TransportUnavailableException

A transport a port called could not be reached, or did not answer in time: a broker, leader or coordinator not available, a network failure, too few replicas in sync, a request that timed out. The same call may succeed once the transport is back.

A transport throws it, with the client's exception as cause, from TopicProducer.send and its admin's calls; a subscription pauses its partition on it, as on a store that is unavailable. Any other failure stays the client's own exception.