kafka_group doesn't react on adding partitions to a topic

[mfvanek]

Hi there.

When we add partitions to a topic, fluent-plugin-kafka doesn't detect these changes and doesn't start consuming messages from new partitions.
We use kafka_group.
As the workaround we just restart our fluentd instances. It is enough to restart at least one instance.

Is it expected behavior?

[mfvanek]

Repro steps
We send logs to Kafka, consumes them with Fluentd and persist them into Elastic.

Fluentd configuration

<source>
 @type kafka_group
 brokers broker:9092
 consumer_group my-consumer-group
 topics my-topic
 format json
</source>

<match **>
  @type stdout
</match>

1. Create a topic with one partition

kafka-topics \
--create \
--bootstrap-server broker:9092 \
--replication-factor 1 \
--partitions 1 \
--topic my-topic

2. Run fluetnd

3. Send logs to Kafka:

kafka-console-producer --broker-list broker:9092 --topic my-topic
>1
>2
>3
>4

Logs:

2020-10-19 14:08:57.042342952 +0000 my-topic: 1
2020-10-19 14:08:57.042384921 +0000 my-topic: 2
2020-10-19 14:08:57.042392425 +0000 my-topic: 3
2020-10-19 14:09:00.070712536 +0000 my-topic: 4

4. Visible list of partitions in consumer-groups

kafka-consumer-groups --bootstrap-server broker:9092 --group my-consumer-group --describe
GROUP              TOPIC           PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG             CONSUMER-ID                                HOST            CLIENT-ID
my-consumer-group  my-topic        0          -               0               -               kafka-975c4e8e-ae7a-437c-988e-c740923e5834 /10.116.1.147   kafka

5. Increase partitions count up to 2:

kafka-topics --zookeeper zookeeper:2181 \
--alter \
--topic my-topic \
--partitions 2

6. Send logs to Kafka:

kafka-console-producer --broker-list broker:9092 --topic my-topic
>1
>2
>3
>4

Logs:

2020-10-19 14:09:57.132099077 +0000 my-topic: 1
2020-10-19 14:09:57.132111450 +0000 my-topic: 2
2020-10-19 14:09:57.132118563 +0000 my-topic: 3

7. List of partitions in consumer-groups is the same

kafka-consumer-groups --bootstrap-server broker:9092 --group my-consumer-group --describe
GROUP              TOPIC           PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG             CONSUMER-ID                                HOST            CLIENT-ID
my-consumer-group  my-topic        0          7               7               0               kafka-975c4e8e-ae7a-437c-988e-c740923e5834 /10.116.1.147   kafka

Actual behavior: data is read from only one partition.

[github-actions]

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[Gourds]

+1