Persistence Query for LevelDB

This is documentation for the LevelDB implementation of the Persistence Query API. Note that implementations for other journals may have different semantics.

Dependencies

Akka persistence LevelDB query implementation is bundled in the akka-persistence-query artifact. Make sure that you have the following dependency in your project:

sbt
"com.typesafe.akka" %% "akka-persistence-query" % "2.5.6"
Gradle
compile group: 'com.typesafe.akka', name: 'akka-persistence-query_2.12', version: '2.5.6'
Maven
<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-persistence-query_2.12</artifactId>
  <version>2.5.6</version>
</dependency>

How to get the ReadJournal

The ReadJournal is retrieved via the akka.persistence.query.PersistenceQuery extension:

Scala
import akka.persistence.query.PersistenceQuery
import akka.persistence.query.journal.leveldb.scaladsl.LeveldbReadJournal

val queries = PersistenceQuery(system).readJournalFor[LeveldbReadJournal](
  LeveldbReadJournal.Identifier)
Java
final ActorMaterializer mat = ActorMaterializer.create(system);

LeveldbReadJournal queries =
  PersistenceQuery.get(system).getReadJournalFor(LeveldbReadJournal.class, 
      LeveldbReadJournal.Identifier());

Supported Queries

EventsByPersistenceIdQuery and CurrentEventsByPersistenceIdQuery

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

Scala
implicit val mat = ActorMaterializer()(system)
val queries = PersistenceQuery(system).readJournalFor[LeveldbReadJournal](
  LeveldbReadJournal.Identifier)

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

val events: Source[Any, NotUsed] = src.map(_.event)
Java
LeveldbReadJournal queries =
    PersistenceQuery.get(system).getReadJournalFor(LeveldbReadJournal.class, 
        LeveldbReadJournal.Identifier());

Source<EventEnvelope, NotUsed> source =
    queries.eventsByPersistenceId("some-persistence-id", 0, Long.MAX_VALUE);

You can retrieve a subset of all events by specifying fromSequenceNr and toSequenceNr or use 0L and Long.MaxValueLong.MAX_VALUE 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.

The stream is not completed when it reaches the end of the currently stored events, but it continues to push new events when new events are persisted. Corresponding query that is completed when it reaches the end of the currently stored events is provided by currentEventsByPersistenceId.

The LevelDB write journal is notifying the query side as soon as events are persisted, but for efficiency reasons the query side retrieves the events in batches that sometimes can be delayed up to the configured refresh-interval or given RefreshInterval hint.

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

AllPersistenceIdsQuery and CurrentPersistenceIdsQuery

allPersistenceIds is used for retrieving all persistenceIds of all persistent actors.

Scala
implicit val mat = ActorMaterializer()(system)
val queries = PersistenceQuery(system).readJournalFor[LeveldbReadJournal](
  LeveldbReadJournal.Identifier)

val src: Source[String, NotUsed] = queries.persistenceIds()
Java
LeveldbReadJournal queries =
    PersistenceQuery.get(system).getReadJournalFor(LeveldbReadJournal.class, 
        LeveldbReadJournal.Identifier());

Source<String, NotUsed> source = queries.persistenceIds();

The returned event stream is unordered and you can expect different order for multiple executions of the 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. Corresponding query that is completed when it reaches the end of the currently used persistenceIds is provided by currentPersistenceIds.

The LevelDB write journal is notifying the query side as soon as new persistenceIds are created and there is no periodic polling or batching involved in this query.

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

EventsByTag and CurrentEventsByTag

eventsByTag is used for retrieving events that were marked with a given tag, e.g. all domain events of an Aggregate Root type.

Scala
implicit val mat = ActorMaterializer()(system)
val queries = PersistenceQuery(system).readJournalFor[LeveldbReadJournal](
  LeveldbReadJournal.Identifier)

val src: Source[EventEnvelope, NotUsed] =
  queries.eventsByTag(tag = "green", offset = Sequence(0L))
Java
LeveldbReadJournal queries =
    PersistenceQuery.get(system).getReadJournalFor(LeveldbReadJournal.class, 
        LeveldbReadJournal.Identifier());

Source<EventEnvelope, NotUsed> source =
    queries.eventsByTag("green", new Sequence(0L));

To tag events you create an Event Adapters that wraps the events in a akka.persistence.journal.Tagged with the given tags.

