[
https://issues.apache.org/jira/browse/KAFKA-2063?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15404147#comment-15404147
]
Jun Rao commented on KAFKA-2063:
--------------------------------
For #3, the main reason for people to customize partition-level fetch size is
to accommodate large messages. We can make the default max_reponse_bytes sth
like 10MB, which is 10 times larger than default message size. If
max_reponse_bytes >= partition-level fetch size, we just ignore the latter.
Otherwise, we can error out and advise users to increase max_reponse_bytes if
truly needed.
> 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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
