kafka producer does not distribute messages to partitions evenly?

[Yang]

we have 10 partitions for a topic, and omit the explicit partition param in
the message creation:

KeyedMessage<String, String> data = new KeyedMessage<String, String>
(mytopic,   myMessageContent);   // partition key need to be polished
producer.send(data);



but on average 3--5 of the partitions are empty.



what went wrong?

thanks
Yang