Akka Persistence CassandraReadJournal: Unable to get events by tags

Akka Libraries Persistence / Event Sourcing
1

I am unable to get events by tags, I am using akka 2.5.23, akka-persistence-cassandra 0.98, and Cassandra 3.6

This is part of my config file (I removed persistence snapshot config for brevity):

cassandra {
      host = "localhost"
      host = ${?CASSANDRA_HOST}
      port = 9042
      port = ${?CASSANDRA_PORT}
      number-of-retries = 500
    }
    
    cassandra-journal {
      contact-points = [${cassandra.host}]
      port = ${cassandra.port}
      tags = {
        account = 1
        accountcreated = 2
      }
    
      event-adapters{
        proto = "MyProtobufAdapter"
      }
    
      event-adapter-bindings{
        "com.google.protobuf.Message" = proto
      }
    }
    
    akka {
      actor {
        serializers {
          proto = "akka.remote.serialization.ProtobufSerializer"
        }
    
        serialization-bindings {
          "com.google.protobuf.Message" = proto
        }
      }
    
      persistence {
        journal.plugin = "cassandra-journal"
      }
    }

The events are successfully persisted in Cassandra:

cqlsh:akka> select persistence_id, tags from messages;
    
     persistence_id                       | tags
    --------------------------------------+-------------------------------
     b34f6158-b693-44af-a01d-be4f9e0463d9 | {'account', 'accountcreated'}

I can successfully read all persistenceIds:

 import akka.persistence.query.{EventEnvelope, PersistenceQuery, Sequence}
    import akka.persistence.cassandra.query.scaladsl.CassandraReadJournal
    
    val journal: CassandraReadJournal = PersistenceQuery(system)
        .readJournalFor[CassandraReadJournal](CassandraReadJournal.Identifier)
    
    journal.persistenceIds.runWith(Sink.foreach(println(_)))

But when I try to get a live stream of events by tags, it is empty, but as you can see from the cqlsh output, I have tagged messages.

journal.eventsByTag("account", Sequence(0L)).runWith(Sink.foreach(envelope => println(envelope.persistenceId)))

I am unable to find the problem, please help.

2

If you check the result of the event by tag stream it will have failed as the Cassandra plugin doesn’t use Sequence offsets. Use NoOffset instead.

1 Like
3

Thank you so much for your reply. It worked with NoOffset, I got this log info:

[INFO] [07/23/2019 07:35:51.032] [accounts-system-akka.actor.default-dispatcher-7] [EventsByTagStage(akka://accounts-system)] [c619a08b-b3f2-4c5d-a8f4-9c77216b69a3]: EventsByTag query [account] starting with EC delay 5000ms: fromOffset [a4698000-8f19-11e5-8080-808080808080 (2015-11-20 00:00:00:000)] toOffset [None]

But, if I can not specify an offset, how can I create a Resumable Projection ?
I am reading the journal from an actor in charge of populating a Read database (Elasticsearch), for CQRS purpose. Am I forced to read all events every time with Cassandra ?

4

I found the issue.

There are 3 types of Offset:

  • akka.persistence.query.Offset.sequence(value: Long),
  • akka.persistence.query.Offset.timeBasedUUID(value: UUID)
  • and finally NoOffset if not offset should be used.

But Cassandra does not support the first type (Sequence). So I have to use a time based UUID as offset:

val offset = com.datastax.driver.core.utils.UUIDs.startOf(0L)

journal.eventsByTag("account", offset)
   .runWith(Sink.foreach(envelope => println(envelope.persistenceId)))

This works.
Please let me know if there is a better way.

Thanks.

5

Thank you, this is what I needed.