[jira] [Resolved] (KAFKA-6607) Kafka Streams lag not zero when input topic transactional

["Matthias J. Sax (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KAFKA-6607?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Matthias J. Sax resolved KAFKA-6607.
------------------------------------
    Fix Version/s: 2.5.0
       Resolution: Fixed

> Kafka Streams lag not zero when input topic transactional
> ---------------------------------------------------------
>
>                 Key: KAFKA-6607
>                 URL: https://issues.apache.org/jira/browse/KAFKA-6607
>             Project: Kafka
>          Issue Type: Improvement
>          Components: streams
>    Affects Versions: 1.0.0
>            Reporter: Matthias J. Sax
>            Assignee: Matthias J. Sax
>            Priority: Minor
>             Fix For: 2.5.0
>
>
> When an input topic for a Kafka Streams application is written using transaction, Kafka Streams commits an "incorrect" offset, ie, it commits "lastProcessedMessageOffset + 1" that is smaller than "endOffset" if it reaches the end of topic. The reason is the commit marker that is the last "message" in the topic; Streams does not take commit markers into account when committing.
> This is not a correctness issue, but when one inspect the consumer lag via {{bin/kafka-consumer.group.sh}} the lag is shown as 1 instead of 0 – what is correct from consumer-group tool point of view.
> Note that all applications using a plain consumer may face the same issue if they use `KafkaConsumer#commitSync(Map<TopicPartition, OffsetAndMetadata> offsets)`: to address the issue, the correct pattern is to either commit "nextRecord.offset()" (if the next record is available already, ie, was returned by `poll()`, or use `consumer.position()` that takes the commit marker into account and would "step over it").



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