Events from Akka Persistence

A typical source for Projections is events stored with EventSourcedBehaviorEventSourcedBehavior in Akka Persistence. Events can be tagged and then consumed with the eventsByTag query.

Akka Projections has integration with eventsByTag, which is described here.

Dependencies

The Akka dependencies are available from Akka’s library repository. To access them there, you need to configure the URL for this repository.

sbt

resolvers += "Akka library repository".at("https://repo.akka.io/maven")

Maven

<project>
  ...
  <repositories>
    <repository>
      <id>akka-repository</id>
      <name>Akka library repository</name>
      <url>https://repo.akka.io/maven</url>
    </repository>
  </repositories>
</project>

Gradle

repositories {
    mavenCentral()
    maven {
        url "https://repo.akka.io/maven"
    }
}

To use the Event Sourced module of Akka Projections add the following dependency in your project:

sbt

libraryDependencies += "com.lightbend.akka" %% "akka-projection-eventsourced" % "1.4.2"

Maven

<properties>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencies>
  <dependency>
    <groupId>com.lightbend.akka</groupId>
    <artifactId>akka-projection-eventsourced_${scala.binary.version}</artifactId>
    <version>1.4.2</version>
  </dependency>
</dependencies>

Gradle

def versions = [
  ScalaBinary: "2.13"
]
dependencies {
  implementation "com.lightbend.akka:akka-projection-eventsourced_${versions.ScalaBinary}:1.4.2"
}

Akka Projections require Akka 2.8.1 or later, see Akka version.

Project Info: Akka Projections Eventsourced
Artifact	com.lightbend.akka akka-projection-eventsourced 1.4.2 Snapshots are available
JDK versions	AdoptOpenJDK 8 AdoptOpenJDK 11
Scala versions	2.13.10, 2.12.17, 3.2.2
JPMS module name	akka.projection.eventsourced
License	BUSL-1.1
Readiness level	Supported, Lightbend Subscription provides support Since 1.0.0, 2020-09-10
Home page	https://akka.io
API documentation	API (Scaladoc)
Forums	Lightbend Discuss akka/akka Gitter channel
Release notes	GitHub releases
Issues	GitHub issues
Sources	https://github.com/akka/akka-projection

Transitive dependencies

The table below shows akka-projection-eventsourced’s direct dependencies and the second tab shows all libraries it depends on transitively.

Direct dependencies

Organization	Artifact	Version
com.lightbend.akka	akka-projection-core_2.13	1.4.2
com.typesafe.akka	akka-persistence-query_2.13	2.8.1
org.scala-lang	scala-library	2.13.10

Dependency tree

com.lightbend.akka    akka-projection-core_2.13    1.4.2
    com.typesafe.akka    akka-actor-typed_2.13    2.8.1    BUSL-1.1
        com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
            com.typesafe    config    1.4.2    Apache-2.0
            org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        com.typesafe.akka    akka-slf4j_2.13    2.8.1    BUSL-1.1
            com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
                com.typesafe    config    1.4.2    Apache-2.0
                org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                    org.scala-lang    scala-library    2.13.10    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.slf4j    slf4j-api    1.7.36
        org.scala-lang    scala-library    2.13.10    Apache-2.0
        org.slf4j    slf4j-api    1.7.36
    com.typesafe.akka    akka-persistence-query_2.13    2.8.1    BUSL-1.1
        com.typesafe.akka    akka-persistence_2.13    2.8.1    BUSL-1.1
            com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
                com.typesafe    config    1.4.2    Apache-2.0
                org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                    org.scala-lang    scala-library    2.13.10    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            com.typesafe.akka    akka-stream_2.13    2.8.1    BUSL-1.1
                com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
                    com.typesafe    config    1.4.2    Apache-2.0
                    org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                        org.scala-lang    scala-library    2.13.10    Apache-2.0
                    org.scala-lang    scala-library    2.13.10    Apache-2.0
                com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
                com.typesafe    ssl-config-core_2.13    0.6.1    Apache-2.0
                    com.typesafe    config    1.4.2    Apache-2.0
                    org.scala-lang    scala-library    2.13.10    Apache-2.0
                org.reactivestreams    reactive-streams    1.0.4    MIT-0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
        com.typesafe.akka    akka-stream_2.13    2.8.1    BUSL-1.1
            com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
                com.typesafe    config    1.4.2    Apache-2.0
                org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                    org.scala-lang    scala-library    2.13.10    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
            com.typesafe    ssl-config-core_2.13    0.6.1    Apache-2.0
                com.typesafe    config    1.4.2    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.reactivestreams    reactive-streams    1.0.4    MIT-0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        org.scala-lang    scala-library    2.13.10    Apache-2.0
    com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
    com.typesafe.akka    akka-stream_2.13    2.8.1    BUSL-1.1
        com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
            com.typesafe    config    1.4.2    Apache-2.0
            org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
        com.typesafe    ssl-config-core_2.13    0.6.1    Apache-2.0
            com.typesafe    config    1.4.2    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        org.reactivestreams    reactive-streams    1.0.4    MIT-0
        org.scala-lang    scala-library    2.13.10    Apache-2.0
    org.scala-lang    scala-library    2.13.10    Apache-2.0
com.typesafe.akka    akka-persistence-query_2.13    2.8.1    BUSL-1.1
    com.typesafe.akka    akka-persistence_2.13    2.8.1    BUSL-1.1
        com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
            com.typesafe    config    1.4.2    Apache-2.0
            org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        com.typesafe.akka    akka-stream_2.13    2.8.1    BUSL-1.1
            com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
                com.typesafe    config    1.4.2    Apache-2.0
                org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                    org.scala-lang    scala-library    2.13.10    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
            com.typesafe    ssl-config-core_2.13    0.6.1    Apache-2.0
                com.typesafe    config    1.4.2    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.reactivestreams    reactive-streams    1.0.4    MIT-0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        org.scala-lang    scala-library    2.13.10    Apache-2.0
    com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
    com.typesafe.akka    akka-stream_2.13    2.8.1    BUSL-1.1
        com.typesafe.akka    akka-actor_2.13    2.8.1    BUSL-1.1
            com.typesafe    config    1.4.2    Apache-2.0
            org.scala-lang.modules    scala-java8-compat_2.13    1.0.0    Apache-2.0
                org.scala-lang    scala-library    2.13.10    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        com.typesafe.akka    akka-protobuf-v3_2.13    2.8.1    BUSL-1.1
        com.typesafe    ssl-config-core_2.13    0.6.1    Apache-2.0
            com.typesafe    config    1.4.2    Apache-2.0
            org.scala-lang    scala-library    2.13.10    Apache-2.0
        org.reactivestreams    reactive-streams    1.0.4    MIT-0
        org.scala-lang    scala-library    2.13.10    Apache-2.0
    org.scala-lang    scala-library    2.13.10    Apache-2.0
org.scala-lang    scala-library    2.13.10    Apache-2.0

