== Kafka: partitions are the ceiling on consumer parallelism ==

topic 'orders-scaling', 3 partitions, 5 consumers in one group

  consumer-1 -> partitions [0]
  consumer-2 -> partitions [1]
  consumer-3 -> partitions [2]
  consumer-4 -> no partitions (idle)
  consumer-5 -> no partitions (idle)

consumers with no partitions: 2

A partition is assigned to at most one consumer in a group, so the number
of partitions is a hard ceiling on consumer parallelism. Adding consumers
beyond it adds idle processes, not throughput.