView {
  override def persistenceId: String = "some-persistence-id"
  override def viewId: String = "some-persistence-id-view"

  def receive: Receive = {
    case payload if isPersistent =>
    // handle message from journal...
    case payload                 =>
    // handle message from user-land...
  }
}

The persistenceId identifies the persistent actor from which the view receives journaled messages. It is not necessary that the referenced persistent actor is actually running. Views read messages from a persistent actor's journal directly. When a persistent actor is started later and begins to write new messages, by default the corresponding view is updated automatically.

It is possible to determine if a message was sent from the Journal or from another actor in user-land by calling the isPersistent method. Having that said, very often you don't need this information at all and can simply apply the same logic to both cases (skip the if isPersistent check).

Updates

The default update interval of all views of an actor system is configurable:

akka.persistence.view.auto-update-interval = 5s

PersistentView implementation classes may also override the autoUpdateInterval method to return a custom update interval for a specific view class or view instance. Applications may also trigger additional updates at any time by sending a view an Update message.

val view = system.actorOf(Props[MyView])
view ! Update(await = true)

If the await parameter is set to true, messages that follow the Update request are processed when the incremental message replay, triggered by that update request, completed. If set to false (default), messages following the update request may interleave with the replayed message stream. Automated updates always run with await = false.

Automated updates of all persistent views of an actor system can be turned off by configuration:

akka.persistence.view.auto-update = off

Implementation classes may override the configured default value by overriding the autoUpdate method. To limit the number of replayed messages per update request, applications can configure a custom akka.persistence.view.auto-update-replay-max value or override the autoUpdateReplayMax method. The number of replayed messages for manual updates can be limited with the replayMax parameter of the Update message.

Recovery

Initial recovery of persistent views works the very same way as for persistent actors (i.e. by sending a Recover message to self). The maximum number of replayed messages during initial recovery is determined by autoUpdateReplayMax. Further possibilities to customize initial recovery are explained in section Recovery.

Identifiers

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

The viewId must differ from the referenced persistenceId, unless Snapshots of a view and its persistent actor should be shared (which is what applications usually do not want).

Snapshots

Snapshots can dramatically reduce recovery times of persistent actors and views. The following discusses snapshots in context of persistent actors but this is also applicable to persistent views.

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

var state: Any = _

override def receiveCommand: Receive = {
  case "snap"                                => saveSnapshot(state)
  case SaveSnapshotSuccess(metadata)         => // ...
  case SaveSnapshotFailure(metadata, reason) => // ...
}

where metadata is of type SnapshotMetadata:

final case class SnapshotMetadata(persistenceId: String, sequenceNr: Long, timestamp: Long = 0L)

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

var state: Any = _

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

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 SnapshotSelectionCriteria that can be specified for recovery.

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

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

Note

In order to use snapshots, a default snapshot-store (akka.persistence.snapshot-store.plugin) must be configured, or the PersistentActor can pick a snapshot store explicitly by overriding def snapshotPluginId: String.

Since it is acceptable for some applications to not use any snapshotting, it is legal to not configure a snapshot store. However, Akka will log a warning message when this situation is detected and then continue to operate until an actor tries to store a snapshot, at which point the operation will fail (by replying with an SaveSnapshotFailure for example).

Note that Cluster Sharding is using snapshots, so if you use Cluster Sharding you need to define a snapshot store plugin.

Snapshot deletion

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

To bulk-delete a range of snapshots matching SnapshotSelectionCriteria, persistent actors should use the deleteSnapshots method.

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.

At-Least-Once Delivery

To send messages with at-least-once delivery semantics to destinations you can mix-in AtLeastOnceDelivery trait to your PersistentActor 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 trait 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.

