On Thu, Aug 3, 2017 at 6:08 PM, Eugene Kirpichov <kirpic...@google.com.invalid> wrote: > https://github.com/apache/beam/pull/3649 has landed. The main contribution > of this PR is deprecating PTransform.getDefaultOutputCoder(). > > Next steps are to get rid of all setCoder() calls in the SDK, and deprecate > setCoder(). > Nearly all setCoder() calls (perhaps simply all?) I found are on the output > of mapping transforms, such as ParDo, Map/FlatMapElements, WithKeys. > I think we should simply make these transforms optionally configurable with > an output coder: e.g. input.apply(ParDo.of(new > SomeFn<>()).withOutputCoder(SomeCoder.of())) > For multi-output ParDo this is a little more complex API-wise, but doable > too. > > (another minor next step is to say in PTransform Style Guide that the > transform must set a coder on all its outputs) > > Sounds reasonable?
+1 I'd like to do this in a way that lowers the burden for all PTransform authors. Can't think of a better way than a special subclass of PTransform that has the setters that one would subclass... > On Thu, Aug 3, 2017 at 5:34 AM Lukasz Cwik <lc...@google.com.invalid> wrote: > >> I'm for (1) and am not sure about the feasibility of (2) without having an >> escape hatch that allows a pipeline author to specify a coder to handle >> their special case. >> >> On Tue, Aug 1, 2017 at 2:15 PM, Reuven Lax <re...@google.com.invalid> >> wrote: >> >> > One interesting wrinkle: I'm about to propose a set of semantics for >> > snapshotting/in-place updating pipelines. Part of this proposal is the >> > ability to write pipelines to "upgrade" snapshots to make them compatible >> > with new graphs. This relies on the ability to have two separate coders >> for >> > the same type - the old coder and the new coder - in order to handle the >> > case where the user has changed coders in the new pipeline. >> > >> > On Tue, Aug 1, 2017 at 2:12 PM, Robert Bradshaw >> > <rober...@google.com.invalid >> > > wrote: >> > >> > > There are two concerns in this thread: >> > > >> > > (1) Getting rid of PCollection.setCoder(). Everyone seems in favor of >> > this >> > > (right?) >> > > >> > > (2) Deprecating specifying Coders in favor of specifying >> TypeDescriptors. >> > > I'm generally in favor, but it's unclear how far we can push this >> > through. >> > > >> > > Let's at least do (1), and separately state a preference for (2), >> seeing >> > > how fare we can push it. >> > > >> > > On Thu, Jul 27, 2017 at 9:13 PM, Kenneth Knowles >> <k...@google.com.invalid >> > > >> > > wrote: >> > > >> > > > Another thought on this: setting a custom coder to support a special >> > data >> > > > distribution is likely often a property of the input to the pipeline. >> > So >> > > > setting a coder during pipeline construction - more generally, when >> > > writing >> > > > a composite transform for reuse - you might not actually have the >> > needed >> > > > information. But setting up a special indicator type descriptor lets >> > your >> > > > users map that type descriptor to a coder that works well for their >> > data. >> > > > >> > > > But Robert's example of RawUnionValue seems like a deal breaker for >> all >> > > > approaches. It really requires .getCoder() during expand() and >> > explicitly >> > > > building coders encoding information that is cumbersome to get into a >> > > > TypeDescriptor. While making up new type languages is a comfortable >> > > > activity for me :-) I don't think we should head down that path, for >> > our >> > > > users' sake. So I'll stop hoping we can eliminate this pain point for >> > > now. >> > > > >> > > > Kenn >> > > > >> > > > On Thu, Jul 27, 2017 at 8:48 PM, Kenneth Knowles <k...@google.com> >> > wrote: >> > > > >> > > > > On Thu, Jul 27, 2017 at 11:18 AM, Thomas Groh >> > <tg...