On Thu, Feb 13, 2020 at 10:12 AM Robert Burke <rob...@frantil.com> wrote: > > One thing that doesn't appear to have been suggested yet is we could "batch" > urns together under a "super urn" so that adding one super urn is like adding > each of the represented batch of features. This prevents needing to send > dozens of urns to be individually sent over. > > > The super urns would need to be static after definition to avoid mismatched > definitions down the road. > > We collect together urns what is reasonably consider "vX" support, and can > then increment that later. > > This would simplify new SDKs, as they can have a goal of initial v1 support > as we define what level of feature support it has, and doesn't prevent new > capabilities from being added incrementally.
Yes, this is a very good idea. I've also been thinking of certain sets of common operations/well known DoFns that often occur on opposite sides of GBKs (e.g. the pair-with-one, sum-ints, drop-keys, ...) that are commonly supported that could be grouped under these meta-urns. Note that these need not be monotonic, for example a current v1 might be requiring LengthPrefixCoderV1, but if a more efficient LengthPrefixCoderV2 comes along eventually v2 could require that and *not* require the old, now rarely used LengthPrefixCoderV1. Probably makes sense to defer adding such super-urns until we notice a set that is commonly used together in practice. Of course there's still value in SDKs being able to support features piecemeal as well, which is the big reason we're avoiding a simple monotonically-increasing version number. > Similarly, certain features sets could stand alone, eg around SQL. It's > benefitial for optimization reasons if an SDK has native projection and UDF > support for example, which a runner could take advantage of by avoiding extra > cross language hops. These could then also be grouped under a SQL super urn. > > This is from the SDK capability side of course, rather than the SDK pipeline > requirements side. > > ------- > Related to that last point, it might be good to nail down early the > perspective used when discussing these things, as there's a dual between > "what and SDK can do", and "what the runner will do to a pipeline that the > SDK can understand" (eg. Combiner lifting, and state backed iterables), as > well as "what the pipeline requires from the runner" and "what the runner is > able to do" (eg. Requires sorted input) > > > On Thu, Feb 13, 2020, 9:06 AM Luke Cwik <lc...@google.com> wrote: >> >> >> >> On Wed, Feb 12, 2020 at 2:24 PM Kenneth Knowles <k...@apache.org> wrote: >>> >>> >>> >>> On Wed, Feb 12, 2020 at 12:04 PM Robert Bradshaw <rober...@google.com> >>> wrote: >>>> >>>> On Wed, Feb 12, 2020 at 11:08 AM Luke Cwik <lc...@google.com> wrote: >>>> > >>>> > We can always detect on the runner/SDK side whether there is an unknown >>>> > field[1] within a payload and fail to process it but this is painful in >>>> > two situations: >>>> > 1) It doesn't provide for a good error message since you can't say what >>>> > the purpose of the field is. With a capability URN, the runner/SDK could >>>> > say which URN it doesn't understand. >>>> > 2) It doesn't allow for the addition of fields which don't impact >>>> > semantics of execution. For example, if the display data feature was >>>> > being developed, a runner could ignore it and still execute the pipeline >>>> > correctly. >>>> >>>> Yeah, I don't think proto reflection is a flexible enough tool to do >>>> this well either. >>>> >>>> > If we think this to be common enough, we can add capabilities list to >>>> > the PTransform so each PTransform can do this and has a natural way of >>>> > being extended for additions which are forwards compatible. The >>>> > alternative to having capabilities on PTransform (and other constructs) >>>> > is that we would have a new URN when the specification of the transform >>>> > changes. For forwards compatible changes, each SDK/runner would map >>>> > older versions of the URN onto the latest and internally treat it as the >>>> > latest version but always downgrade it to the version the other party >>>> > expects when communicating with it. Backwards incompatible changes would >>>> > always require a new URN which capabilities at the PTransform level >>>> > would not help with. >>>> >>>> As you point out, stateful+splittable may not be a particularly useful >>>> combination, but as another example, we have >>>> (backwards-incompatible-when-introduced) markers on DoFn as to whether >>>> it requires finalization, stable inputs, and now time sorting. I don't >>>> think we should have a new URN for each combination. >>> >>> >>> Agree with this. I don't think stateful, splittable, and "plain" ParDo are >>> comparable to these. Each is an entirely different computational paradigm: >>> per-element independent processing, per-key-and-window linear processing, >>> and per-element-and-restriction splittable processing. Most relevant IMO is >>> the nature of the parallelism. If you added state to splittable processing, >>> it would still be splittable processing. Just as Combine and ParDo can >>> share the SideInput specification, it is easy to share relevant >>> sub-structures like state declarations. But it is a fair point that the >>> ability to split can be ignored and run as a plain-old ParDo. It brings up >>> the question of whether a runner that doesn't know SDF is should have to >>> reject it or should be allowed to run poorly. >> >> >> Being splittable means that the SDK could choose to return a continuation >> saying please process the rest of my element in X amount of time which would >> require the runner to inspect certain fields on responses. One example would >> be I don't have many more messages to read from this message stream at the >> moment and another