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https://issues.apache.org/jira/browse/FLINK-4280?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15866051#comment-15866051
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ASF GitHub Bot commented on FLINK-4280:
---------------------------------------
Github user rmetzger commented on a diff in the pull request:
https://github.com/apache/flink/pull/2509#discussion_r101075570
--- Diff: docs/dev/connectors/kafka.md ---
@@ -161,6 +161,46 @@ For convenience, Flink provides the following schemas:
The KeyValue objectNode contains a "key" and "value" field which
contain all fields, as well as
an optional "metadata" field that exposes the offset/partition/topic
for this message.
+#### Kafka Consumers Start Position Configuration
+
+By default, the Flink Kafka Consumer starts reading partitions from the
consumer group's (`group.id` setting in the
+consumer properties) committed offsets in Kafka brokers (or Zookeeper for
Kafka 0.8).
+
+This behaviour can be explicitly overriden, as demonstrated below:
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+{% highlight java %}
+final StreamExecutionEnvironment env =
StreamExecutionEnvironment.getExecutionEnvironment();
+
+FlinkKafkaConsumer08<String> myConsumer = new FlinkKafkaConsumer08<>(...);
+myConsumer.setStartFromEarliest(); // start from the earliest record
possible
+myConsumer.setStartFromLatest(); // start from the latest record
+myConsumer.setStartFromGroupOffsets(); // the default behaviour
--- End diff --
Okay, cool. Can you add that to the docs as well?
> 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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