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From Jan Lukavský <je...@seznam.cz>
Subject Re: Semantics of PCollection.isBounded
Date Wed, 15 May 2019 19:01:37 GMT
Hmmm, looking into the code of FlinkRunner (and also by observing 
results from the stateful ParDo), it seems, that I got it wrong from the 
beginning. The data is not sorted before the stateful ParDo, but that a 
little surprises me. How the operator should work in this case? It would 
mean, that in the batch case I have to hold arbitrarily long 
allowedLateness inside the BagState, which seems to be kind of 
suboptimal. Or am I missing something obvious here? I'll describe the 
use case in more detail, let's suppose I have a series of ones and zeros 
and I want emit at each time point value of 1 if value changes from 0 to 
1, value of -1 if changes from 1 to 0 and 0 otherwise. So:

  0, 1, 1, 0, 0, 1 -> 0, 1, 0, -1, 0, 1

Does anyone have a better idea how to solve it? And if not, how to make 
it running on batch, without possibly infinite buffer? Should the input 
to stateful ParDo be sorted in batch case? My intuition would be that it 
should be, because in my understanding of "batch as a special case of 
streaming" in batch case, there is (by default) single window, time 
advances from -inf to +inf at the end, and the data contains no out of 
order data, in places where this might matter (which therefore enables 
some optimizations). The order would be relevant only in the stateful 
ParDo, I'd say.

Jan

On 5/15/19 8:34 PM, Jan Lukavský wrote:
> Just to clarify, I understand, that changing semantics of the 
> PCollection.isBounded,  is probably impossible now, because would 
> probably introduce chicken egg problem. Maybe I will state it more 
> clearly - would it be better to be able to run bounded pipelines using 
> batch semantics on DirectRunner (including sorting before stateful 
> ParDos), or would it be better to come up with some way to notify the 
> pipeline that it will be running in a streaming way although it 
> consists only of bounded inputs? And I'm not saying how to do it, just 
> trying to find out if anyone else ever had such a need.
>
> Jan
>
> On 5/15/19 5:20 PM, Jan Lukavský wrote:
>> Hi,
>>
>> I have come across unexpected (at least for me) behavior of some 
>> apparent inconsistency of how a PCollection is processed in 
>> DirectRunner and what PCollection.isBounded signals. Let me explain:
>>
>>  - I have a stateful ParDo, which needs to make sure that elements 
>> arrive in order - it accomplishes this by defining BagState for 
>> buffering input elements and sorting them inside this buffer, it also 
>> keeps track of element with highest timestamp to somehow estimate 
>> local watermark (minus some allowed lateness), to know when to remove 
>> elements from the buffer, sort them by time and pass them to some 
>> (time ordered) processing
>>
>>  - this seems to work well for streaming (unbounded) data
>>
>>  - for batch (bounded) data the semantics of stateful ParDo should be 
>> (please correct me if I'm wrong) that elements always arrive in 
>> order, because the runner can sort them by timestamp
>>
>>  - this implies that for batch processed input (bounded) the 
>> allowedLateness can be set to zero, so that the processing is little 
>> more effective, because it doesn't have to use the BagState at all
>>
>>  - now, the trouble seems to be, that DirectRunner always uses 
>> streaming processing, even if the input is bounded (that is by 
>> definition possible), but there is no way now to know when it is 
>> possible to change allowed lateness to zero (because input will 
>> arrive ordered)
>>
>>  - so - it seems to me, that either DirectRunner should apply sorting 
>> to stateful ParDo, when it processes bounded data (the same way that 
>> other runners do), or it can apply streaming processing, but then it 
>> should change PCollection.isBounded to UNBOUNDED, even if the input 
>> is originally bounded
>>
>>  - that way, the semantics of PCollection.isBounded, would be not if 
>> the data are known in advance to be finite, but *how* the data are 
>> going to be processed, which is much more valuable (IMO)
>>
>> Any thoughts?
>>
>>  Jan
>>

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