Persistence Plugins

Storage backends for journals, snapshot stores, durable state stores and persistence queries are pluggable in the Akka persistence extension. The following plugins are maintained by the Akka team.

R2DBC plugin

The Reactive database drivers (R2DBC) support relational databases like PostgreSQL, H2 (As a minimal in-process memory or file based database) and Yugabyte.

The Akka Persistence R2DBC plugin supports the latest feature additions of Akka Persistence and is generally recommended over the JDBC-based plugin.

Cassandra plugin

Akka supports Cassandra's data model through Akka Persistence Cassandra.

Some later Akka Persistence feature additions (including @ref:Durable State) are not supported by the Cassandra plugin (see below).

AWS DynamoDB plugin

AWS DynamoDB can be used as backend for Akka Persistence with the Akka Persistence DynamoDB plugin.

@ref:Durable State is not supported by the DynamoDB plugin.

Recovery from only last event is not supported by the DynamoDB plugin.

JDBC plugin

Relational databases with JDBC-drivers are supported through Akka Persistence JDBC. For new projects, the @ref:R2DBC plugin is recommended.

Some later Akka Persistence feature additions are not supported by the Akka Persistence JDBC plugin (see below).

Feature limitations

Example of concrete features not supported by the Cassandra and JDBC plugins:

• eventsBySlices query
• Projections over gRPC
• Replicated Event Sourcing over gRPC
• Dynamic scaling of number of Projection instances
• Low latency Projections
• Projections starting from snapshots
• Scalability of many Projections
• Durable State entities (partly supported by JDBC plugin)
• Recovery from only last event

Enabling a plugin

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, snapshot-store and durable-state plugins configured in reference.conf:

akka.persistence.journal.plugin = ""
akka.persistence.snapshot-store.plugin = ""
akka.persistence.state.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 @ref:Local LevelDB journal.
• For an example of a snapshot store plugin which writes snapshots as individual files to the local filesystem see @ref:Local snapshot store.
• The state store is relatively new, one available implementation is the akka-persistence-jdbc-plugin.

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 is deprecated and it is not advised to build new applications with it. As a replacement we recommend using Akka Persistence JDBC.

The LevelDB journal plugin config entry is akka.persistence.journal.leveldb. Enable this plugin by defining config property:

@@snip PersistencePluginDocSpec.scala { #leveldb-plugin-config }

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

@@dependency[sbt,Maven,Gradle] { group="org.fusesource.leveldbjni" artifact="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:

@@snip PersistencePluginDocSpec.scala { #journal-config }

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:

@@snip PersistencePluginDocSpec.scala { #compaction-intervals-config }

Shared 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 in a multi node environment the "inmem" journal together with the @refproxy plugin can be used, but the actual journal used in production of applications is also a good choice.

@@@ note This plugin has been supplanted by @ref:Persistence Plugin Proxy. @@@

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

Scala : @@snip PersistencePluginDocSpec.scala { #shared-store-creation }

Java : @@snip LambdaPersistencePluginDocTest.java { #shared-store-creation }

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:

@@snip PersistencePluginDocSpec.scala { #shared-store-config }

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

@@snip PersistencePluginDocSpec.scala { #shared-journal-config }

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 : @@snip PersistencePluginDocSpec.scala { #shared-store-usage }

Java : @@snip LambdaPersistencePluginDocTest.java { #shared-store-usage }

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:

@@snip PersistencePluginDocSpec.scala { #leveldb-snapshot-plugin-config }

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:

@@snip PersistencePluginDocSpec.scala { #snapshot-config }

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 a journal and snapshot store on a single node 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 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.inmem). 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 @ref:extension, or by calling the PersistencePluginProxy.start method. @@@

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