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From "Jay Kreps (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-2063) Bound fetch response size
Date Fri, 29 Jul 2016 19:43:20 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-2063?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15399909#comment-15399909
] 

Jay Kreps commented on KAFKA-2063:
----------------------------------

[~benstopford] 
1. I guess the question is how do you load balance over topics. So say your request level
max bytes is 10MB, and let's say you have lot's of backlog of data on all partitions, does
it make sense to get 10MB of a single partition or would you rather get a little from each
(I'm actually not sure, I think what you're proposing may be fine)? There may be some implications
for Streams since it tries to align partition consumption by time (cc [~guozhang]).
2. I agree that could potentially be better though we've generally preferred putting the smarts
on the server rather than the clients.

> Bound fetch response size
> -------------------------
>
>                 Key: KAFKA-2063
>                 URL: https://issues.apache.org/jira/browse/KAFKA-2063
>             Project: Kafka
>          Issue Type: Improvement
>            Reporter: Jay Kreps
>
> Currently the only bound on the fetch response size is max.partition.fetch.bytes * num_partitions.
There are two problems:
> 1. First this bound is often large. You may chose max.partition.fetch.bytes=1MB to enable
messages of up to 1MB. However if you also need to consume 1k partitions this means you may
receive a 1GB response in the worst case!
> 2. The actual memory usage is unpredictable. Partition assignment changes, and you only
actually get the full fetch amount when you are behind and there is a full chunk of data ready.
This means an application that seems to work fine will suddenly OOM when partitions shift
or when the application falls behind.
> We need to decouple the fetch response size from the number of partitions.
> The proposal for doing this would be to add a new field to the fetch request, max_bytes
which would control the maximum data bytes we would include in the response.
> The implementation on the server side would grab data from each partition in the fetch
request until it hit this limit, then send back just the data for the partitions that fit
in the response. The implementation would need to start from a random position in the list
of topics included in the fetch request to ensure that in a case of backlog we fairly balance
between partitions (to avoid first giving just the first partition until that is exhausted,
then the next partition, etc).
> This setting will make the max.partition.fetch.bytes field in the fetch request much
less useful and we  should discuss just getting rid of it.
> I believe this also solves the same thing we were trying to address in KAFKA-598. The
max_bytes setting now becomes the new limit that would need to be compared to max_message
size. This can be much larger--e.g. setting a 50MB max_bytes setting would be okay, whereas
now if you set 50MB you may need to allocate 50MB*num_partitions.
> This will require evolving the fetch request protocol version to add the new field and
we should do a KIP for it.



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