Changes from Durable State

A typical source for Projections is the change stored with DurableStateBehaviorDurableStateBehavior in Akka Persistence. Durable state changes can be tagged and then consumed with the changes query.

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

Alternative

When using the R2DBC plugin an alternative to using a Projection is to store the query representation directly from the write side.

Dependencies

To use the Durable State module of Akka Projections, add the following dependency in your project:

sbt

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

Maven

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

Gradle

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

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

Project Info: Akka Projections Durable State
Artifact	com.lightbend.akka akka-projection-durable-state 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.durable-state
License	BUSL-1.1
Readiness level	Supported, Lightbend Subscription provides support Since 1.2.2, 2021-08-19
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 the akka-projection-durable-state direct dependencies.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 changesByTag

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

Scala

sourceimport akka.persistence.jdbc.state.scaladsl.JdbcDurableStateStore
import akka.persistence.query.DurableStateChange
import akka.persistence.query.Offset
import akka.projection.state.scaladsl.DurableStateSourceProvider
import akka.projection.scaladsl.SourceProvider

val sourceProvider: SourceProvider[Offset, DurableStateChange[AccountEntity.Account]] =
  DurableStateSourceProvider
    .changesByTag[AccountEntity.Account](system, JdbcDurableStateStore.Identifier, "bank-accounts-1")

Java

sourceimport akka.persistence.jdbc.state.javadsl.JdbcDurableStateStore;
import akka.persistence.query.DurableStateChange;
import akka.persistence.query.Offset;
import akka.projection.state.javadsl.DurableStateSourceProvider;
import akka.projection.javadsl.SourceProvider;

SourceProvider<Offset, DurableStateChange<AccountEntity.Account>> sourceProvider =
    DurableStateSourceProvider.changesByTag(
        system, JdbcDurableStateStore.Identifier(), "bank-accounts-1");

This example is using the DurableStateStore JDBC plugin for Akka Persistence. You will use the same plugin that you configured for the write side. The one that is used by the DurableStateBehavior.

This source is consuming all the changes from the Account DurableStateBehavior that are tagged with "bank-accounts-1". In a production application, you would need to start as many instances as the number of different tags you used. That way you consume the changes from all entities.

The DurableStateChange[AccountEntity.Account]DurableStateChange<AccountEntity.Account> is what the Projection handler will process. It contains the State and additional meta data, such as the offset that will be stored by the Projection. See DurableStateChangeDurableStateChange for full details of what it contains.

SourceProvider for changesBySlices

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

Scala

sourceimport akka.persistence.query.DurableStateChange
import akka.persistence.query.Offset
import akka.projection.state.scaladsl.DurableStateSourceProvider
import akka.projection.scaladsl.SourceProvider

// Slit the slices into 4 ranges
val numberOfSliceRanges: Int = 4
val sliceRanges =
  DurableStateSourceProvider.sliceRanges(system, R2dbcDurableStateStore.Identifier, numberOfSliceRanges)

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

val sourceProvider: SourceProvider[Offset, DurableStateChange[AccountEntity.Account]] =
  DurableStateSourceProvider
    .changesBySlices[AccountEntity.Account](
      system,
      R2dbcDurableStateStore.Identifier,
      entityType,
      minSlice,
      maxSlice)

Java

sourceimport akka.japi.Pair;
import akka.persistence.query.DurableStateChange;
import akka.persistence.query.Offset;
import akka.projection.eventsourced.javadsl.EventSourcedProvider;
import akka.projection.javadsl.SourceProvider;
import akka.projection.state.javadsl.DurableStateSourceProvider;

// Slit the slices into 4 ranges
int numberOfSliceRanges = 4;
List<Pair<Integer, Integer>> sliceRanges =
    EventSourcedProvider.sliceRanges(
        system, R2dbcDurableStateStore.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, DurableStateChange<AccountEntity.Account>> sourceProvider =
    DurableStateSourceProvider.changesBySlices(
        system, R2dbcDurableStateStore.Identifier(), entityType, minSlice, maxSlice);

This example is using the R2DBC plugin for Akka Persistence. You will use the same plugin that you configured for the write side. The one that is used by the DurableStateBehavior.

This source is consuming all the changes from the Account DurableStateBehavior 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 changes from all entities.