Recently, in a concerted effort to make Kafka's rebalances less painful, various approaches has been used to reduce the number of and impact of rebalances. Often, the trigger of a rebalance is a failure of some sort or a thrown exception during processing, in which case, the workload will be redistributed among surviving threads. Working to reduce rebalances due to random consumer crashes, a recent change to Kafka internals had been made (which introduces the concept of static membership) that prevents a rebalance from occurring within session.timeout.ms in the hope that the consumer thread which crashed would recover in that time interval and rejoin the group.
However, in some cases, some consumer threads would permanently go down or remain dead for long periods of time. In these scenarios, users of Kafka would possibly not be aware of such a crash until hours later after it happened which forces Kafka users to manually start a new KafkaConsumer process a considerable period of time after the failure had occurred. That is where the addition of a callback such as onConsumerFailure would help. There are multiple use cases for this callback (which is defined by the user). onConsumerFailure is called when a particular consumer thread goes under for some specified time interval (i.e. a config called acceptable.consumer.failure.timeout.ms). When called, this method could be used to log a consumer failure or should the user wish it, create a new thread which would then rejoin the consumer group (which could also include the required group.instance.id so that a rebalance wouldn't be re-triggered –- we would need to think about that).
Should the old thread recover and attempt to rejoin the consumer group (with the substitute thread being part of the group), the old thread will be denied access and an exception would be thrown (to indicate that another process has already taken its place).