Currently, the camel-kafka producer requires the definition of kafka.PARTITION_KEY and kafka.KEY to specify the partition to send to. While the key is usually used to determine the partition to send to, I don't understand the hard limitation on this header, when specifically defining the exact partition to send to anyway (via kafka.PARTITION_KEY).
Also, this is not clearly stated in the documentation.
Not sure, if this qualifies as bug or improvement, looks like a limitation to me. I can create a PR, if desired.