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From Jay Kreps <>
Subject Re: Offset commit api
Date Thu, 20 Dec 2012 22:03:59 GMT
I actually recommend we just punt on implementing persistence in zk
entirely, otherwise we have to have an upgrade path to grandfather over
existing zk data to the new format. Let's just add it in the API and only
actually store it out when we redo the backend. We can handle the size
limit then too.


On Thu, Dec 20, 2012 at 1:30 PM, David Arthur <> wrote:

> No particular objection, though in order to support atomic writes of
> (offset, metadata), we will need to define a protocol for the ZooKeeper
> payloads. Something like:
>   OffsetPayload => Offset [Metadata]
>   Metadata => length prefixed string
> should suffice. Otherwise we would have to rely on the multi-write
> mechanism to keep parallel znodes in sync (I generally don't like things
> like this).
> +1 for limiting the size (1kb sounds reasonable)
> On 12/20/12 4:03 PM, Jay Kreps wrote:
>> Okay I did some assessment of use cases we have which aren't using the
>> default offset storage API and came up with one generalization. I would
>> like to propose--add a generic metadata field to the offset api on a
>> per-partition basis. So that would leave us with the following:
>> OffsetCommitRequest => ConsumerGroup [TopicName [Partition Offset
>> Metadata]]
>> OffsetFetchResponse => [TopicName [Partition Offset Metadata ErrorCode]]
>>    Metadata => string
>> If you want to store a reference to any associated state (say an HDFS file
>> name) so that if the consumption fails over the new consumer can start up
>> with the same state, this would be a place to store that. It would not be
>> intended to support large stuff (we could enforce a 1k limit or something,
>> just something small or a reference on where to find the state (say a file
>> name).
>> Objections?
>> -Jay
>> On Mon, Dec 17, 2012 at 10:45 AM, Jay Kreps <> wrote:
>>  Hey Guys,
>>> David has made a bunch of progress on the offset commit api
>>> implementation.
>>> Since this is a public API it would be good to do as much thinking
>>> up-front as possible to minimize future iterations.
>>> It would be great if folks could do the following:
>>> 1. Read the wiki here:
>>> 2. Check out the code David wrote here:
>>> In particular our hope is that this API can act as the first step in
>>> scaling the way we store offsets (ZK is not really very appropriate for
>>> this). This of course requires having some plan in mind for offset
>>> storage.
>>> I have written (and then after getting some initial feedback, rewritten)
>>> a
>>> section in the above wiki on how this might work.
>>> If no one says anything I will be taking a slightly modified patch that
>>> adds this functionality on trunk as soon as David gets in a few minor
>>> tweaks.
>>> -Jay

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