Typesafe Activator

Akka Persistence Samples with Scala

Akka Persistence Samples with Scala

Akka Team
September 30, 2015
akka persistence scala sample

Akka Persistence Samples with Scala

How to get "Akka Persistence Samples with Scala" on your computer

There are several ways to get this template.

Option 1: Choose akka-sample-persistence-scala in the Typesafe Activator UI.

Already have Typesafe Activator (get it here)? Launch the UI then search for akka-sample-persistence-scala in the list of templates.

Option 2: Download the akka-sample-persistence-scala project as a zip archive

If you haven't installed Activator, you can get the code by downloading the template bundle for akka-sample-persistence-scala.

  1. Download the Template Bundle for "Akka Persistence Samples with Scala"
  2. Extract the downloaded zip file to your system
  3. The bundle includes a small bootstrap script that can start Activator. To start Typesafe Activator's UI:

    In your File Explorer, navigate into the directory that the template was extracted to, right-click on the file named "activator.bat", then select "Open", and if prompted with a warning, click to continue:

    Or from a command line:

     C:\Users\typesafe\akka-sample-persistence-scala> activator ui 
    This will start Typesafe Activator and open this template in your browser.

Option 3: Create a akka-sample-persistence-scala project from the command line

If you have Typesafe Activator, use its command line mode to create a new project from this template. Type activator new PROJECTNAME akka-sample-persistence-scala on the command line.

Option 4: View the template source

The creator of this template maintains it at https://github.com/akka/akka.

Option 5: Preview the tutorial below

We've included the text of this template's tutorial below, but it may work better if you view it inside Activator on your computer. Activator tutorials are often designed to be interactive.

Preview the tutorial

This tutorial contains examples that illustrate a subset of Akka Persistence features.

  • persistent actor
  • persistent actor snapshots
  • persistent actor recovery
  • persistent actor views

Custom storage locations for the journal and snapshots can be defined in application.conf.

Persistent actor

PersistentActorExample.scala is described in detail in the Event sourcing section of the user documentation. With every application run, the ExamplePersistentActor is recovered from events stored in previous application runs, processes new commands, stores new events and snapshots and prints the current persistent actor state to stdout.

To run this example, go to the Run tab, and run the application main class sample.persistence.PersistentActorExample several times.

Persistent actor snapshots

SnapshotExample.scala demonstrates how persistent actors can take snapshots of application state and recover from previously stored snapshots. Snapshots are offered to persistent actors at the beginning of recovery, before any messages (younger than the snapshot) are replayed.

To run this example, go to the Run tab, and run the application main class sample.persistence.SnapshotExample several times. With every run, the state offered by the most recent snapshot is printed to stdout, followed by the updated state after sending new persistent messages to the persistent actor.

Persistent actor recovery

PersistentActorFailureExample.scala shows how a persistent actor can throw an exception, restart and restore the state by replaying the events.

To run this example, go to the Run tab, and run the application main class sample.persistence.PersistentActorFailureExample several times.

Processor failure handling

ProcessorFailureExample.scala shows how a processor can delete persistent messages from the journal if they threw an exception. Throwing an exception restarts the processor and replays messages. In order to prevent that the message that caused the exception is replayed, it is marked as deleted in the journal (during invocation of preRestart). This is a common pattern in command-sourcing to compensate write-ahead logging of messages.

To run this example, go to the Run tab, and run the application main class sample.persistence.ProcessorFailureExample several times.

Event sourcing on the other hand, does not persist commands directly but rather events that have been derived from received commands (not shown here). These events are known to be successfully applicable to current processor state i.e. there's no need for deleting them from the journal. Event sourced processors usually have a lower throughput than command sourced processors, as the maximum size of a write batch is limited by the number of persisted events per received command.

Processor views

Persistent actor views

ViewExample.scala demonstrates how a view (ExampleView) is updated with the persistent message stream of a persistent actor (ExamplePersistentActor). Messages sent to the persistent actor are scheduled periodically. Views also support snapshotting to reduce recovery time.

To run this example, go to the Run tab, and run the application main class sample.persistence.ViewExample.

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