Kenn
On Tue, Apr 9, 2024 at 9:19 AM Jan Lukavský <je...@seznam.cz> wrote:
On 4/6/24 21:23, Reuven Lax via dev wrote:
So the problem here is that windowFn is a property of the
PCollection, not the element, and the result of Flatten is a
single PCollection.
Yes. That is the cause of why Flatten.pCollections() needs the
same windowFn.
In various cases, there is a notion of "compatible" windows.
Basically given window functions W1 and W2, provide a W3 that
"works" with both.
Exactly this would be a nice feature for Flatten, something like
'windowFn resolve strategy', so that if use does not know the
windowFn of upstream PCollections this can be somehow resolved at
pipeline construction time. Alternatively only as a small
syntactic sugar, something like:
Flatten.pCollections().withWindowingStrategy(WindowResolution.into(oneInput.getWindowingStrategy()))
or anything similar. This can be done in user code, so it is not
something deeper, but might help in some cases. It would be cool
if we could reuse concepts from other cases where such mechanism
is needed.
Note that Beam already has something similar with side inputs,
since the side input often is in a different window than the main
input. However main input elements are supposed to see side input
elements in the same window (and in fact main inputs are blocked
until the side-input window is ready), so we must do a mapping.
If for example (and very commonly!) the side input is in the
global window and the main input is in a fixed window, by default
we will remap the global-window elements into the main-input's
fixed window.
This is a one-sided merge function, there is a 'main' and 'side'
input, but the generic symmetric merge might be possible as well.
E.g. if one PCollection of Flatten is in GlobalWindow, I wonder if
there are cases where users would actually want to do anything
else then apply the same global windowing strategy to all input
PCollections.
Jan
In Side input we also allow the user to control this mapping, so
for example side input elements could always map to the previous
fixed window (e.g. while processing window 12-1, you want to see
summary data of all records in the previous window 11-12). Users
can do this by providing a WindowMappingFunction to the View -
essentially a function from window to window. Unfortunately this
is hard to use (one must create their own PCollectionView class)
and very poorly documented, so I doubt many users know about this!
Reuven
On Sat, Apr 6, 2024 at 7:09 AM Jan Lukavský <je...@seznam.cz> wrote:
Immediate self-correction, although setting the strategy
directly via
setWindowingStrategyInternal() *seemed* to be working during
Pipeline
construction time, during runtime it obviously does not work,
because
the PCollection was still windowed using the old windowFn.
Make sense to
me, but there remains the other question if we can make
flattening
PCollections with incompatible windowFns more user-friendly.
The current
approach where we require the same windowFn for all input
PCollections
creates some unnecessary boilerplate code needed on user side.
Jan
On 4/6/24 15:45, Jan Lukavský wrote:
> Hi,
>
> I came across a case where using
> PCollection#applyWindowingStrategyInternal seems legit in
user core.
> The case is roughly as follows:
>
> a) compute some streaming statistics
>
> b) apply the same transform (say
ComputeWindowedAggregation) with
> different parameters on these statistics yielding two windowed
> PCollections - first is global with early trigger, the
other is
> sliding window, the specific parameters of the windowFns are
> encapsulated in the ComputeWindowedAggregation transform
>
> c) apply the same transform on both of the above
PCollections,
> yielding two PCollections with the same types, but
different windowFns
>
> d) flatten these PCollections into single one (e.g. for
downstream
> processing - joining - or flushing to sink)
>
> Now, the flatten will not work, because these PCollections
have
> different windowFns. It would be possible to restore the
windowing for
> either of them, but it requires to somewhat break the
encapsulation of
> the transforms that produce the windowed outputs. A more
natural
> solution is to take the WindowingStrategy from the global
aggregation
> and set it via setWindowingStrategyInternal() to the other
> PCollection. This works, but it uses API that is marked as
@Internal
> (and obviously, the name as well suggests it is not
intended for
> client-code usage).
>
> The question is, should we make a legitimate version of
this call? Or
> should we introduce a way for Flatten.pCollections() to
re-window the
> input PCollections appropriately? In the case of conflicting
> WindowFns, where one of them is GlobalWindowing strategy,
it seems to
> me that the user's intention is quite well-defined (this
might extend
> to some 'flatten windowFn resolution strategy', maybe).
>
> WDYT?
>
> Jan
>