Note

At-least-once delivery implies that original message sending order is not always preserved, and the destination may receive duplicate messages. Semantics do not match those of a normal ActorRef send operation:

  • it is not at-most-once delivery
  • message order for the same sender–receiver pair is not preserved due to possible resends
  • after a crash and restart of the destination messages are still delivered to the new actor incarnation

These semantics are similar to what an ActorPath represents (see Actor Lifecycle), therefore you need to supply a path and not a reference when delivering messages. The messages are sent to the path with an actor selection.

Use the deliver method to send a message to a destination. Call the confirmDelivery 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 same``deliveryId`` wrapped in a confirmation message back to the sender. The sender will then use it to call confirmDelivery method to complete the delivery routine.

import 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)
  }
}

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 trait 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 deliver and confirmDelivery 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 getDeliverySnapshot and setDeliverySnapshot. The AtLeastOnceDeliverySnapshot 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 redeliverInterval 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 redeliveryBurstLimit 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 AtLeastOnceDelivery.UnconfirmedWarning 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 warnAfterNumberOfUnconfirmedAttempts 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 trait 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 maxUnconfirmedMessages method. If this limit is exceed the deliver method will not accept more messages and it will throw AtLeastOnceDelivery.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 toJournal function is usually an identity function, however the fromJournal is implemented as v1.Event=>v2.Event, performing the neccessary mapping inside the fromJournal method. This technique is sometimes refered 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 stright forward:

class 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
}

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:

akka.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.empty. The adapted events are then delivered in-order to the PersistentActor during replay.

Note

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

Persistent FSM

PersistentFSM handles the incoming messages in an FSM like fashion. Its internal state is persisted as a sequence of changes, later referred to as domain events. Relationship between incoming messages, FSM's states and transitions, persistence of domain events is defined by a DSL.

Warning

PersistentFSM is marked as “experimental” as of its introduction in Akka 2.4.0. We will continue to improve this API based on our users’ feedback, which implies that while we try to keep incompatible changes to a minimum the binary compatibility guarantee for maintenance releases does not apply to the contents of the classes related to ``PersistentFSM`.

A Simple Example

To demonstrate the features of the PersistentFSM trait, consider an actor which represents a Web store customer. The contract of our "WebStoreCustomerFSMActor" is that it accepts the following commands:

sealed trait Command
case class AddItem(item: Item) extends Command
case object Buy extends Command
case object Leave extends Command
case object GetCurrentCart extends Command

AddItem sent when the customer adds an item to a shopping cart Buy - when the customer finishes the purchase Leave - when the customer leaves the store without purchasing anything GetCurrentCart allows to query the current state of customer's shopping cart

The customer can be in one of the following states:

sealed trait UserState extends FSMState
case object LookingAround extends UserState {
  override def identifier: String = "Looking Around"
}
case object Shopping extends UserState {
  override def identifier: String = "Shopping"
}
case object Inactive extends UserState {
  override def identifier: String = "Inactive"
}
case object Paid extends UserState {
  override def identifier: String = "Paid"
}

LookingAround customer is browsing the site, but hasn't added anything to the shopping cart Shopping customer has recently added items to the shopping cart Inactive customer has items in the shopping cart, but hasn't added anything recently Paid customer has purchased the items

Note

PersistentFSM states must inherit from trait PersistentFSM.FSMState and implement the def identifier: String method. This is required in order to simplify the serialization of FSM states. String identifiers should be unique!

Customer's actions are "recorded" as a sequence of "domain events" which are persisted. Those events are replayed on an actor's start in order to restore the latest customer's state:

sealed trait DomainEvent
case class ItemAdded(item: Item) extends DomainEvent
case object OrderExecuted extends DomainEvent
case object OrderDiscarded extends DomainEvent

Customer state data represents the items in a customer's shopping cart:

case class Item(id: String, name: String, price: Float)

sealed trait ShoppingCart {
  def addItem(item: Item): ShoppingCart
  def empty(): ShoppingCart
}
case object EmptyShoppingCart extends ShoppingCart {
  def addItem(item: Item) = NonEmptyShoppingCart(item :: Nil)
  def empty() = this
}
case class NonEmptyShoppingCart(items: Seq[Item]) extends ShoppingCart {
  def addItem(item: Item) = NonEmptyShoppingCart(items :+ item)
  def empty() = EmptyShoppingCart
}