SourceProvider for eventsByTag

A SourceProviderSourceProvider defines the source of the event envelopes that the Projection will process. A SourceProvider for the eventsByTag query can be defined with the EventSourcedProviderEventSourcedProvider like this:

Scala

sourceimport akka.projection.eventsourced.EventEnvelope
import akka.persistence.cassandra.query.scaladsl.CassandraReadJournal
import akka.persistence.query.Offset
import akka.projection.eventsourced.scaladsl.EventSourcedProvider
import akka.projection.scaladsl.SourceProvider

val sourceProvider: SourceProvider[Offset, EventEnvelope[ShoppingCart.Event]] =
  EventSourcedProvider
    .eventsByTag[ShoppingCart.Event](system, readJournalPluginId = CassandraReadJournal.Identifier, tag = "carts-1")

Java

sourceimport akka.persistence.cassandra.query.javadsl.CassandraReadJournal;
import akka.persistence.query.Offset;
import akka.projection.eventsourced.EventEnvelope;
import akka.projection.eventsourced.javadsl.EventSourcedProvider;
import akka.projection.javadsl.SourceProvider;

SourceProvider<Offset, EventEnvelope<ShoppingCart.Event>> sourceProvider =
    EventSourcedProvider.eventsByTag(system, CassandraReadJournal.Identifier(), "carts-1");

This example is using the Cassandra plugin for Akka Persistence, but same code can be used for other Akka Persistence plugins by replacing the CassandraReadJournal.Identifier. For example the JDBC plugin can be used. You will use the same plugin as you have configured for the write side that is used by the EventSourcedBehavior.

This source is consuming all events from the ShoppingCart EventSourcedBehavior that are tagged with "cart-1".

The tags are assigned as described in Tagging Events in EventSourcedBehavior.

The EventEnvelope[ShoppingCart.Event]EventEnvelope<ShoppingCart.Event> is what the Projection handler will process. It contains the Event and additional meta data, such as the offset that will be stored by the Projection. See EventEnvelopeEventEnvelope for full details of what the envelope contains.

SourceProvider for eventsBySlices

A SourceProviderSourceProvider defines the source of the event envelopes that the Projection will process. A SourceProvider for the eventsBySlices query can be defined with the EventSourcedProviderEventSourcedProvider like this:

Scala

sourceimport akka.persistence.query.typed.EventEnvelope
import akka.persistence.query.Offset
import akka.projection.eventsourced.scaladsl.EventSourcedProvider
import akka.projection.scaladsl.SourceProvider

// Slit the slices into 4 ranges
val numberOfSliceRanges: Int = 4
val sliceRanges = EventSourcedProvider.sliceRanges(system, R2dbcReadJournal.Identifier, numberOfSliceRanges)

// Example of using the first slice range
val minSlice: Int = sliceRanges.head.min
val maxSlice: Int = sliceRanges.head.max
val entityType: String = "ShoppingCart"

val sourceProvider: SourceProvider[Offset, EventEnvelope[ShoppingCart.Event]] =
  EventSourcedProvider
    .eventsBySlices[ShoppingCart.Event](
      system,
      readJournalPluginId = R2dbcReadJournal.Identifier,
      entityType,
      minSlice,
      maxSlice)

Java

sourceimport akka.japi.Pair;
import akka.persistence.query.Offset;
import akka.persistence.query.typed.EventEnvelope;
import akka.projection.eventsourced.javadsl.EventSourcedProvider;
import akka.projection.javadsl.SourceProvider;

// Slit the slices into 4 ranges
int numberOfSliceRanges = 4;
List<Pair<Integer, Integer>> sliceRanges =
    EventSourcedProvider.sliceRanges(
        system, R2dbcReadJournal.Identifier(), numberOfSliceRanges);

// Example of using the first slice range
int minSlice = sliceRanges.get(0).first();
int maxSlice = sliceRanges.get(0).second();
String entityType = "MyEntity";

SourceProvider<Offset, EventEnvelope<ShoppingCart.Event>> sourceProvider =
    EventSourcedProvider.eventsBySlices(
        system, R2dbcReadJournal.Identifier(), entityType, minSlice, maxSlice);

This example is using the R2DBC plugin for Akka Persistence. You will use the same plugin as you have configured for the write side that is used by the EventSourcedBehavior.

This source is consuming all events from the ShoppingCart EventSourcedBehavior for the given slice range. In a production application, you would need to start as many instances as the number of slice ranges. That way you consume the events from all entities.