[jira] [Commented] (FLINK-4280) New Flink-specific option to set starting position of Kafka consumer without respecting external offsets in ZK / Broker

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-4280?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15852752#comment-15852752 ] 

ASF GitHub Bot commented on FLINK-4280:
---------------------------------------

Github user tzulitai commented on the issue:

    https://github.com/apache/flink/pull/2509
  
    Note about 3rd commit: fixed failing `FlinkKafkaConsumerBaseMigrationTest`s after the rebase.
    
    The tests were failing due to the removal of `AbstractFetcher#restoreOffsets(...)` method as part of the refactoring of offset restoration in this PR. On the other hand, the previous implementation of tests in `FlinkKafkaConsumerBaseMigrationTest` were too tightly coupled with how the connector was implemented, i.e. it was testing how the `AbstractFetcher` methods are called, whether `MAX_VALUE` watermark was emitted (which will likely change as features are added to the connector) etc, even though the actual purpose of the tests was simply to test states were restored correctly.
    
    The 3rd commit therefore attempts to simplify `FlinkKafkaConsumerBaseMigrationTest` to only test legacy state restore behaviour. The deleted parts, IMHO, are already covered in other tests.


> New Flink-specific option to set starting position of Kafka consumer without respecting external offsets in ZK / Broker
> -----------------------------------------------------------------------------------------------------------------------
>
>                 Key: FLINK-4280
>                 URL: https://issues.apache.org/jira/browse/FLINK-4280
>             Project: Flink
>          Issue Type: New Feature
>          Components: Kafka Connector
>            Reporter: Tzu-Li (Gordon) Tai
>            Assignee: Tzu-Li (Gordon) Tai
>             Fix For: 1.3.0
>
>
> Currently, to start reading from the "earliest" and "latest" position in topics for the Flink Kafka consumer, users set the Kafka config {{auto.offset.reset}} in the provided properties configuration.
> However, the way this config actually works might be a bit misleading if users were trying to find a way to "read topics from a starting position". The way the {{auto.offset.reset}} config works in the Flink Kafka consumer resembles Kafka's original intent for the setting: first, existing external offsets committed to the ZK / brokers will be checked; if none exists, then will {{auto.offset.reset}} be respected.
> I propose to add Flink-specific ways to define the starting position, without taking into account the external offsets. The original behaviour (reference external offsets first) can be changed to be a user option, so that the behaviour can be retained for frequent Kafka users that may need some collaboration with existing non-Flink Kafka consumer applications.
> How users will interact with the Flink Kafka consumer after this is added, with a newly introduced {{flink.starting-position}} config:
> {code}
> Properties props = new Properties();
> props.setProperty("flink.starting-position", "earliest/latest");
> props.setProperty("auto.offset.reset", "..."); // this will be ignored (log a warning)
> props.setProperty("group.id", "...") // this won't have effect on the starting position anymore (may still be used in external offset committing)
> ...
> {code}
> Or, reference external offsets in ZK / broker:
> {code}
> Properties props = new Properties();
> props.setProperty("flink.starting-position", "external-offsets");
> props.setProperty("auto.offset.reset", "earliest/latest"); // default will be latest
> props.setProperty("group.id", "..."); // will be used to lookup external offsets in ZK / broker on startup
> ...
> {code}
> A thing we would need to decide on is what would the default value be for {{flink.starting-position}}.
> Two merits I see in adding this:
> 1. This compensates the way users generally interpret "read from a starting position". As the Flink Kafka connector is somewhat essentially a "high-level" Kafka consumer for Flink users, I think it is reasonable to add Flink-specific functionality that users will find useful, although it wasn't supported in Kafka's original consumer designs.
> 2. By adding this, the idea that "the Kafka offset store (ZK / brokers) is used only to expose progress to the outside world, and not used to manipulate how Kafka topics are read in Flink (unless users opt to do so)" is even more definite and solid. There was some discussion in this PR (https://github.com/apache/flink/pull/1690, FLINK-3398) on this aspect. I think adding this "decouples" more Flink's internal offset checkpointing from the external Kafka's offset store.



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