@google.com.invalid >> > > > >> > > > > wrote: >> > > > > >> > > > >> introduce a >> > > > >> new, specialized type to represent the restricted >> > > > >> (alternatively-distributed?) data. The TypeDescriptor for this >> type >> > > can >> > > > >> map >> > > > >> to the specialized coder, without having to perform a significant >> > > degree >> > > > >> of >> > > > >> potentially wasted encoding work, plus it includes the assumptions >> > > that >> > > > >> are >> > > > >> being made about the distribution of data. >> > > > >> >> > > > > >> > > > > This is a very cool idea, in theory :-) >> > > > > >> > > > > For complex types with a few allocations involved and/or nontrivial >> > > > > deserialization, or when a pipeline does a lot of real work, I >> think >> > > the >> > > > > wrapper cost won't be perceptible. >> > > > > >> > > > > But for more primitive types in pipelines that don't really do >> much >> > > > > computation but just move data around, I think it could matter. >> > > Certainly >> > > > > there are languages with constructs to allow type wrappers at zero >> > cost >> > > > > (Haskell's `newtype`). >> > > > > >> > > > > This is all just speculation until we measure, like most of this >> > > thread. >> > > > > >> > > > > Kenn >> > > > > >> > > > > >> > > > >> > On Thu, Jul 27, 2017 at 11:00 AM, Thomas Groh >> > > > <tg...@google.com.invalid >> > > > >> > >> > > > >> > wrote: >> > > > >> > >> > > > >> > > +1 on getting rid of setCoder; just from a Java SDK >> perspective, >> > > my >> > > > >> ideal >> > > > >> > > world contains PCollections which don't have a user-visible >> way >> > to >> > > > >> mutate >> > > > >> > > them. >> > > > >> > > >> > > > >> > > My preference would be to use TypeDescriptors everywhere >> within >> > > > >> Pipeline >> > > > >> > > construction (where possible), and utilize the CoderRegistry >> > > > >> everywhere >> > > > >> > to >> > > > >> > > actually extract the appropriate type. The unfortunate >> > difficulty >> > > of >> > > > >> > having >> > > > >> > > to encode a union type and the lack of variable-length >> generics >> > > does >> > > > >> > > complicate that. We could consider some way of constructing >> > coders >> > > > in >> > > > >> the >> > > > >> > > registry from a collection of type descriptors (which should >> be >> > > > >> > accessible >> > > > >> > > from the point the union-type is being constructed), e.g. >> > > something >> > > > >> like >> > > > >> > > `getCoder(TypeDescriptor output, TypeDescriptor... >> components)` >> > - >> > > > that >> > > > >> > does >> > > > >> > > only permit a single flat level (but since this is being >> invoked >> > > by >> > > > >> the >> > > > >> > SDK >> > > > >> > > during construction it could also pass Coder...). >> > > > >> > > >> > > > >> > > >> > > > >> > > >> > > > >> > > On Thu, Jul 27, 2017 at 10:22 AM, Robert Bradshaw < >> > > > >> > > rober...@google.com.invalid> wrote: >> > > > >> > > >> > > > >> > > > On Thu, Jul 27, 2017 at 10:04 AM, Kenneth Knowles >> > > > >> > > > <k...@google.com.invalid> wrote: >> > > > >> > > > > On Thu, Jul 27, 2017 at 2:22 AM, Lukasz Cwik >> > > > >> > <lc...@google.com.invalid >> > > > >> > > > >> > > > >> > > > > wrote: >> > > > >> > > > >> >> > > > >> > > > >> Ken/Robert, I believe users will want the ability to set >> > the >> > > > >> output >> > > > >> > > > coder >> > > > >> > > > >> because coders may have intrinsic properties where the >> type >> > > > isn't >> > > > >> > > enough >> > > > >> > > > >> information to fully specify what I want as a user. Some >> > > cases >> > > > I >> > > > >> can >> > > > >> > > see >> > > > >> > > > >> are: >> > > > >> > > > >> 1) I have a cheap and fast non-deterministic coder but a >> > > > >> different >> > > > >> > > > slower >> > > > >> > > > >> coder when I want to use it as the key to a GBK, For >> > example >> > > > >> with a >> > > > >> > > set >> > > > >> > > > >> coder, it would need to consistently order the values of >> > the >> > > > set >> > > > >> > when >> > > > >> > > > used >> > > > >> > > > >> as the key. >> > > > >> > > > >> 2) I know a property of the data which allows me