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https://issues.apache.org/jira/browse/KAFKA-3775?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15312250#comment-15312250
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Matthias J. Sax commented on KAFKA-3775:
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I have some concerns about this:
1) a KStreams application should process the whole topic and not parts of it
-- limiting the number of partitions is kinda artificial from my point of view
2) even if we limit the number of partitions, it is quite random which would
get processed which not -- I would assume that users would like to have a more
transparent assignment
3) last but not least, under the hood we are using the standard Java
KafkaConsumer: looking at your patch (just briefly), it seems you changed the
task assignment -- however, this is independent from the partitions assignment
of the used consumer -- thus, the consumer would still poll all partitions but
would not be able to assign records for some partitions as the corresponding
tasks are missing.
Haven elaborated 3) this raises a more complex problem: Right now, KafkaStreams
relies on Kafka's internal partition assignment. If you want to assign only
some partitions, we cannot use standard high level Java KafkaConsumer and would
need to implement an own assignment strategy to allow for partial assignment
within a consumer group (ie, allow a consumer group that does not assign all
partitions).
> Throttle maximum number of tasks assigned to a single KafkaStreams
> ------------------------------------------------------------------
>
> Key: KAFKA-3775
> URL: https://issues.apache.org/jira/browse/KAFKA-3775
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Affects Versions: 0.10.0.0
> Reporter: Yuto Kawamura
> Assignee: Yuto Kawamura
> Fix For: 0.10.1.0
>
>
> As of today, if I start a Kafka Streams app on a single machine which
> consists of single KafkaStreams instance, that instance gets all partitions
> of the target topic assigned.
> As we're using it to process topics which has huge number of partitions and
> message traffic, it is a problem that we don't have a way of throttling the
> maximum amount of partitions assigned to a single instance.
> In fact, when we started a Kafka Streams app which consumes a topic which has
> more than 10MB/sec traffic of each partition we saw that all partitions
> assigned to the first instance and soon the app dead by OOM.
> I know that there's some workarounds considerable here. for example:
> - Start multiple instances at once so the partitions distributed evenly.
> => Maybe works. but as Kafka Streams is a library but not an execution
> framework, there's no predefined procedure of starting Kafka Streams apps so
> some users might wanna take an option to start the first single instance and
> check if it works as expected with lesster number of partitions(I want :p)
> - Adjust config parameters such as {{buffered.records.per.partition}},
> {{max.partition.fetch.bytes}} and {{max.poll.records}} to reduce the heap
> pressure.
> => Maybe works. but still have two problems IMO:
> - Still leads traffic explosion with high throughput processing as it
> accepts all incoming messages from hundreads of partitions.
> - In the first place, by the distributed system principle, it's wired that
> users don't have a away to control maximum "partitions" assigned to a single
> shard(an instance of KafkaStreams here). Users should be allowed to provide
> the maximum amount of partitions that is considered as possible to be
> processed with single instance(or host).
> Here, I'd like to introduce a new configuration parameter
> {{max.tasks.assigned}}, which limits the number of tasks(a notion of
> partition) assigned to the processId(which is the notion of single
> KafkaStreams instance).
> At the same time we need to change StreamPartitionAssignor(TaskAssignor) to
> tolerate the incomplete assignment. That is, Kafka Streams should continue
> working for the part of partitions even there are some partitions left
> unassigned, in order to satisfy this> "user may want to take an option to
> start the first single instance and check if it works as expected with
> lesster number of partitions(I want :p)".
> I've implemented the rough POC for this. PTAL and if it make sense I will
> continue sophisticating it.
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