Constant PARTITION_KEY_HEADER 

pub const PARTITION_KEY_HEADER: &str = "amqp-partition-key";

Header carrying a message’s partition key, read by Partitioned for keyed worker lanes.

Set it on an outgoing message’s Headers to route deliveries that share a key to the same worker lane under workers(n, by_key). It rides in the AMQP header table like any other header; nothing else in the broker interprets it.