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 > > > >> > > > >> > > > > >> > > > >> > > > >> > > > > > > >> > > > > > >> > > > > >> > > > > > > > > > > > > > >
