Persistence 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

Two popular plugins are:

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

When a persistent actor 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.

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 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.

For example, if you want eager initialization for the leveldb journal plugin and the local snapshot store plugin, your configuration should look like this:

akka {

  extensions = [akka.persistence.Persistence]

  persistence {

    journal {
      plugin = "akka.persistence.journal.leveldb"
      auto-start-journals = ["akka.persistence.journal.leveldb"]
    }

    snapshot-store {
      plugin = "akka.persistence.snapshot-store.local"
      auto-start-snapshot-stores = ["akka.persistence.snapshot-store.local"]
    }

  }

}

Pre-packaged plugins

The Akka Persistence module comes with few built-in persistence plugins, but none of these are suitable for production usage in an Akka Cluster.

Local LevelDB journal

This plugin writes events to a local LevelDB instance.

Warning

The LevelDB plugin cannot be used in an Akka Cluster since the storage is in a local file system.

The LevelDB journal plugin config entry is akka.persistence.journal.leveldb. 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:

sbt
libraryDependencies += "org.fusesource.leveldbjni" % "leveldbjni-all" % "1.8"
Maven
<dependency>
  <groupId>org.fusesource.leveldbjni</groupId>
  <artifactId>leveldbjni-all</artifactId>
  <version>1.8</version>
</dependency>
Gradle
dependencies {
  compile group: 'org.fusesource.leveldbjni', name: 'leveldbjni-all', version: '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.

One peculiarity of LevelDB is that the deletion operation does not remove messages from the journal, but adds a “tombstone” for each deleted message instead. In the case of heavy journal usage, especially one including frequent deletes, this may be an issue as users may find themselves dealing with continuously increasing journal sizes. To this end, LevelDB offers a special journal compaction function that is exposed via the following configuration:

# Number of deleted messages per persistence id that will trigger journal compaction
akka.persistence.journal.leveldb.compaction-intervals {
  persistence-id-1 = 100
  persistence-id-2 = 200
  # ...
  persistence-id-N = 1000
  # use wildcards to match unspecified persistence ids, if any
  "*" = 250
}

Shared LevelDB journal

For testing purposes 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.

Note

This plugin has been supplanted by Persistence Plugin Proxy.

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

Scala
import akka.persistence.journal.leveldb.SharedLeveldbStore

val store = system.actorOf(Props[SharedLeveldbStore], "store")
Java
final ActorRef store = system.actorOf(Props.create(SharedLeveldbStore.class), "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.

Scala
trait SharedStoreUsage extends Actor {
  override def preStart(): Unit = {
    context.actorSelection("akka://[email protected]:2552/user/store") ! Identify(1)
  }

  def receive = {
    case ActorIdentity(1, Some(store)) =>
      SharedLeveldbJournal.setStore(store, context.system)
  }
}
Java
class SharedStorageUsage extends AbstractActor {
  @Override
  public void preStart() throws Exception {
    String path = "akka://[email protected]:2552/user/store";
    ActorSelection selection = getContext().actorSelection(path);
    selection.tell(new Identify(1), getSelf());
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .match(
            ActorIdentity.class,
            ai -> {
              if (ai.correlationId().equals(1)) {
                Optional<ActorRef> store = ai.getActorRef();
                if (store.isPresent()) {
                  SharedLeveldbJournal.setStore(store.get(), getContext().getSystem());
                } else {
                  throw new RuntimeException("Couldn't identify store");
                }
              }
            })
        .build();
  }
}

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

This plugin writes snapshot files to the local filesystem.

Warning

The local snapshot store plugin cannot be used in an Akka Cluster since the storage is in a local file system.

The local snapshot store plugin config entry is akka.persistence.snapshot-store.local. 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

For testing purposes 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 therefore 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.

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.

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