to have >> a >> > > > >> cheaper >> > > > >> > > > >> encoding. Imagine I know that all the strings have a >> common >> > > > >> prefix >> > > > >> > or >> > > > >> > > > >> integers that are in a certain range, or that a matrix is >> > > > >> > > sparse/dense. >> > > > >> > > > Not >> > > > >> > > > >> all PCollections of strings / integers / matrices in the >> > > > pipeline >> > > > >> > will >> > > > >> > > > have >> > > > >> > > > >> this property, just some. >> > > > >> > > > >> 3) Sorting comes up occasionally, traditionally in Google >> > > this >> > > > >> was >> > > > >> > > done >> > > > >> > > > by >> > > > >> > > > >> sorting the encoded version of the object >> lexicographically >> > > > >> during a >> > > > >> > > > GBK. >> > > > >> > > > >> There are good lexicographical byte representations for >> > ASCII >> > > > >> > strings, >> > > > >> > > > >> integers, and for some IEEE number representations which >> > > could >> > > > be >> > > > >> > done >> > > > >> > > > by >> > > > >> > > > >> the use of a special coder. >> > > > >> > > > >> >> > > > >> > > > > >> > > > >> > > > > Items (1) and (3) do not require special knowledge from >> the >> > > > user. >> > > > >> > They >> > > > >> > > > are >> > > > >> > > > > easily observed properties of a pipeline. My proposal >> > included >> > > > >> full >> > > > >> > > > > automation for both. The suggestion is new methods >> > > > >> > > > > .getDeterministicCoder(TypeDescriptor) and >> > > > >> > > > > .getLexicographicCoder(TypeDescriptor). >> > > > >> > > > >> > > > >> > > > Completely agree--usecases (1) and (3) are an indirect use >> of >> > > > Coders >> > > > >> > > > that are used to achieve an effect that would be better >> > > expressed >> > > > >> > > > directly. >> > > > >> > > > >> > > > >> > > > > (2) is an interesting hypothetical for massive scale where >> > > tiny >> > > > >> > > > incremental >> > > > >> > > > > optimization represents a lot of cost _and_ your data has >> > > > >> sufficient >> > > > >> > > > > structure to realize a benefit _and_ it needs to be >> > pinpointed >> > > > to >> > > > >> > just >> > > > >> > > > some >> > > > >> > > > > PCollections. I think our experience with coders so far is >> > > that >> > > > >> their >> > > > >> > > > > existence is almost entirely negative. It would be nice to >> > > > support >> > > > >> > this >> > > > >> > > > > vanishingly rare case without inflicting a terrible pain >> > point >> > > > on >> > > > >> the >> > > > >> > > > model >> > > > >> > > > > and all other users. >> > > > >> > > > >> > > > >> > > > (2) is not just about cheapness, sometimes there's other >> > > structure >> > > > >> in >> > > > >> > > > the data we can leverage. Consider the UnionCoder used in >> > > > >> > > > CoGBK--RawUnionValue has an integer value that specifies >> > > indicates >> > > > >> the >> > > > >> > > > type of it's raw Object field. Unless we want to extend the >> > type >> > > > >> > > > language, there's not a sufficient type descriptor that can >> be >> > > > used >> > > > >> to >> > > > >> > > > infer the coder. I'm dubious going down the road of adding >> > > special >> > > > >> > > > cases is the right thing here. >> > > > >> > > > >> > > > >> > > > > For example, in those cases you could encode in your >> > > > >> > > > > DoFn so the type descriptor would just be byte[]. >> > > > >> > > > >> > > > >> > > > As well as being an extremely cumbersome API, this would >> incur >> > > the >> > > > >> > > > cost of coding/decoding at that DoFn boundary even if it is >> > > fused >> > > > >> > > > away. >> > > > >> > > > >> > > > >> > > > >> On Thu, Jul 27, 2017 at 1:34 AM, Jean-Baptiste Onofré < >> > > > >> > > j...