xingHu created FLINK-6790:
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Summary: Flink Kafka Consumer Cannot Round Robin Fetch Records
Key: FLINK-6790
URL: https://issues.apache.org/jira/browse/FLINK-6790
Project: Flink
Issue Type: Bug
Components: DataStream API
Affects Versions: 1.2.1
Reporter: xingHu
The Java consumer fails consume messages in a round robin fashion. This can
lead to an unbalance consumption.
In our use case we have a set of consumer that can take a significant amount of
time consuming messages off a topic. For this reason, we are using the
pause/poll/resume pattern to ensure the consumer session is not timeout. The
topic that is being consumed has been preloaded with message. That means there
is a significant message lag when the consumer is first started. To limit how
many messages are consumed at a time, the consumer has been configured with
max.poll.records=1.
The first initial observation is that the client receive a large batch of
messages for the first partition it decides to consume from and will consume
all those messages before moving on, rather than returning a message from a
different partition for each call to poll.
We solved this issue by configuring max.partition.fetch.bytes to be small
enough that only a single message will be returned by the broker on each fetch,
although this would not be feasible if message size were highly variable.
The behavior of the consumer after this change is to largely consume from a
small number of partitions, usually just two, iterating between them, until it
exhausts them, before moving to another partition. This behavior is problematic
if the messages have some rough time semantics and need to be process roughly
time ordered across all partitions.
It would be useful if source has a pluggable API that allowed custom logic to
select which partition to consume from next, thus enabling the creation of a
round robin partition consumer.
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