KIP-739 — Block Less on KafkaProducer#send

Discussion Producer

Proposes a truly non-blocking `KafkaProducer.send()` overload that returns a `CompletableFuture` and never blocks the calling thread, even when partition metadata is unavailable or the record accumulator is full. The existing `send()` returns a `Future` but can silently block on metadata fetches or buffer-full conditions, causing deadlocks in async runtimes like Netty or gRPC that prohibit blocking the I/O thread.

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