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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-4280) New Flink-specific option to set starting position of Kafka consumer without respecting external offsets in ZK / Broker
Date Mon, 28 Nov 2016 10:39:58 GMT

    [ https://issues.apache.org/jira/browse/FLINK-4280?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15701584#comment-15701584
] 

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

Github user tzulitai commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2509#discussion_r89755327
  
    --- Diff: flink-streaming-connectors/flink-connector-kafka-0.10/src/main/java/org/apache/flink/streaming/connectors/kafka/FlinkKafkaConsumer010.java
---
    @@ -128,6 +129,7 @@ public FlinkKafkaConsumer010(List<String> topics, KeyedDeserializationSchema<T>
     	protected AbstractFetcher<T, ?> createFetcher(
     			SourceContext<T> sourceContext,
     			List<KafkaTopicPartition> thisSubtaskPartitions,
    +			HashMap<KafkaTopicPartition, Long> restoredSnapshotState,
    --- End diff --
    
    To make the startup mode logic cleaner, I've changed the `AbstractFetcher` life cycle
a bit.
    Now, restored state is provided when constructing the `AbstractFetcher`, instead of explicitly
calling `AbstractFetcher#restoreOffsets()` as a separate call.
    
    This allows the AbstractFetcher to have a final `isRestored` flag that version-specific
implementations can use.
    
    The startup offset configuring logic is much simpler now with this flag:
    ```
    if (isRestored) {
      // all subscribed partition states should have defined offset
      // setup the KafkaConsumer client we're using to respect these restored offsets
    } else {
      // all subscribed partition states have no defined offset
      // (1) set offsets depending on whether startup mode is EARLIEST, LATEST, or GROUP_OFFSET
      // (2) use the fetched offsets from Kafka to set the initial partition states we use
in Flink.
    }
    ```


> 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: 1.2.0
>
>
> 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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