Persistence Query

How to get the ReadJournal using Scala
How to get the ReadJournal using Java
Supported persistence queries

How to get the ReadJournal using Scala

The ReadJournal is retrieved via the org.apache.pekko.persistence.query.PersistenceQuery extension:

import org.apache.pekko.persistence.query.PersistenceQuery
import org.apache.pekko.persistence.postgres.query.scaladsl.PostgresReadJournal

val readJournal: PostgresReadJournal = PersistenceQuery(system).readJournalFor[PostgresReadJournal](PostgresReadJournal.Identifier)

How to get the ReadJournal using Java

The ReadJournal is retrieved via the org.apache.pekko.persistence.query.PersistenceQuery extension:

import org.apache.pekko.persistence.query.PersistenceQuery;
import org.apache.pekko.persistence.postgres.query.javadsl.PostgresReadJournal;

final PostgresReadJournal readJournal = PersistenceQuery.get(system).getReadJournalFor(PostgresReadJournal.class, PostgresReadJournal.Identifier());

Supported persistence queries

The plugin supports the following queries:

AllPersistenceIdsQuery and CurrentPersistenceIdsQuery

allPersistenceIds and currentPersistenceIds are used for retrieving all persistenceIds of all persistent actors.

import org.apache.pekko.actor.ActorSystem
import org.apache.pekko.stream.{Materializer, ActorMaterializer}
import org.apache.pekko.stream.scaladsl.Source
import org.apache.pekko.persistence.query.PersistenceQuery
import org.apache.pekko.persistence.postgres.query.scaladsl.PostgresReadJournal

implicit val system: ActorSystem = ActorSystem()
implicit val mat: Materializer = ActorMaterializer()(system)
val readJournal: PostgresReadJournal = PersistenceQuery(system).readJournalFor[PostgresReadJournal](PostgresReadJournal.Identifier)

val willNotCompleteTheStream: Source[String, NotUsed] = readJournal.allPersistenceIds()

val willCompleteTheStream: Source[String, NotUsed] = readJournal.currentPersistenceIds()

The returned event stream is unordered and you can expect different order for multiple executions of the query.

When using the allPersistenceIds query, the stream is not completed when it reaches the end of the currently used persistenceIds, but it continues to push new persistenceIds when new persistent actors are created.

When using the currentPersistenceIds query, the stream is completed when the end of the current list of persistenceIds is reached, thus it is not a live query.

The stream is completed with failure if there is a failure in executing the query in the backend journal.

EventsByPersistenceIdQuery and CurrentEventsByPersistenceIdQuery

eventsByPersistenceId and currentEventsByPersistenceId is used for retrieving events for a specific PersistentActor identified by persistenceId.

import org.apache.pekko.actor.ActorSystem
import org.apache.pekko.stream.{Materializer, ActorMaterializer}
import org.apache.pekko.stream.scaladsl.Source
import org.apache.pekko.persistence.query.{ PersistenceQuery, EventEnvelope }
import org.apache.pekko.persistence.postgres.query.scaladsl.PostgresReadJournal

implicit val system: ActorSystem = ActorSystem()
implicit val mat: Materializer = ActorMaterializer()(system)
val readJournal: PostgresReadJournal = PersistenceQuery(system).readJournalFor[PostgresReadJournal](PostgresReadJournal.Identifier)

val willNotCompleteTheStream: Source[EventEnvelope, NotUsed] = readJournal.eventsByPersistenceId("some-persistence-id", 0L, Long.MaxValue)

val willCompleteTheStream: Source[EventEnvelope, NotUsed] = readJournal.currentEventsByPersistenceId("some-persistence-id", 0L, Long.MaxValue)

You can retrieve a subset of all events by specifying fromSequenceNr and toSequenceNr or use 0L and Long.MaxValue respectively to retrieve all events. Note that the corresponding sequence number of each event is provided in the EventEnvelope, which makes it possible to resume the stream at a later point from a given sequence number.

The returned event stream is ordered by sequence number, i.e. the same order as the PersistentActor persisted the events. The same prefix of stream elements (in same order) are returned for multiple executions of the query, except for when events have been deleted.