@nanthrax.net> >> > > > >> > > > >> wrote: >> > > > >> > > > >> >> > > > >> > > > >> > Hi, >> > > > >> > > > >> > >> > > > >> > > > >> > That's an interesting thread and I was wondering the >> > > > >> relationship >> > > > >> > > > between >> > > > >> > > > >> > type descriptor and coder for a while ;) >> > > > >> > > > >> > >> > > > >> > > > >> > Today, in a PCollection, we can set the coder and we >> also >> > > > have >> > > > >> a >> > > > >> > > > >> > getTypeDescriptor(). It sounds weird to me: it should >> be >> > > one >> > > > or >> > > > >> > the >> > > > >> > > > >> other. >> > > > >> > > > >> > >> > > > >> > > > >> > Basically, if the Coder is not used to define the type, >> > > > than, I >> > > > >> > > fully >> > > > >> > > > >> > agree with Eugene. >> > > > >> > > > >> > >> > > > >> > > > >> > Basically, the PCollection should define only the type >> > > > >> descriptor, >> > > > >> > > not >> > > > >> > > > >> the >> > > > >> > > > >> > coder by itself: the coder can be found using the type >> > > > >> descriptor. >> > > > >> > > > >> > >> > > > >> > > > >> > With both coder and type descriptor on the PCollection, >> > it >> > > > >> sounds >> > > > >> > a >> > > > >> > > > big >> > > > >> > > > >> > "decoupled" to me and it would be possible to have a >> > coder >> > > on >> > > > >> the >> > > > >> > > > >> > PCollection that doesn't match the type descriptor. >> > > > >> > > > >> > >> > > > >> > > > >> > I think PCollection type descriptor should be defined, >> > and >> > > > the >> > > > >> > coder >> > > > >> > > > >> > should be implicit based on this type descriptor. >> > > > >> > > > >> > >> > > > >> > > > >> > Thoughts ? >> > > > >> > > > >> > >> > > > >> > > > >> > Regards >> > > > >> > > > >> > JB >> > > > >> > > > >> > >> > > > >> > > > >> > >> > > > >> > > > >> > On 07/26/2017 05:25 AM, Eugene Kirpichov wrote: >> > > > >> > > > >> > >> > > > >> > > > >> >> Hello, >> > > > >> > > > >> >> >> > > > >> > > > >> >> I've worked on a few different things recently and ran >> > > > >> repeatedly >> > > > >> > > > into >> > > > >> > > > >> the >> > > > >> > > > >> >> same issue: that we do not have clear guidance on who >> > > should >> > > > >> set >> > > > >> > > the >> > > > >> > > > >> Coder >> > > > >> > > > >> >> on a PCollection: is it responsibility of the >> PTransform >> > > > that >> > > > >> > > outputs >> > > > >> > > > >> it, >> > > > >> > > > >> >> or is it responsibility of the user, or is it >> sometimes >> > > one >> > > > >> and >> > > > >> > > > >> sometimes >> > > > >> > > > >> >> the other? >> > > > >> > > > >> >> >> > > > >> > > > >> >> I believe that the answer is "it's responsibility of >> the >> > > > >> > transform" >> > > > >> > > > and >> > > > >> > > > >> >> moreover that ideally PCollection.setCoder() should >> not >> > > > >> exist. >> > > > >> > > > Instead: >> > > > >> > > > >> >> >> > > > >> > > > >> >> - Require that all transforms set a Coder on the >> > > > PCollection's >> > > > >> > they >> > > > >> > > > >> >> produce >> > > > >> > > > >> >> - i.e. it should never be responsibility of the user >> to >> > > "fix >> > > > >> up" >> > > > >> > a >> > > > >> > > > coder >> > > > >> > > > >> >> on >> > > > >> > > > >> >> a PCollection produced by a transform. >> > > > >> > > > >> >> >> > > > >> > > > >> >> - Since all transforms are composed of primitive >> > > transforms, >> > > > >> > saying >> > > > >> > > > >> >> "transforms must set a Coder" means simply that all >> > > > >> *primitive* >> > > > >> > > > >> transforms >> > > > >> > > > >> >> must set a Coder on their output. >> > > > >> > > > >> >> >> > > > >> > > > >> >> - In some cases, a primitive PTransform currently >> > doesn't >> > > > have >> > > > >> > > enough >> > > > >> > > > >> >> information to infer a coder for its output >> collection - >> > > > e.g. >> > > > >> > > > >> >> ParDo.of(DoFn<InputT, OutputT>) might be unable to >> > infer a >> > > > >> coder >> > > > >> > > for >> > > > >> > > > >> >> OutputT. In that case such transforms should allow the >> > > user >> > > > to >> > > > >> > > > provide a >> > > > >> > > > >> >> coder: ParDo.of(DoFn).withOutputCoder(...) [note that >> > > this >> > > > >> > differs >> > > > >> > > > from >> > > > >> > > > >> >> requiring the user to set a coder on the resulting >> > > > collection] >> > > > >> > > > >> >> >> > > > >> > > > >> >> - Corollary: composite transforms need to only >> configure >> > > > their >> > > > >> > > > primitive >> > > > >> > > > >> >> transforms (and composite sub-transforms) properly, >> and >> > > give >> > > > >> > them a >> > > > >> > > > >> Coder >> > > > >> > > > >> >> if needed. >> > > > >> > > > >> >> >> > > > >> > > > >> >> - Corollary: a PTransform with type parameters <FooT, >> > > BarT, >> > > > >> ...