Cassandra Projection

[octonato]

The goal is to provide a projection that is backed by a Cassandra offset storage that can be used to track offsets.

The offset storage should support different kinds of offsets: Long, akka.persistence.Sequence, akka.persistence.TimeBasedUUID.

This projection won't support transactions and therefore can be offer delayed commits (commit every 100 events for example).

Use cases:

• read from any source and track the offset on a Cassandra table (non-transactional)
• read from Akka Persistence journal (Cassandra or JDBC) and track the offset on a Cassandra table (non-transactional). Specialization of the above but since very common we can provide a boilerplate free variant.

Specs:

• Offset is managed in a Cassandra table
• needs an offset extraction function to extract the offset from the envelope
• requires a Flow that commits to a predefined Cassandra table (non-transactional), supports at-least-once and at-most-once
  since non-transactional, event handler can be a function E => Future[Done] where E is any envelope
• Provide a Cassandra Offset Store based on Long. When reading Kafka, the offset will be a Long. Other offset typed may be supported and can be added when needed (or implemented by user)
• Convenience API when reading from Akka Persistence Journal.
  We have a privileged situation when reading from Akka Persistence Journal, as we know how the envelope looks like, we know how to extract the offset (no need for extractor). However, we need a special offset store that knows how to write and read akka.persistence.Offset

Tasks:

• OffsetStore Cassandra Projection, #21 #54
• atLeastOnce Cassandra Projection, #21 #54
• atMostOnce Cassandra at-most-once projection, #21 #62
• Convenience for eventsByTag source
• javadsl javadsl for CassandraProjection #84