[jira] [Created] (KAFKA-7410) Rack aware partitions assignment create unbalanced broker assignments on unbalanced racks

["Lucas Bradstreet (JIRA)" <ji...@apache.org>]

Lucas Bradstreet created KAFKA-7410:
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             Summary: Rack aware partitions assignment create unbalanced broker assignments on unbalanced racks
                 Key: KAFKA-7410
                 URL: https://issues.apache.org/jira/browse/KAFKA-7410
             Project: Kafka
          Issue Type: Bug
          Components: admin
    Affects Versions: 1.1.1
            Reporter: Lucas Bradstreet
         Attachments: AdminUtilsTest.scala

AdminUtils creates a bad partition assignment when the number of brokers on each rack is unbalanced, e.g. 80 brokers rack A, 20 brokers rack B, 15 brokers rack C. Under such a scenario, a single broker from rack C may be assigned over and over again, when more balanced allocations exist.

kafka.admin.AdminUtils.getRackAlternatedBrokerList is supposed to create a list of brokers alternating by rack, however once it runs out of brokers on the racks with fewer brokers, it ends up placing a run of brokers from the same rack together as rackIterator.hasNext will return false for the other racks.
{code:java}
while (result.size < brokerRackMap.size) {
  val rackIterator = brokersIteratorByRack(racks(rackIndex))
  if (rackIterator.hasNext)
    result += rackIterator.next()
  rackIndex = (rackIndex + 1) % racks.length
}{code}
Once assignReplicasToBrokersRackAware hits the run of brokers from the same rack, when choosing the replicas to go along with the leader on the rack with the most brokers e.g. C, it will skip all of the C brokers until it wraps around to the first broker in the alternated list, and choose the first broker in the alternated list.

 
{code:java}
if ((!racksWithReplicas.contains(rack) || racksWithReplicas.size == numRacks)
&& (!brokersWithReplicas.contains(broker) || brokersWithReplicas.size == numBrokers)) {
replicaBuffer += broker
racksWithReplicas += rack
brokersWithReplicas += broker
done = true
}
k += 1
{code}
It does so for each of the remaining brokers for C, choosing the first broker in the alternated list until it's allocated all of the partitions.

See the attached sample code for more details.



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