At a top level `setWindowingStrategyInternal` exists to set up the metadata without actually assigning windows. If we were more clever we might have found a way for it to not be public... it is something that can easily lead to an invalid pipeline.
I think "compatible windows" today in Beam doesn't have very good uses anyhow. I do see how when you are flattening PCollections you might also want to explicitly have a function that says "and here is how to reconcile their different metadata". But is it not reasonable to use Window.into(global window)? It doesn't seem like boilerplate to me actually, but something you really want to know is happening. 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 >> > >> >