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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