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From "Tzu-Li (Gordon) Tai (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-4280) New Flink-specific option to set starting position of Kafka consumer without respecting external offsets in ZK / Broker
Date Fri, 29 Jul 2016 09:41:20 GMT
Tzu-Li (Gordon) Tai created FLINK-4280:
------------------------------------------

             Summary: 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
             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:

{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
...
{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 definition 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. 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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