I’ve written up a test using the Akka/Akka Streams framework in Scala (along with htm.java 0.6.13). Akka stream nodes are push and pull based. I can give you snippets if you’re interested - you would have to be familiar with Scala, though. There is a Java API as well, though I haven’t used it.
The basic approach to the actor is to wrap computeImmediate(..) in the receive method of the actor, under the case when an envelope from our Kafka topic is sent. The result from this is then “asked” back to the sender. An actor is single threaded and guarantees order so each actor processes receive synchronously.
The actor constructor can then be wrapped up to create a Future object, the Future can then be wrapped into a Flow object, and then the flow can be wired into a Graph by setting up the Source and Sink. These can be built without Kafka (for test purposes) or wired up to Kafka for production.