Here is how everything is wired together:

startWith(LookingAround, EmptyShoppingCart)

when(LookingAround) {
  case Event(AddItem(item), _) 
    goto(Shopping) applying ItemAdded(item) forMax (1 seconds)
  case Event(GetCurrentCart, data) 
    stay replying data
}

when(Shopping) {
  case Event(AddItem(item), _) 
    stay applying ItemAdded(item) forMax (1 seconds)
  case Event(Buy, _) 
    goto(Paid) applying OrderExecuted andThen {
      case NonEmptyShoppingCart(items) 
        reportActor ! PurchaseWasMade(items)
        saveStateSnapshot()
      case EmptyShoppingCart  saveStateSnapshot()
    }
  case Event(Leave, _) 
    stop applying OrderDiscarded andThen {
      case _ 
        reportActor ! ShoppingCardDiscarded
        saveStateSnapshot()
    }
  case Event(GetCurrentCart, data) 
    stay replying data
  case Event(StateTimeout, _) 
    goto(Inactive) forMax (2 seconds)
}

when(Inactive) {
  case Event(AddItem(item), _) 
    goto(Shopping) applying ItemAdded(item) forMax (1 seconds)
  case Event(StateTimeout, _) 
    stop applying OrderDiscarded andThen {
      case _  reportActor ! ShoppingCardDiscarded
    }
}

when(Paid) {
  case Event(Leave, _)  stop()
  case Event(GetCurrentCart, data) 
    stay replying data
}

Note

State data can only be modified directly on initialization. Later it's modified only as a result of applying domain events. Override the applyEvent method to define how state data is affected by domain events, see the example below

override def applyEvent(event: DomainEvent, cartBeforeEvent: ShoppingCart): ShoppingCart = {
  event match {
    case ItemAdded(item)  cartBeforeEvent.addItem(item)
    case OrderExecuted    cartBeforeEvent
    case OrderDiscarded   cartBeforeEvent.empty()
  }
}

andThen can be used to define actions which will be executed following event's persistence - convenient for "side effects" like sending a message or logging. Notice that actions defined in andThen block are not executed on recovery:

goto(Paid) applying OrderExecuted andThen {
  case NonEmptyShoppingCart(items) 
    reportActor ! PurchaseWasMade(items)
}

A snapshot of state data can be persisted by calling the saveStateSnapshot() method:

stop applying OrderDiscarded andThen {
  case _ 
    reportActor ! ShoppingCardDiscarded
    saveStateSnapshot()
}

On recovery state data is initialized according to the latest available snapshot, then the remaining domain events are replayed, triggering the applyEvent method.

Storage plugins

Storage backends for journals and snapshot stores are pluggable in the Akka persistence extension.

A directory of persistence journal and snapshot store plugins is available at the Akka Community Projects page, see Community plugins

Plugins can be selected either by "default" for all persistent actors and views, or "individually", when a persistent actor or view defines its own set of plugins.

When a persistent actor or view does NOT override the journalPluginId and snapshotPluginId methods, the persistence extension will use the "default" journal and snapshot-store plugins configured in reference.conf:

akka.persistence.journal.plugin = ""
akka.persistence.snapshot-store.plugin = ""

However, these entries are provided as empty "", and require explicit user configuration via override in the user application.conf. For an example of a journal plugin which writes messages to LevelDB see Local LevelDB journal. For an example of a snapshot store plugin which writes snapshots as individual files to the local filesystem see Local snapshot store.

