Status

Current state: Under Discussion 

Discussion thread: here

JIRA: KAFKA-9205

Motivation

One regularly used healing operation on Kafka clusters is replica reassignments for topic partitions. For example, when there is a skew in inbound/outbound traffic of a broker replica reassignment can be used to move some leaders/followers from the broker; or if there is a skew in disk usage of brokers, replica reassignment can more some partitions to other brokers that have more disk space available.

In Kafka clusters that span across multiple data centers (or availability zones), high availability is a priority; in the sense that when a data center goes offline the cluster should be able to resume normal operation by guaranteeing partition replicas in all data centers.

This guarantee is currently the responsibility of the on-call engineer that performs the reassignment or the tool that automatically generates the reassignment plan for improving the cluster health (e.g. by considering the rack configuration value of each broker in the cluster). the former, is quite error-prone, and the latter, would lead to duplicate code in all such admin tools (which are not error free either). Not all use cases can make use the default assignment strategy that is used by --generate option; and current rack aware enforcement applies to this option only.

It would be great for the built-in replica assignment API and tool provided by Kafka to support a rack aware verification option for --execute scenario that would simply return an error when [some] brokers in any replica set share a common rack. 

Public Interfaces

Existing API will be updated under Admin tools

def executeAssignment(zkClient: KafkaZkClient, adminClientOpt: Option[JAdminClient], opts: ReassignPartitionsCommandOptions)

Rack-aware option should be acceptable as an optional parameter while use kafka-reassign-partitions with --execute command. 

For example:

./kafka-reassign-partitions.sh --reassignment-json-file partitions-to-move.json --execute --zookeeper localhost:2181 --disable-rack-aware <True/False> 

$ cat partitions-to-move.json
{
  "partitions": [
    {
      "topic": "foo",
      "partition": 6,
      "replicas": [
        1004,
        1003,
        1005
      ],
      "log_dirs": [
        "any",
        "any",
        "any"
      ]
    }
  ],
  "version": 1
}

Proposed Changes

Before this proposal:

If kafka-reassign-partitions is used with custom reassignment algorithm or if the reassignment is manually generated on ad-hoc basic, the tool does not enforce rack awareness when run with --execute option.

After this proposal:

The caller can choose rack awareness on custom or manually generated reassignment planner, by choosing optional parameter --disable-rack-aware along with --execute command.

By doing this if the custom generated reassignment planner has conflicts along with the racks then it can throw the error msg with appropriate reason and conflict of partitions along with the racks info, so that the users can re-generate the reassignment planner accordingly.

Compatibility, Deprecation, and Migration Plan

There is no impact to current usage, --disable-rack-aware along with --execute is not enabled by default, so there is no compatibility concern or migration plan needed for this change.

Rejected Alternatives

None.