ORSet example, MovieWatchList
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Patrik Nordwall
Christopher Batey
parent
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commit
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6 changed files with 244 additions and 17 deletions
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@ -134,13 +134,41 @@ The factory returns a `Behavior` that can be spawned like any other behavior.
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### Conflict free replicated data types
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The following CRDTs are included that can be used to build your own data model:
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Writing code to resolve conflicts can be complicated to get right.
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One well-understood technique to create eventually-consistent systems is to
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model your state as a Conflict Free Replicated Data Type, a CRDT. There are two types of CRDTs;
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operation-based and state-based. For Replicated Event Sourcing the operation-based is a good fit,
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since the events represent the operations. Note that this is distinct from the CRDT's implemented
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in @ref:[Akka Distributed Data](distributed-data.md), which are state-based rather than operation-based.
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The rule for operation-based CRDT's is that the operations must be commutative — in other words, applying the same events
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(which represent the operations) in any order should always produce the same final state. You may assume each event
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is applied only once, with @ref:[causal delivery order](#causal-delivery-order).
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The following CRDTs are included that can you can use as the state or part of the state in the entity:
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* @apidoc[LwwTime]
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* @apidoc[Counter]
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* @apidoc[akka.persistence.typed.crdt.ORSet]
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Akka serializers are included for all these types and can be used to serialize when @ref[embedded in Jackson](../serialization-jackson.md#using-akka-serialization-for-embedded-types).
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Akka serializers are included for all these types and can be used to serialize when
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@ref[embedded in Jackson](../serialization-jackson.md#using-akka-serialization-for-embedded-types).
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An example would be a movies watch list that is represented by the general purpose
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@apidoc[akka.persistence.typed.crdt.ORSet] CRDT. `ORSet` is short for Observed Remove Set. Elements can be added and
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removed any number of times. Concurrent add wins over remove. It is an operation based CRDT where the delta of an
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operation (add/remove) can be represented as an event.
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Such movies watch list example:
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Scala
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: @@snip [movie](/akka-persistence-typed-tests/src/test/scala/docs/akka/persistence/typed/ReplicatedMovieWatchListExampleSpec.scala) { #movie-entity }
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Java
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: @@snip [movie](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/ReplicatedMovieExample.java) { #movie-entity }
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The @ref[Auction example](replicated-eventsourcing-auction.md) is a more comprehensive example that illustrates how application-specific
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rules can be used to implement an entity with CRDT semantics.
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### Last writer wins
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@ -26,7 +26,7 @@ interface ReplicatedBlogExample {
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public final class BlogEntity
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extends ReplicatedEventSourcedBehavior<
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BlogEntity.Command, BlogEntity.Event, BlogEntity.BlogState> {
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BlogEntity.Command, BlogEntity.Event, BlogEntity.BlogState> {
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private final ActorContext<Command> context;
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@ -0,0 +1,105 @@
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/*
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* Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
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*/
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package jdocs.akka.persistence.typed;
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import akka.actor.typed.ActorRef;
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import akka.actor.typed.Behavior;
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import akka.persistence.testkit.query.javadsl.PersistenceTestKitReadJournal;
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import akka.persistence.typed.ReplicaId;
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import akka.persistence.typed.crdt.ORSet;
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import akka.persistence.typed.javadsl.CommandHandler;
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import akka.persistence.typed.javadsl.EventHandler;
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import akka.persistence.typed.javadsl.ReplicatedEventSourcedBehavior;
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import akka.persistence.typed.javadsl.ReplicatedEventSourcing;
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import akka.persistence.typed.javadsl.ReplicationContext;
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import java.util.Collections;
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import java.util.Set;
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interface ReplicatedMovieExample {
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// #movie-entity
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public final class MovieWatchList
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extends ReplicatedEventSourcedBehavior<MovieWatchList.Command, ORSet.DeltaOp, ORSet<String>> {
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interface Command {}
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public static class AddMovie implements Command {
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public final String movieId;
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public AddMovie(String movieId) {
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this.movieId = movieId;
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}
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}
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public static class RemoveMovie implements Command {
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public final String movieId;
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public RemoveMovie(String movieId) {
