I have a question about this. My scenario is:

* PubSub input with a timestampAttribute named doc_timestamp
* Fixed windowing of one hour size.
* Keys are an internal attribute of the messages (the type)
* Messages of one particular type are way more frequent than the others, so
it is likely a hot key

Will it help if I add a string representation of the doc_timestamp (to the
hour) to the key, in order to increase the range of keys and therefore make
it more parallelisable?

I wonder if it will help or not, as the final result will be the same (type
+ window), but not sure if it would help before the point where the
windowing is applied.


On Mon, Feb 12, 2018 at 6:41 PM Carlos Alonso <car...@mrcalonso.com> wrote:

> Ok, that makes a lot of sense. Thanks Kenneth!
> On Mon, Feb 12, 2018 at 5:41 PM Kenneth Knowles <k...@google.com> wrote:
>> Hi Carlos,
>> You are quite correct that choosing the keys is important for work to be
>> evenly distributed. The reason you need to have a KvCoder is that state is
>> partitioned per key (to give natural & automatic parallelism) and window
>> (to allow reclaiming expired state so you can process unbounded data with
>> bounded storage, and also more parallelism). To a Beam runner, most data in
>> the pipeline is "just bytes" that it cannot interpret. KvCoder is a special
>> case where a runner knows the binary layout of encoded data so it can pull
>> out the keys in order to shuffle data of the same key to the same place, so
>> that is why it has to be a KvCoder.
>> Kenn
>> On Mon, Feb 12, 2018 at 5:52 AM, Carlos Alonso <car...@mrcalonso.com>
>> wrote:
>>> I was refactoring my solution a bit and tried to make my stateful
>>> transform to work on simple case classes and I got this exception:
>>> https://pastebin.com/x4xADmvL . I'd like to understand the rationale
>>> behind this as I think carefully choosing the keys would be very important
>>> in order for the work to be properly distributed.
>>> Thanks!

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