TopicMessage
class TopicMessage @InternalWowApi constructor(val key: String, val value: ByteArray, val headers: Map<String, String> = emptyMap(), val partition: Int? = null)
A message to write to a topic: its key, which picks its partition (the same key always lands in the same partition, so the messages of one key keep their order), its headers and its value bytes.
partition, when set, is the partition the message goes to instead of the one its key picks: a command topic's partitions are the context's log partitions, chosen by the log's own partitioner.