Applications can provide their own plugins by implementing a plugin API and activating them by configuration. Plugin development requires the following imports:

import akka.persistence._
import akka.persistence.journal._
import akka.persistence.snapshot._

Eager initialization of persistence plugin

By default, persistence plugins are started on-demand, as they are used. In some case, however, it might be beneficial to start a certain plugin eagerly. In order to do that, you should first add the akka.persistence.Persistence under the akka.extensions key. Then, specify the IDs of plugins you wish to start automatically under akka.persistence.journal.auto-start-journals and akka.persistence.snapshot-store.auto-start-snapshot-stores.

Journal plugin API

A journal plugin extends AsyncWriteJournal.

AsyncWriteJournal is an actor and the methods to be implemented are:

/**
 * Plugin API: asynchronously writes a batch (`Seq`) of persistent messages to the
 * journal.
 *
 * The batch is only for performance reasons, i.e. all messages don't have to be written
 * atomically. Higher throughput can typically be achieved by using batch inserts of many
 * records compared to inserting records one-by-one, but this aspect depends on the
 * underlying data store and a journal implementation can implement it as efficient as
 * possible. Journals should aim to persist events in-order for a given `persistenceId`
 * as otherwise in case of a failure, the persistent state may be end up being inconsistent.
 *
 * Each `AtomicWrite` message contains the single `PersistentRepr` that corresponds to
 * the event that was passed to the `persist` method of the `PersistentActor`, or it
 * contains several `PersistentRepr` that corresponds to the events that were passed
 * to the `persistAll` method of the `PersistentActor`. All `PersistentRepr` of the
 * `AtomicWrite` must be written to the data store atomically, i.e. all or none must
 * be stored. If the journal (data store) cannot support atomic writes of multiple
 * events it should reject such writes with a `Try` `Failure` with an
 * `UnsupportedOperationException` describing the issue. This limitation should
 * also be documented by the journal plugin.
 *
 * If there are failures when storing any of the messages in the batch the returned
 * `Future` must be completed with failure. The `Future` must only be completed with
 * success when all messages in the batch have been confirmed to be stored successfully,
 * i.e. they will be readable, and visible, in a subsequent replay. If there is
 * uncertainty about if the messages were stored or not the `Future` must be completed
 * with failure.
 *
 * Data store connection problems must be signaled by completing the `Future` with
 * failure.
 *
 * The journal can also signal that it rejects individual messages (`AtomicWrite`) by
 * the returned `immutable.Seq[Try[Unit]]`. It is possible but not mandatory to reduce
 * number of allocations by returning `Future.successful(Nil)` for the happy path,
 * i.e. when no messages are rejected. Otherwise the returned `Seq` must have as many elements
 * as the input `messages` `Seq`. Each `Try` element signals if the corresponding
 * `AtomicWrite` is rejected or not, with an exception describing the problem. Rejecting
 * a message means it was not stored, i.e. it must not be included in a later replay.
 * Rejecting a message is typically done before attempting to store it, e.g. because of
 * serialization error.
 *
 * Data store connection problems must not be signaled as rejections.
 *
 * It is possible but not mandatory to reduce number of allocations by returning
 * `Future.successful(Nil)` for the happy path, i.e. when no messages are rejected.
 *
 * Calls to this method are serialized by the enclosing journal actor. If you spawn
 * work in asynchronous tasks it is alright that they complete the futures in any order,
 * but the actual writes for a specific persistenceId should be serialized to avoid
 * issues such as events of a later write are visible to consumers (query side, or replay)
 * before the events of an earlier write are visible.
 * A PersistentActor will not send a new WriteMessages request before the previous one
 * has been completed.
 *
 * Please note that the `sender` field of the contained PersistentRepr objects has been
 * nulled out (i.e. set to `ActorRef.noSender`) in order to not use space in the journal
 * for a sender reference that will likely be obsolete during replay.
 *
 * Please also note that requests for the highest sequence number may be made concurrently
 * to this call executing for the same `persistenceId`, in particular it is possible that
 * a restarting actor tries to recover before its outstanding writes have completed. In
 * the latter case it is highly desirable to defer reading the highest sequence number
 * until all outstanding writes have completed, otherwise the PersistentActor may reuse
 * sequence numbers.
 *
 * This call is protected with a circuit-breaker.
 */
def asyncWriteMessages(messages: immutable.Seq[AtomicWrite]): Future[immutable.Seq[Try[Unit]]]

