== Kafka: ordering is per partition ==

produced 12 records, keys D/A/F round-robin, values 1..12 in order
(keys chosen so that murmur2 spreads them: D->0, A->1, F->2. A, B and C
 all hash to partition 1 with three partitions, which is worth knowing
 before you decide your keys are well distributed.)

consumed in this order:
  F=2@p2
  F=5@p2
  F=8@p2
  F=11@p2
  A=1@p1
  A=4@p1
  A=7@p1
  A=10@p1
  D=3@p0
  D=6@p0
  D=9@p0
  D=12@p0

per key:
  F -> partition [2]  values [2, 5, 8, 11]
  A -> partition [1]  values [1, 4, 7, 10]
  D -> partition [0]  values [3, 6, 9, 12]

Order is preserved within each key because a key hashes to one partition.
Across keys it is not: the values above are not 1..12 in order, because a
consumer drains one partition's buffer before the next.