== RabbitMQ: FIFO per queue, per consumer ==

one queue, one consumer, 12 messages
  completion order : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

one queue, two consumers, prefetch 1, consumer-1 slower than consumer-2
  completion order : [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 1, 12]
  out-of-order steps: 1

A queue is FIFO and a single consumer sees it that way. The moment a second
consumer is added the broker hands the next message to whichever consumer
is free, so the order in which work finishes is no longer the order in
which it was published. There is no key: RabbitMQ has no notion of a
partition to which related messages could be pinned. Ordering across
related messages means one queue and one consumer, and therefore no
horizontal scaling for that queue -- or a consistent-hash exchange, which
is a plugin.