/**
 * Plugin API: asynchronously deletes all persistent messages up to `toSequenceNr`
 * (inclusive).
 *
 * This call is protected with a circuit-breaker.
 * Message deletion doesn't affect the highest sequence number of messages, journal must maintain the highest sequence number and never decrease it.
 */
def asyncDeleteMessagesTo(persistenceId: String, toSequenceNr: Long): Future[Unit]

/**
 * Plugin API
 *
 * Allows plugin implementers to use `f pipeTo self` and
 * handle additional messages for implementing advanced features
 *
 */
def receivePluginInternal: Actor.Receive = Actor.emptyBehavior

If the storage backend API only supports synchronous, blocking writes, the methods should be implemented as:

def asyncWriteMessages(messages: immutable.Seq[AtomicWrite]): Future[immutable.Seq[Try[Unit]]] =
  Future.fromTry(Try {
    // blocking call here
    ???
  })

A journal plugin must also implement the methods defined in AsyncRecovery for replays and sequence number recovery:

/**
 * Plugin API: asynchronously replays persistent messages. Implementations replay
 * a message by calling `replayCallback`. The returned future must be completed
 * when all messages (matching the sequence number bounds) have been replayed.
 * The future must be completed with a failure if any of the persistent messages
 * could not be replayed.
 *
 * The `replayCallback` must also be called with messages that have been marked
 * as deleted. In this case a replayed message's `deleted` method must return
 * `true`.
 *
 * The `toSequenceNr` is the lowest of what was returned by [[#asyncReadHighestSequenceNr]]
 * and what the user specified as recovery [[akka.persistence.Recovery]] parameter.
 * This does imply that this call is always preceded by reading the highest sequence
 * number for the given `persistenceId`.
 *
 * This call is NOT protected with a circuit-breaker because it may take long time
 * to replay all events. The plugin implementation itself must protect against
 * an unresponsive backend store and make sure that the returned Future is
 * completed with success or failure within reasonable time. It is not allowed
 * to ignore completing the future.
 *
 * @param persistenceId persistent actor id.
 * @param fromSequenceNr sequence number where replay should start (inclusive).
 * @param toSequenceNr sequence number where replay should end (inclusive).
 * @param max maximum number of messages to be replayed.
 * @param recoveryCallback called to replay a single message. Can be called from any
 *                       thread.
 *
 * @see [[AsyncWriteJournal]]
 */
def asyncReplayMessages(persistenceId: String, fromSequenceNr: Long, toSequenceNr: Long,
                        max: Long)(recoveryCallback: PersistentRepr  Unit): Future[Unit]

/**
 * Plugin API: asynchronously reads the highest stored sequence number for the
 * given `persistenceId`. The persistent actor will use the highest sequence
 * number after recovery as the starting point when persisting new events.
 * This sequence number is also used as `toSequenceNr` in subsequent call
 * to [[#asyncReplayMessages]] unless the user has specified a lower `toSequenceNr`.
 * Journal must maintain the highest sequence number and never decrease it.
 *
 * This call is protected with a circuit-breaker.
 *
 * Please also note that requests for the highest sequence number may be made concurrently
 * to writes executing for the same `persistenceId`, in particular it is possible that
 * a restarting actor tries to recover before its outstanding writes have completed.
 *
 * @param persistenceId persistent actor id.
 * @param fromSequenceNr hint where to start searching for the highest sequence
 *                       number. When a persistent actor is recovering this
 *                       `fromSequenceNr` will be the sequence number of the used
 *                       snapshot or `0L` if no snapshot is used.
 */
def asyncReadHighestSequenceNr(persistenceId: String, fromSequenceNr: Long): Future[Long]

A journal plugin can be activated with the following minimal configuration:

# Path to the journal plugin to be used
akka.persistence.journal.plugin = "my-journal"

# My custom journal plugin
my-journal {
  # Class name of the plugin.
  class = "docs.persistence.MyJournal"
  # Dispatcher for the plugin actor.
  plugin-dispatcher = "akka.actor.default-dispatcher"
}

The specified plugin class must have a no-arg constructor. The plugin-dispatcher is the dispatcher used for the plugin actor. If not specified, it defaults to akka.persistence.dispatchers.default-plugin-dispatcher.

