[jira] [Created] (KAFKA-6758) Default "" consumer group tracks committed offsets, but is otherwise not a real group

["David van Geest (JIRA)" <ji...@apache.org>]

David van Geest created KAFKA-6758:
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             Summary: Default "" consumer group tracks committed offsets, but is otherwise not a real group
                 Key: KAFKA-6758
                 URL: https://issues.apache.org/jira/browse/KAFKA-6758
             Project: Kafka
          Issue Type: Bug
          Components: clients
    Affects Versions: 0.11.0.2
            Reporter: David van Geest


*To reproduce:*
 * Use the default config for `group.id` of "" (the empty string)
 * Use the default config for `enable.auto.commit` of `true`
 * Use manually assigned partitions with `assign`

*Actual (unexpected) behaviour:*

Consumer offsets are stored for the "" group. Example:

{{~ $ /opt/kafka/kafka_2.11-0.11.0.2/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group ""}}
{{Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).Consumer group '' has no active members.TOPIC PARTITION CURRENT-OFFSET LOG-END-OFFSET LAG CONSUMER-ID HOST CLIENT-ID}}
{{my_topic 54 7859593 7865082 5489 - - -}}
{{my_topic 5 14252813 14266419 13606 - - -}}
{{my_topic 39 19099099 19122441 23342 - - -}}
{{my_topic 43 16434573 16449180 14607 - - -.}}

....

 

However, the "" is not a real group. It doesn't show up with:

{{~ $ /opt/kafka/kafka_2.11-0.11.0.2/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --list}}

You also can't do dynamic partition assignment with it - if you try to `subscribe` when using the default "" group ID, you get:

{{AbstractCoordinator: Attempt to join group  failed due to fatal error: The configured groupId is invalid}}

*Better behaviours:*

(any of these would be preferable, in my opinion)
 * Don't commit offsets with the "" group, and log a warning telling the user that `enable.auto.commit = true` is meaningless in this situation. This is what I would have expected.
 * Don't have a default `group.id`. Some of my reading indicates that the new consumer basically needs a `group.id` to function. If so, force users to choose a group ID so that they're more aware of what will happen.
 * Have a default `group.id` of `default`, and make it a real consumer group. That is, it shows up in lists of groups, it has dynamic partitioning, etc.

As a user, when I don't set `group.id` I expect that I'm not using consumer groups. Therefore, I expect that there will be no offset tracking in Kafka.

In my specific application, I was wanting `auto.offset.reset` to kick in so that a failed consumer would start at the `latest` offset. However, it started at this unexpectedly stored offset instead.

 



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