Seamus O Ceanainn created KAFKA-13448:
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Summary: Kafka Producer Client Callback behaviour does not align
with Javadoc
Key: KAFKA-13448
URL: https://issues.apache.org/jira/browse/KAFKA-13448
Project: Kafka
Issue Type: Bug
Components: clients
Affects Versions: 1.1.0
Reporter: Seamus O Ceanainn
In PR [#4188|https://github.com/apache/kafka/pull/4188] as part of KAFKA-6180,
a breaking change was accidentally introduced in the behaviour of Callbacks for
the producer client.
Previously, whenever an exception was thrown when producing an event, the value
for 'metadata' passed to the Callback.onCompletion method was always null. In
PR [#4188|https://github.com/apache/kafka/pull/4188], in one part of the code
where Callback.onCompletion is called, the behaviour was changed so that
instead of passing a null value for metadata, a 'placeholder' value was
provided instead (see
[here|https://github.com/apache/kafka/pull/4188/files#diff-42d8f5166459ee28f201ff9cec0080fc7845544a0089ac9e8f3e16864cc1193eR1196]
and
[here|https://github.com/apache/kafka/pull/4188/files#diff-42d8f5166459ee28f201ff9cec0080fc7845544a0089ac9e8f3e16864cc1193eR1199]).
This placeholder contained only topic and partition information, and with all
other fields set to '-1'.
This change only impacted a subset of exceptions, so that in the case of
ApiExceptions metadata would still be null (see
[here|https://github.com/apache/kafka/commit/aa42a11dfd99ee9ab24d2e9a7521ef1c97ae1ff4#diff-42d8f5166459ee28f201ff9cec0080fc7845544a0089ac9e8f3e16864cc1193eR843]),
but for all other exceptions the placeholder value would be used. The
behaviour at the time of writing remains the same.
This issue was first reported in KAFKA-7412 when a user first noticed the
difference between the documented behaviour of Callback.onCompletion and the
implemented behaviour.
At the time it was assumed that the behaviour when errors occur was to always
provide a placeholder metadata value to Callback.onCompletion, and the
documentation was updated at that time to reflect this assumption in [PR
#5798|https://github.com/apache/kafka/pull/5798]. The documentation now states
that when an exception occurs that the method will be called with an empty
metadata value (see
[here|https://github.com/apache/kafka/blob/3.1/clients/src/main/java/org/apache/kafka/clients/producer/Callback.java#L30-L31]).
However, there is still one case where Callback.onCompletion is called with a
null value for metadata (see
[here|https://github.com/apache/kafka/blob/3.1/clients/src/main/java/org/apache/kafka/clients/producer/KafkaProducer.java#L1002]),
so there is still a discrepancy between the documented behaviour and the
implementation of Callback.onCompletion.
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