Producer.MsgOpt

< rdkafka will free(3) \p payload * when it is done with it.

< the \p payload data will be copied * and the \p payload pointer will not * be used by rdkafka after the * call returns.

< Block produce*() on message queue full. WARNING: If a delivery report callback is used the application MUST call rd_kafka_poll() (or equiv.) to make sure delivered messages are drained from the internal delivery report queue. Failure to do so will result in indefinately blocking on the produce() call when the message queue is full.