Scala
import akka.persistence.journal.WriteEventAdapter
import akka.persistence.journal.Tagged

class MyTaggingEventAdapter extends WriteEventAdapter {
  val colors = Set("green", "black", "blue")
  override def toJournal(event: Any): Any = event match {
    case s: String =>
      var tags = colors.foldLeft(Set.empty[String]) { (acc, c) =>
        if (s.contains(c)) acc + c else acc
      }
      if (tags.isEmpty) event
      else Tagged(event, tags)
    case _ => event
  }

  override def manifest(event: Any): String = ""
}
Java
public class MyTaggingEventAdapter implements WriteEventAdapter {

  @Override
  public Object toJournal(Object event) {
    if (event instanceof String) {
      String s = (String) event;
      Set<String> tags = new HashSet<String>();
      if (s.contains("green")) tags.add("green");
      if (s.contains("black")) tags.add("black");
      if (s.contains("blue")) tags.add("blue");
      if (tags.isEmpty())
        return event;
      else
        return new Tagged(event, tags);
    } else {
      return event;
    }
  }
  
  @Override
  public String manifest(Object event) {
    return "";
  }
}

You can use NoOffset to retrieve all events with a given tag or retrieve a subset of all events by specifying a Sequence offset. The offset corresponds to an ordered sequence number for the specific tag. Note that the corresponding offset of each event is provided in the EventEnvelope, which makes it possible to resume the stream at a later point from a given offset.

The offset is exclusive, i.e. the event with the exact same sequence number will not be included in the returned stream. This means that you can use the offset that is returned in EventEnvelope as the offset parameter in a subsequent query.

In addition to the offset the EventEnvelope also provides persistenceId and sequenceNr for each event. The sequenceNr is the sequence number for the persistent actor with the persistenceId that persisted the event. The persistenceId + sequenceNr is an unique identifier for the event.

The returned event stream is ordered by the offset (tag sequence number), which corresponds to the same order as the write journal stored the events. The same stream elements (in same order) are returned for multiple executions of the query. Deleted events are not deleted from the tagged event stream.

Note

Events deleted using deleteMessages(toSequenceNr) are not deleted from the “tagged stream”.

The stream is not completed when it reaches the end of the currently stored events, but it continues to push new events when new events are persisted. Corresponding query that is completed when it reaches the end of the currently stored events is provided by currentEventsByTag.

The LevelDB write journal is notifying the query side as soon as tagged events are persisted, but for efficiency reasons the query side retrieves the events in batches that sometimes can be delayed up to the configured refresh-interval or given RefreshInterval hint.

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

Configuration

Configuration settings can be defined in the configuration section with the absolute path corresponding to the identifier, which is "akka.persistence.query.journal.leveldb" for the default LeveldbReadJournal.Identifier.

It can be configured with the following properties:

# Configuration for the LeveldbReadJournal
akka.persistence.query.journal.leveldb {
  # Implementation class of the LevelDB ReadJournalProvider
  class = "akka.persistence.query.journal.leveldb.LeveldbReadJournalProvider"
  
  # Absolute path to the write journal plugin configuration entry that this 
  # query journal will connect to. That must be a LeveldbJournal or SharedLeveldbJournal.
  # If undefined (or "") it will connect to the default journal as specified by the
  # akka.persistence.journal.plugin property.
  write-plugin = ""
  
  # The LevelDB write journal is notifying the query side as soon as things
  # are persisted, but for efficiency reasons the query side retrieves the events 
  # in batches that sometimes can be delayed up to the configured `refresh-interval`.
  refresh-interval = 3s
  
  # How many events to fetch in one query (replay) and keep buffered until they
  # are delivered downstreams.
  max-buffer-size = 100
}
The source code for this page can be found here.