The journal plugin instance is an actor so the methods corresponding to requests from persistent actors are executed sequentially. It may delegate to asynchronous libraries, spawn futures, or delegate to other actors to achive parallelism.

The journal plugin class must have a constructor without parameters or a constructor with one com.typesafe.config.Config parameter. The plugin section of the actor system's config will be passed in the config constructor parameter.

Don't run journal tasks/futures on the system default dispatcher, since that might starve other tasks.

Snapshot store plugin API

A snapshot store plugin must extend the SnapshotStore actor and implement the following methods:

/**
 * Plugin API: asynchronously loads a snapshot.
 *
 * If the future `Option` is `None` then all events will be replayed,
 * i.e. there was no snapshot. If snapshot could not be loaded the `Future`
 * should be completed with failure. That is important because events may
 * have been deleted and just replaying the events might not result in a valid
 * state.
 *
 * This call is protected with a circuit-breaker.
 *
 * @param persistenceId id of the persistent actor.
 * @param criteria selection criteria for loading.
 */
def loadAsync(persistenceId: String, criteria: SnapshotSelectionCriteria): Future[Option[SelectedSnapshot]]

/**
 * Plugin API: asynchronously saves a snapshot.
 *
 * This call is protected with a circuit-breaker.
 *
 * @param metadata snapshot metadata.
 * @param snapshot snapshot.
 */
def saveAsync(metadata: SnapshotMetadata, snapshot: Any): Future[Unit]

/**
 * Plugin API: deletes the snapshot identified by `metadata`.
 *
 * This call is protected with a circuit-breaker.
 *
 * @param metadata snapshot metadata.
 */
def deleteAsync(metadata: SnapshotMetadata): Future[Unit]

/**
 * Plugin API: deletes all snapshots matching `criteria`.
 *
 * This call is protected with a circuit-breaker.
 *
 * @param persistenceId id of the persistent actor.
 * @param criteria selection criteria for deleting.
 */
def deleteAsync(persistenceId: String, criteria: SnapshotSelectionCriteria): Future[Unit]

/**
 * Plugin API
 * Allows plugin implementers to use `f pipeTo self` and
 * handle additional messages for implementing advanced features
 */
def receivePluginInternal: Actor.Receive = Actor.emptyBehavior

A snapshot store plugin can be activated with the following minimal configuration:

# Path to the snapshot store plugin to be used
akka.persistence.snapshot-store.plugin = "my-snapshot-store"

# My custom snapshot store plugin
my-snapshot-store {
  # Class name of the plugin.
  class = "docs.persistence.MySnapshotStore"
  # Dispatcher for the plugin actor.
  plugin-dispatcher = "akka.persistence.dispatchers.default-plugin-dispatcher"
}

The specified plugin class must have a no-arg constructor. The plugin-dispatcher is the dispatcher used for the plugin actor. If not specified, it defaults to akka.persistence.dispatchers.default-plugin-dispatcher.

The snapshot store instance is an actor so the methods corresponding to requests from persistent actors are executed sequentially. It may delegate to asynchronous libraries, spawn futures, or delegate to other actors to achive parallelism.

The snapshot store plugin class must have a constructor without parameters or a constructor with one com.typesafe.config.Config parameter. The plugin section of the actor system's config will be passed in the config constructor parameter.

Don't run snapshot store tasks/futures on the system default dispatcher, since that might starve other tasks.