example could be that I detected that this filesystem is >> throttling me or is down and I would like to resume processing later. >> >>> >>> It isn't a huge deal. Three different top-level URNS versus three different >>> sub-URNs will achieve the same result in the end if we get this >>> "capability" thing in place. >>> >>> Kenn >>> >>>> >>>> >>>> >> > I do think that splittable ParDo and stateful ParDo should have >>>> >> > separate PTransform URNs since they are different paradigms than >>>> >> > "vanilla" ParDo. >>>> >> >>>> >> Here I disagree. What about one that is both splittable and stateful? >>>> >> Would one have a fourth URN for that? If/when another flavor of DoFn >>>> >> comes out, would we then want 8 distinct URNs? (SplitableParDo in >>>> >> particular can be executed as a normal ParDo as long as the output is >>>> >> bounded.) >>>> > >>>> > I agree that you could have stateful and splittable dofns where the >>>> > element is the key and you share state and timers across restrictions. >>>> > No runner is capable of executing this efficiently. >>>> > >>>> >> >> > On the SDK requirements side: the constructing SDK owns the >>>> >> >> > Environment proto completely, so it is in a position to ensure the >>>> >> >> > involved docker images support the necessary features. >>>> >> >> >>>> >> >> Yes. >>>> > >>>> > >>>> > I believe capabilities do exist on a Pipeline and it informs runners >>>> > about new types of fields to be aware of either within Components or on >>>> > the Pipeline object itself but for this discussion it makes sense that >>>> > an environment would store most "capabilities" related to execution. >>>> > >>>> >> [snip] >>>> > >>>> > As for the proto clean-ups, the scope is to cover almost all things >>>> > needed for execution now and to follow-up with optional transforms, >>>> > payloads, and coders later which would exclude job managment APIs and >>>> > artifact staging. A formal enumeration would be useful here. Also, we >>>> > should provide formal guidance about adding new fields, adding new types >>>> > of transforms, new types of proto messages, ... (best to describe this >>>> > on a case by case basis as to how people are trying to modify the protos >>>> > and evolve this guidance over time). >>>> >>>> What we need is the ability for (1) runners to reject future pipelines >>>> they cannot faithfully execute and (2) runners to be able to take >>>> advantage of advanced features/protocols when interacting with those >>>> SDKs that understand them while avoiding them for older (or newer) >>>> SDKs that don't. Let's call (1) (hard) requirements and (2) (optional) >>>> capabilities. >>>> >>>> Where possible, I think this is best expressed inherently in the set >>>> of transform (and possibly other component) URNs. For example, when an >>>> SDK uses a combine_per_key composite, that's a signal that it >>>> understands the various related combine_* transforms. Similarly, a >>>> pipeline with a test_stream URN would be rejected by pipelines not >>>> recognizing/supporting this primitive. However, this is not always >>>> possible, e.g. for (1) we have the aforementioned boolean flags on >>>> ParDo and for (2) we have features like large iterable and progress >>>> support. >>>> >>>> For (1) we have to enumerate now everywhere a runner must look a far >>>> into the future as we want to remain backwards compatible. This is why >>>> I suggested putting something on the pipeline itself, but we could >>>> (likely in addition) add it to Transform and/or ParDoPayload if we >>>> think that'd be useful now. (Note that a future pipeline-level >>>> requirement could be "inspect (previously non-existent) requirements >>>> field attached to objects of type X.") >>>> >>>> For (2) I think adding a capabilities field to the environment for now >>>> makes the most sense, and as it's optional to inspect them adding it >>>> elsewhere if needed is backwards compatible. (The motivation to do it >>>> now is that there are some capabilities that we'd like to enumerate >>>> now rather than make part of the minimal set of things an SDK must >>>> support.) >>>> >> >> Agree on the separation of requirements from capabilities where requirements >> is a set of MUST understand while capabilities are a set of MAY understand. >> >>>> >>>> > All in all, I think "capabilities" is about informing a runner about >>>> > what they should know about and what they are allowed to do. If we go >>>> > with a list of "capabilities", we could always add a "parameterized >>>> > capabilities" urn which would tell runners they need to also look at >>>> > some other field. >>>> >>>> Good point. That lets us keep it as a list for now. (The risk is that >>>> it makes possible the bug of populating parameters without adding the >>>> required notification to the list.) >>>> >>>> > I also believe capabilities should NOT be "inherited". For example if we >>>> > define capabilities on a ParDoPayload, and on a PTransform and on >>>> > Environment, then ParDoPayload capabilities shouldn't be copied to >>>> > PTransform and PTransform specific capabilities shouldn't be copied to >>>> > the Environment. My reasoning about this is that some "capabilities" can >>>> > only be scoped to a single ParDoPayload or a single PTransform and >>>> > wouldn't apply generally everywhere. The best example I could think of >>>> > is that Environment A supports progress reporting while Environment B >>>> > doesn't so it wouldn't have made sense to say the "Pipeline" supports >>>> > progress reporting. >>>> > >>>> > Are capabilities strictly different from "resources" (transform needs >>>> > python package X) or "execution hints" (e.g. deploy on machines that >>>> > have GPUs, some generic but mostly runner specific hints)? At first >>>> > glance I would say yes. >>>> >>>> Agreed.
