Topic Subscription with offset info for exactly-once semantics

Consuming from a topic may provide the topic offset so that consumers can track it and have an exactly-once implementation.

Kafka docs, for example, suggest that approach:

So what about exactly once semantics (i.e. the thing you actually want)? […]this can be handled more simply and generally by simply letting the consumer store its offset in the same place as its output.

This could require adding an exactlyOncemethod on the subscriber API which required a Flow of (message,offset). We may have to model this offset as a (offset:Long, partitionId:Int) since each partition of the same topic has it’s offset.

[TimMoore]

There’s some example code at http://doc.akka.io/docs/akka-stream-kafka/current/consumer.html#external-offset-storage

[ignasi35]

we need to take full control of the distribution of partitions across nodes.

We probably need https://github.com/akka/reactive-kafka/blob/776a6ec9e320415bb72486612cc90d33f3557fc4/core/src/main/scala/akka/kafka/ConsumerSettings.scala#L30-L30