Cameron Redpath created KAFKA-16277:
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Summary: CooperativeStickyAssignor does not spread topics evenly
among consumer group
Key: KAFKA-16277
URL: https://issues.apache.org/jira/browse/KAFKA-16277
Project: Kafka
Issue Type: Bug
Reporter: Cameron Redpath
Attachments: image-2024-02-19-13-00-28-306.png
Consider the following scenario:
`topic-1`: 12 partitions
`topic-2`: 12 partitions
Of note, `topic-1` gets approximately 10 times more messages through it than
`topic-2`.
Both of these topics are consumed by a single application, single consumer
group, which scales under load. Each member of the consumer group subscribes to
both topics. The `partition.assignment.strategy` being used is
`org.apache.kafka.clients.consumer.CooperativeStickyAssignor`. The application
may start with one consumer. It consumes all partitions from both topics.
The problem begins when the application scales up to two consumers. What is
seen is that all partitions from `topic-1` go to one consumer, and all
partitions from `topic-2` go to the other consumer. In the case with one topic
receiving more messages than the other, this results in a very imbalanced group
where one consumer is receiving 10x the traffic of the other due to partition
assignment.
This is the issue being seen in our cluster at the moment. See this graph of
the number of messages being processed by each consumer as the group scales
from one to four consumers:
!image-2024-02-19-13-00-28-306.png|width=537,height=612!
Things to note from this graphic:
* With two consumers, the partitions for a topic all go to a single consumer
each
* With three consumers, the partitions for a topic are split between two
consumers each
* With four consumers, the partitions for a topic are split between three
consumers each
With regard to the number of _partitions_ being assigned to each consumer, the
group is balanced. However, the assignment appears to be biased so that
partitions from the same topic go to the same consumer. In our scenario, this
leads to very bad partition assignment.
I question if the behaviour of the assignor should be revised, so that each
topic has its partitions maximally spread across all available members of the
consumer group. In the above scenario, this would result in much more even
distribution of load. The behaviour would then be:
* With two consumers, 6 partitions from each topic go to each consumer
* With three consumers, 4 partitions from each topic go to each consumer
* With four consumers, 3 partitions from each topic go to each consumer
Of note, we only saw this behaviour after migrating to the
`CooperativeStickyAssignor`. It was not an issue with the default partition
assignment strategy.
It is possible this may be intended behaviour. In which case, what is the
preferred workaround for our scenario? Our current workaround if we decide to
go ahead with the update to `CooperativeStickyAssignor` may be to limit our
consumers so they only subscribe to one topic, and have two consumer threads
per instance of the application.
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