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this.movieId = movieId;
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}
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}
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public static class GetMovieList implements Command {
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public final ActorRef<MovieList> replyTo;
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public GetMovieList(ActorRef<MovieList> replyTo) {
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this.replyTo = replyTo;
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}
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}
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public static class MovieList {
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public final Set<String> movieIds;
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public MovieList(Set<String> movieIds) {
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this.movieIds = Collections.unmodifiableSet(movieIds);
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}
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}
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public static Behavior<Command> create(
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String entityId, ReplicaId replicaId, Set<ReplicaId> allReplicas) {
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return ReplicatedEventSourcing.withSharedJournal(
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"movies",
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entityId,
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replicaId,
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allReplicas,
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PersistenceTestKitReadJournal.Identifier(),
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MovieWatchList::new);
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}
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private MovieWatchList(ReplicationContext replicationContext) {
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super(replicationContext);
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}
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@Override
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public ORSet<String> emptyState() {
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return ORSet.empty(getReplicationContext().replicaId());
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}
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@Override
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public CommandHandler<Command, ORSet.DeltaOp, ORSet<String>> commandHandler() {
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return newCommandHandlerBuilder()
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.forAnyState()
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.onCommand(
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AddMovie.class, (state, command) -> Effect().persist(state.add(command.movieId)))
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.onCommand(
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RemoveMovie.class,
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(state, command) -> Effect().persist(state.remove(command.movieId)))
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.onCommand(
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GetMovieList.class,
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(state, command) -> {
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command.replyTo.tell(new MovieList(state.getElements()));
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return Effect().none();
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})
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.build();
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}
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@Override
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public EventHandler<ORSet<String>, ORSet.DeltaOp> eventHandler() {
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return newEventHandlerBuilder().forAnyState().onAnyEvent(ORSet::applyOperation);
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}
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}
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// #movie-entity
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}
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@ -9,9 +9,6 @@ import java.time.Instant
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import scala.concurrent.duration._
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import docs.akka.persistence.typed.ReplicatedAuctionExampleSpec.AuctionEntity
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import org.scalatest.concurrent.Eventually
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import org.scalatest.concurrent.ScalaFutures
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import org.scalatest.matchers.should.Matchers
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import org.scalatest.wordspec.AnyWordSpecLike
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import akka.actor.testkit.typed.scaladsl.LogCapturing
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@ -295,10 +292,7 @@ object ReplicatedAuctionExampleSpec {
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class ReplicatedAuctionExampleSpec
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extends ScalaTestWithActorTestKit(PersistenceTestKitPlugin.config)
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with AnyWordSpecLike
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with Matchers
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with LogCapturing
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with ScalaFutures
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with Eventually {
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with LogCapturing {
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import ReplicatedAuctionExampleSpec.AuctionEntity._
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"Auction example" should {
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@ -4,9 +4,6 @@
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package docs.akka.persistence.typed
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import org.scalatest.concurrent.Eventually
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import org.scalatest.concurrent.ScalaFutures
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import org.scalatest.matchers.should.Matchers
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import org.scalatest.time.Millis
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import org.scalatest.time.Span
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import org.scalatest.wordspec.AnyWordSpecLike
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@ -140,10 +137,7 @@ object ReplicatedBlogExampleSpec {
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class ReplicatedBlogExampleSpec
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extends ScalaTestWithActorTestKit(PersistenceTestKitPlugin.config)
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with AnyWordSpecLike
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with Matchers
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with LogCapturing
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with ScalaFutures
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with Eventually {
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with LogCapturing {
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import ReplicatedBlogExampleSpec.BlogEntity
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import ReplicatedBlogExampleSpec.BlogEntity._
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@ -0,0 +1,106 @@
