...

def onRevoked(revokedTopicPartitions: Set[TopicPartition], consumer: KafkaConsumer) = {
  for {
    _            <- ZIO.logDebug(s"${revokedTps.size} partitions are revoked")
    state        <- currentStateRef.get
    streamsToEnd = state.assignedStreams.filter(control => revokedTps.contains(control.tp)) // Note, we run 1 stream per partition.
    _            <- ZIO.foreachParDiscardforeachDiscard(streamsToEnd)(_.end(consumer))    // <== Streams will commit not yet committed offsets
    _            <- awaitCommitsCompleted(consumer).timeout(15.seconds)
    _            <- ZIO.logTrace("onRevoked done")
  } yield ()
}

...

Since this code is running in the rebalance listener callback, KafkaConsumer enforces that the commit methods must be invoked from the same thread as the thread that invoked onPartitionsRevoked. Unfortunately, the ZIO runtime is inherently multi-threaded; tasks can be executed from any thread. There is no way Zio could support this limitation without a major rewrite.

The million dollar question: WHY CAN THIS CODE NOT RUN ON A SINGLE THREAD?

We want to use ZIO. ZIO does not support it.

Public Interfaces

Two new methods will be added to org.apache.kafka.clients.consumer.KafkaConsumer: getThreadAccessKey and setThreadAccessKey.

...