Is there a way for a publisher in the delivery callback to know which partition it was sent to?

[winbatch]

If I am letting the broker determine the partition based on key, is there a way the publisher can tell which partition was chosen? I'd like to log it.

[winbatch]

Is that what the topic_set_partitioner_cb() is for? unclear to me.

[edenhill]

The broker does not decide on the partition, that is up to the producer.

If you produce a message through librdkafka with the partition set to -1 (or RD_KAFKA_PARTITION_UA (unavailable)) then the configured partitioner will be called for the message.
If there is no configured partitioner the default partitioner is used which is a random partitioner.

There is unfortunately no support in the delivery report callback (dr_cb) for seeing which partition was used.
But you can configure your own partitioner function to decide which partition to send a message to. It will be called once for each message you produce and it allows you to check if a certain partition is available.
See https://github.com/edenhill/librdkafka/blob/master/rdkafka.h#L287

[edenhill]

Please reopen this issue if anything is unclear.