Plugin TCK

In order to help developers build correct and high quality storage plugins, we provide a Technology Compatibility Kit (TCK for short).

The TCK is usable from Java as well as Scala projects. For Scala you need to include the akka-persistence-tck dependency:

"com.typesafe.akka" %% "akka-persistence-tck" % "2.4-SNAPSHOT" % "test"

To include the Journal TCK tests in your test suite simply extend the provided JournalSpec:

class MyJournalSpec extends JournalSpec(
  config = ConfigFactory.parseString(
    """akka.persistence.journal.plugin = "my.journal.plugin"""")) {

  override def supportsRejectingNonSerializableObjects: CapabilityFlag =
    false // or CapabilityFlag.off
}

Please note that some of the tests are optional, and by overriding the supports... methods you give the TCK the needed information about which tests to run. You can implement these methods using boolean falues or the provided CapabilityFlag.on / CapabilityFlag.off values.

We also provide a simple benchmarking class JournalPerfSpec which includes all the tests that JournalSpec has, and also performs some longer operations on the Journal while printing its performance stats. While it is NOT aimed to provide a proper benchmarking environment it can be used to get a rough feel about your journal's performance in the most typical scenarios.

In order to include the SnapshotStore TCK tests in your test suite simply extend the SnapshotStoreSpec:

class MySnapshotStoreSpec extends SnapshotStoreSpec(
  config = ConfigFactory.parseString(
    """
    akka.persistence.snapshot-store.plugin = "my.snapshot-store.plugin"
    """))

In case your plugin requires some setting up (starting a mock database, removing temporary files etc.) you can override the beforeAll and afterAll methods to hook into the tests lifecycle:

class MyJournalSpec extends JournalSpec(
  config = ConfigFactory.parseString(
    """
    akka.persistence.journal.plugin = "my.journal.plugin"
    """)) {

  override def supportsRejectingNonSerializableObjects: CapabilityFlag =
    true // or CapabilityFlag.on

  val storageLocations = List(
    new File(system.settings.config.getString("akka.persistence.journal.leveldb.dir")),
    new File(config.getString("akka.persistence.snapshot-store.local.dir")))

  override def beforeAll() {
    super.beforeAll()
    storageLocations foreach FileUtils.deleteRecursively
  }

  override def afterAll() {
    storageLocations foreach FileUtils.deleteRecursively
    super.afterAll()
  }

}

We highly recommend including these specifications in your test suite, as they cover a broad range of cases you might have otherwise forgotten to test for when writing a plugin from scratch.

Pre-packaged plugins

Local LevelDB journal

The LevelDB journal plugin config entry is akka.persistence.journal.leveldb. It writes messages to a local LevelDB instance. Enable this plugin by defining config property:

# Path to the journal plugin to be used
akka.persistence.journal.plugin = "akka.persistence.journal.leveldb"

LevelDB based plugins will also require the following additional dependency declaration:

"org.iq80.leveldb"            % "leveldb"          % "0.7"
"org.fusesource.leveldbjni"   % "leveldbjni-all"   % "1.8"

The default location of LevelDB files is a directory named journal in the current working directory. This location can be changed by configuration where the specified path can be relative or absolute:

akka.persistence.journal.leveldb.dir = "target/journal"

With this plugin, each actor system runs its own private LevelDB instance.

Shared LevelDB journal

A LevelDB instance can also be shared by multiple actor systems (on the same or on different nodes). This, for example, allows persistent actors to failover to a backup node and continue using the shared journal instance from the backup node.

Warning

A shared LevelDB instance is a single point of failure and should therefore only be used for testing purposes. Highly-available, replicated journals are available as Community plugins.

Note

This plugin has been supplanted by Persistence Plugin Proxy.