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/*
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* Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
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*/
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package docs.akka.persistence.typed
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import org.scalatest.wordspec.AnyWordSpecLike
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import akka.actor.testkit.typed.scaladsl.LogCapturing
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import akka.actor.testkit.typed.scaladsl.ScalaTestWithActorTestKit
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import akka.actor.typed.ActorRef
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import akka.actor.typed.Behavior
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import akka.persistence.testkit.PersistenceTestKitPlugin
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import akka.persistence.testkit.query.scaladsl.PersistenceTestKitReadJournal
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import akka.persistence.typed.ReplicaId
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import akka.persistence.typed.crdt.ORSet
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import akka.persistence.typed.scaladsl.Effect
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import akka.persistence.typed.scaladsl.EventSourcedBehavior
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import akka.persistence.typed.scaladsl.ReplicatedEventSourcing
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object ReplicatedMovieWatchListExampleSpec {
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//#movie-entity
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object MovieWatchList {
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sealed trait Command
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final case class AddMovie(movieId: String) extends Command
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final case class RemoveMovie(movieId: String) extends Command
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final case class GetMovieList(replyTo: ActorRef[MovieList]) extends Command
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final case class MovieList(movieIds: Set[String])
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def apply(entityId: String, replicaId: ReplicaId, allReplicaIds: Set[ReplicaId]): Behavior[Command] = {
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ReplicatedEventSourcing.withSharedJournal(
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"movies",
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entityId,
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replicaId,
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allReplicaIds,
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PersistenceTestKitReadJournal.Identifier) { replicationContext =>
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EventSourcedBehavior[Command, ORSet.DeltaOp, ORSet[String]](
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replicationContext.persistenceId,
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ORSet.empty(replicationContext.replicaId),
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(state, cmd) => commandHandler(state, cmd),
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(state, event) => eventHandler(state, event))
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}
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}
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private def commandHandler(state: ORSet[String], cmd: Command): Effect[ORSet.DeltaOp, ORSet[String]] = {
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cmd match {
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case AddMovie(movieId) =>
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Effect.persist(state + movieId)
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case RemoveMovie(movieId) =>
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Effect.persist(state - movieId)
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case GetMovieList(replyTo) =>
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replyTo ! MovieList(state.elements)
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Effect.none
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}
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}
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private def eventHandler(state: ORSet[String], event: ORSet.DeltaOp): ORSet[String] = {
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state.applyOperation(event)
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}
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}
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//#movie-entity
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}
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class ReplicatedMovieWatchListExampleSpec
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extends ScalaTestWithActorTestKit(PersistenceTestKitPlugin.config)
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with AnyWordSpecLike
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with LogCapturing {
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import ReplicatedMovieWatchListExampleSpec._
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"MovieWatchList" must {
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"demonstrate ORSet" in {
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import MovieWatchList._
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val Replicas = Set(ReplicaId("DC-A"), ReplicaId("DC-B"))
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val dcAReplica: ActorRef[Command] = spawn(MovieWatchList("mylist", ReplicaId("DC-A"), Replicas))
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val dcBReplica: ActorRef[Command] = spawn(MovieWatchList("mylist", ReplicaId("DC-B"), Replicas))
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val probeA = createTestProbe[MovieList]()
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val probeB = createTestProbe[MovieList]()
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dcAReplica ! AddMovie("movie-15")
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dcAReplica ! AddMovie("movie-17")
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dcBReplica ! AddMovie("movie-20")
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eventually {
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dcAReplica ! GetMovieList(probeA.ref)
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probeA.expectMessage(MovieList(Set("movie-15", "movie-17", "movie-20")))
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dcBReplica ! GetMovieList(probeB.ref)
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probeB.expectMessage(MovieList(Set("movie-15", "movie-17", "movie-20")))
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}
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dcBReplica ! RemoveMovie("movie-17")
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eventually {
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dcAReplica ! GetMovieList(probeA.ref)
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probeA.expectMessage(MovieList(Set("movie-15", "movie-20")))
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dcBReplica ! GetMovieList(probeB.ref)
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probeB.expectMessage(MovieList(Set("movie-15", "movie-20")))
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}
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}
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}
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}
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