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From Jun Rao <jun...@gmail.com>
Subject Re: a few questions from high level consumer documentation.
Date Thu, 09 May 2013 04:15:48 GMT
For #3, we need to checkpoint offsets to a central place so that if a
consumer fails, another consumer in the same group can pick up from where
it's left off.

For #4c, leader change doesn't introduce duplicates.



On Wed, May 8, 2013 at 9:17 AM, Yu, Libo <libo.yu@citi.com> wrote:

> Hi,
> I read this link
> https://cwiki.apache.org/KAFKA/consumer-group-example.html
> and have a few questions (if not too many).
> 1 When you say the iterator may block, do you mean hasNext() may block?
> 2 "Remember, you can only use a single process per Consumer Group."
>     Do you mean we can only use a single process on one node of the
> cluster for a consumer group?
>     Or there can be only one process on the whole cluster for a consumer
> group? Please clarify on this.
> 3 Why save offset to zookeeper? Is it easier to save it to a local file?
> 4 When client exits/crashes or leader for a partition is changed,
> duplicate messages may be replayed. "To help avoid this (replayed duplicate
> messages), make sure you provide a clean way for your client to exit
> instead of assuming it can be 'kill -9'd."
> a.       For client exit, if the client is receiving data at the time, how
> to do a clean exit? How can client tell consumer to write offset to
> zookeepr before exiting?
> b.      For client crash, what can client do to avoid duplicate messages
> when restarted? What I can think of is to read last message from log file
> and ignore the first few received duplicate messages until receiving the
> last read message. But is it possible for client to read log file directly?
> c.       For the change of the partition leader, is there anything that
> clients can do to avoid duplicates?
> Thanks.
> Libo

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