GCP Datastore Akka Persistence Plugin

akka-persistence-gcp-datastore is a journal and snapshot store plugin for akka-persistence using Google Cloud Datastore. It uses the official Google Java Dependency to talk with the datastore.

Scala 2.12 & 2.13, Java 8 & Java 11, akka 2.6.X are supported.

The plugin supports the following functionality:

• serialization of events and snapshots with play-json
• peristence-query api
• custom serialization

Related Blogposts

• Using Google Datastore with akka-persistence
• Was ist Event Sourcing? (de)
• Wie lässt sich das Lesen beim Event Sourcing durch CQRS optimieren? (de)

Usage & Setup

Versions: The table below lists the versions and their main dependencies

Version to use	Scala 2.12	Scala 2.13	Scala 3 / Dotty	Akka	play-json	google-cloud-datastore
1.0.2	✓	✓	?	2.6.x	2.8.x	1.102.x

Dependency

You just need to add the following dependency to you sbt dependencies

libraryDependencies += "de.innfactory" %% "akka-persistence-gcp-datastore" % "X.Y.Z"

Configuration

Take a look at reference.conf under src/main/resources We forked the cqrs cassandra lightbend example with necessary changes for gcp-datastore (@ Demo Test Project available based on the CQRS Example from Lightbend https://github.com/innFactory/akka-persistence-gcp-datastore-example)

Add the following to your application.conf for a basic configuration:

akka {
  # use google cloud datastore as journal and snapshot store
  persistence {

    journal {
      plugin = "gcp-datastore-journal"
      auto-start-journals = ["gcp-datastore-journal"]
    }

    snapshot-store {
      plugin = "gcp-datastore-snapshot"
      auto-start-snapshot-stores = ["gcp-datastore-snapshot"]
    }
  }
}

Datastore Configuration

1. Google Cloud Project with Datastore or FireStore in Datastore mode enabled

2. Create a index.yml file with content bolow in the project that will use this plugin:

   indexes:
     - kind: journal
       properties:
         - name: persistenceId
         - name: sequenceNr
           direction: desc
   
     - kind: snapshot
       properties:
         - name: persistenceId
         - name: timestamp
           direction: desc
   
     - kind: snapshot
       properties:
         - name: persistenceId
         - name: timestamp
   
     - kind: journal
       properties:
       - name: tagsKey
       - name: timestamp
   
     - kind: journal
       properties:
       - name: persistenceId
       - name: sequenceNr
   

   index.yml

3. Open terminal and execute

   gcloud app deploy index.yaml
   

   This is telling the GCP Datastore to build indexes for the plugin based on the yaml file

4. Create a service account for read and write to datastore. Download the json and add it to the project

   src/main/resources/datastore.json
   

Persistence query API

The plugin supports the Persistence query APi, mostly used in CQRS applications to transform/migrate the events from the write side to the read side.

The ReadJournal is retrieved via the akka.persistence.datastore.journal.read.DatastoreScaladslReadJournal and akka.persistence.datastore.journal.read.DatastoreJavadslReadJournal. There is also a DatastoreReadJournalProvider.

import akka.persistence.datastore.journal.read.DatastoreScaladslReadJournal
import akka.persistence.query.{ EventEnvelope, PersistenceQuery }

val system = ??? //ActorSystem akka-classic or akka-typed then system.toClassic is needed. see the example.
val readJournal =
    PersistenceQuery(system).readJournalFor[DatastoreScaladslReadJournal]("gcp-datastore-query")

Supported Queries

All queries are live streams and they are not completed when they reaches the end of the currently stored events, but continue to push new events when new events are persisted.

eventsByTag

eventsByTags is used for retrieving events that were marked with a given tag.

eventsByPersistenceId

eventsByPersistenceId is used for retrieving events for a specific PersistentActor identified by its persistenceId

Testing

To test this plugin

(Source: https://cloud.google.com/datastore/docs/tools/datastore-emulator)

1. gcloud components install cloud-datastore-emulator

2. gcloud beta emulators datastore start --no-store-on-disk --consistency=1.0

3. Set Env Variable DATASTORE_TESTHOST=http://<host>:<port> of datastore emulator

4. Execute sbt run

5. Before executing test reset datastore data: curl -X POST http://<host>:<port>/reset

There is a shell script under .circle ci which runs all of these tests. cqrs tests are outsourced in the example project.

Contribution policy

Contributions via GitHub pull requests are gladly accepted from their original author. Along with any pull requests, please state that the contribution is your original work and that you license the work to the project under the project's open source license. Whether or not you state this explicitly, by submitting any copyrighted material via pull request, email, or other means you agree to license the material under the project's open source license and warrant that you have the legal authority to do so.

Credits

innFactory GmbH is a lightbend partner from germany. We are experts for Apps, BigData & Cloud Computing. If you need help with your next project, feel free to ask for our support.