A shared LevelDB instance is started by instantiating the SharedLeveldbStore actor.

import akka.persistence.journal.leveldb.SharedLeveldbStore

val store = system.actorOf(Props[SharedLeveldbStore], "store")

By default, the shared instance writes journaled messages to a local directory named journal in the current working directory. The storage location can be changed by configuration:

akka.persistence.journal.leveldb-shared.store.dir = "target/shared"

Actor systems that use a shared LevelDB store must activate the akka.persistence.journal.leveldb-shared plugin.

akka.persistence.journal.plugin = "akka.persistence.journal.leveldb-shared"

This plugin must be initialized by injecting the (remote) SharedLeveldbStore actor reference. Injection is done by calling the SharedLeveldbJournal.setStore method with the actor reference as argument.

trait SharedStoreUsage extends Actor {
  override def preStart(): Unit = {
    context.actorSelection("akka.tcp://example@127.0.0.1:2552/user/store") ! Identify(1)
  }

  def receive = {
    case ActorIdentity(1, Some(store)) =>
      SharedLeveldbJournal.setStore(store, context.system)
  }
}

Internal journal commands (sent by persistent actors) are buffered until injection completes. Injection is idempotent i.e. only the first injection is used.

Local snapshot store

The local snapshot store plugin config entry is akka.persistence.snapshot-store.local. It writes snapshot files to the local filesystem. Enable this plugin by defining config property:

# Path to the snapshot store plugin to be used
akka.persistence.snapshot-store.plugin = "akka.persistence.snapshot-store.local"

The default storage location is a directory named snapshots in the current working directory. This can be changed by configuration where the specified path can be relative or absolute:

akka.persistence.snapshot-store.local.dir = "target/snapshots"

Note that it is not mandatory to specify a snapshot store plugin. If you don't use snapshots you don't have to configure it.

Persistence Plugin Proxy

A persistence plugin proxy allows sharing of journals and snapshot stores across multiple actor systems (on the same or on different nodes). This, for example, allows persistent actors to failover to a backup node and continue using the shared journal instance from the backup node. The proxy works by forwarding all the journal/snapshot store messages to a single, shared, persistence plugin instance, and therefor supports any use case supported by the proxied plugin.

Warning

A shared journal/snapshot store is a single point of failure and should therefore only be used for testing purposes. Highly-available, replicated persistence plugins are available as Community plugins.

The journal and snapshot store proxies are controlled via the akka.persistence.journal.proxy and akka.persistence.snapshot-store.proxy configuration entries, respectively. Set the target-journal-plugin or target-snapshot-store-plugin keys to the underlying plugin you wish to use (for example: akka.persistence.journal.leveldb). The start-target-journal and start-target-snapshot-store keys should be set to on in exactly one actor system - this is the system that will instantiate the shared persistence plugin. Next, the proxy needs to be told how to find the shared plugin. This can be done by setting the target-journal-address and target-snapshot-store-address configuration keys, or programmatically by calling the PersistencePluginProxy.setTargetLocation method.

Note

Akka starts extensions lazily when they are required, and this includes the proxy. This means that in order for the proxy to work, the persistence plugin on the target node must be instantiated. This can be done by instantiating the PersistencePluginProxyExtension extension, or by calling the PersistencePluginProxy.start method.

Note

The proxied persistence plugin can (and should) be configured using its original configuration keys.

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

akka.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, a default serializer is used.

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

Testing

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

akka.persistence.journal.leveldb.native = off

or

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

in your Akka configuration. The LevelDB Java port is for testing purposes only.

Warning

It is not possible to test persistence provided classes (i.e. PersistentActor and AtLeastOnceDelivery) using TestActorRef due to its synchronous nature. These traits need to be able to perform asynchronous tasks in the background in order to handle internal persistence related events.

When testing Persistence based projects always rely on asynchronous messaging using the TestKit.

Configuration

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

Multiple persistence plugin configurations

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

# 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 or view overrides only the persistenceId method:

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

When the persistent actor or view overrides the journalPluginId and snapshotPluginId methods, the actor or view will be serviced by these specific persistence plugins instead of the defaults:

trait 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"
}

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.:

# 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
}

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