> >> > > > >> > > > needs >> > > > >> > > > >> to >> > > > >> > > > >> >> be configurable with coders for all of these, because >> > the >> > > > >> > > > implementation >> > > > >> > > > >> >> of >> > > > >> > > > >> >> the transform may change and it may introduce >> > intermediate >> > > > >> > > > collections >> > > > >> > > > >> >> involving these types. However, in many cases, some of >> > > these >> > > > >> type >> > > > >> > > > >> >> parameters appear in the type of the transform's >> input, >> > > > e.g. a >> > > > >> > > > >> >> PTransform<PCollection<KV<FooT, BarT>>, >> > > PCollection<MooT>> >> > > > >> will >> > > > >> > > > always >> > > > >> > > > >> be >> > > > >> > > > >> >> able to extract the coders for FooT and BarT from the >> > > input >> > > > >> > > > PCollection, >> > > > >> > > > >> >> so >> > > > >> > > > >> >> the user does not need to provide them. However, a >> coder >> > > for >> > > > >> BarT >> > > > >> > > > must >> > > > >> > > > >> be >> > > > >> > > > >> >> provided. I think in most cases the transform needs to >> > be >> > > > >> > > > configurable >> > > > >> > > > >> >> only >> > > > >> > > > >> >> with coders for its output. >> > > > >> > > > >> >> >> > > > >> > > > >> >> Here's a smooth migration path to accomplish the >> above: >> > > > >> > > > >> >> - Make PCollection.createPrimitiveOutputInternal() >> > take a >> > > > >> Coder. >> > > > >> > > > >> >> - Make all primitive transforms optionally >> configurable >> > > > with a >> > > > >> > > coder >> > > > >> > > > for >> > > > >> > > > >> >> their outputs, such as ParDo.of(DoFn). >> > withOutputCoder(). >> > > > >> > > > >> >> - By using the above, make all composite transforms >> > > shipped >> > > > >> with >> > > > >> > > the >> > > > >> > > > SDK >> > > > >> > > > >> >> set a Coder on the collections they produce; in some >> > > cases, >> > > > >> this >> > > > >> > > will >> > > > >> > > > >> >> require adding a withSomethingCoder() option to the >> > > > transform >> > > > >> and >> > > > >> > > > >> >> propagating that coder to its sub-transforms. If the >> > > option >> > > > is >> > > > >> > > unset, >> > > > >> > > > >> >> that's fine for now. >> > > > >> > > > >> >> - As a result of the above, get rid of all setCoder() >> > > calls >> > > > in >> > > > >> > the >> > > > >> > > > Beam >> > > > >> > > > >> >> repo. The call will still be there, but it will just >> not >> > > be >> > > > >> used >> > > > >> > > > >> anywhere >> > > > >> > > > >> >> in the SDK or examples, and we can mark it deprecated. >> > > > >> > > > >> >> - Add guidance to PTransform Style Guide in line with >> > the >> > > > >> above >> > > > >> > > > >> >> >> > > > >> > > > >> >> Does this sound like a good idea? I'm not sure how >> > urgent >> > > it >> > > > >> > would >> > > > >> > > > be to >> > > > >> > > > >> >> actually do this, but I'd like to know whether people >> > > agree >> > > > >> that >> > > > >> > > this >> > > > >> > > > >> is a >> > > > >> > > > >> >> good goal in general. >> > > > >> > > > >> >> >> > > > >> > > > >> >> >> > > > >> > > > >> > -- >> > > > >> > > > >> > Jean-Baptiste Onofré >> > > > >> > > > >> > jbono...@apache.org >> > > > >> > > > >> > http://blog.nanthrax.net >> > > > >> > > > >> > Talend - http://www.talend.com >> > > > >> > > > >> > >> > > > >> > > > >> >> > > > >> > > > >> > > > >> > > >> > > > >> > >> > > > >> >> > > > > >> > > > > >> > > > >> > > >> > >>
