[jira] [Created] (KAFKA-4886) In a topic with partitions with replication factor of 1, when a partition is added AFTER a new broker is added, Kafka does not always assign new partition to the new broker.

["Prasanna Kumar P.V. (JIRA)" <ji...@apache.org>]

Prasanna Kumar P.V. created KAFKA-4886:
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             Summary: In a topic with partitions with replication factor of 1, when a partition is added AFTER a new broker is added, Kafka does not always assign new partition to the new broker.
                 Key: KAFKA-4886
                 URL: https://issues.apache.org/jira/browse/KAFKA-4886
             Project: Kafka
          Issue Type: Bug
          Components: admin
    Affects Versions: 0.10.1.0
         Environment: A kafka cluster of 3 KVM VM nodes with RHEL 7.2. 
            Reporter: Prasanna Kumar P.V.


Bring up kafka cluster of two brokers (say ids 0 and 1). Create a topic 'X' with two partitions and replication factor 1. Now add a third broker (id 2). Alter the topic X to have one more partition. Issue is, the newly added partition (i.e partition 2) does not go to the newly added broker if partition 0 has been allotted to broker id 1 and partition 1 is allotted to broker id 0. However, if partition 0 leader has been set to broker id 0 and partition 1 to broker id 1 when creating topic 'X', partition 2 would always go to broker id 2 (a uniform distribution). Due to this, we have to always check and follow up with partition reassignment when a new partition is created to spread processing load uniformly in cluster. (During createTopic in AdminUtils.scala, we let fixedStartIndex become -1, in assignReplicasToBrokersRackAware() we have val startIndex = if (fixedStartIndex >= 0) fixedStartIndex else rand.nextInt(arrangedBrokerList.size). So startIndex could be 0 or 1. 
When startIndex is 1, during alter topic, we end up in assignReplicasToBrokersRackAware() which then does: val firstReplicaIndex = (currentPartitionId + startIndex) % arrangedBrokerList.size. the firstReplicaIndex  becomes either 0 or 2. When it becomes 0, we eventually have the new partition ending up on broker that already is hosting a partition instead of getting assigned to broker id 2.



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