...

The goal of this KIP is to allow consumer callbacks to call the consumer again from another thread, while making sure that concurrent access remain remains impossible.

Motivation

The JVM based KafkaConsumer contains a check that rejects nested invocations from different threads (in method acquire). For programs that use an async runtime, this is an almost impossible requirement. Also, the check is more strict than is required; we only need to validate that there is no concurrent access to the consumer.

...