I would rather say that "runners-core" is a utility library with some helpful things. Like other libraries. The runner still decides how to use the library. That was the idea, anyhow. A runner could have a bunch of "if" statements around how it uses some generic runners-core utility, etc. I think at this point the proposal is trying to solve a problem we may not have.
Kenn On Thu, Feb 20, 2020 at 1:25 PM Jan Lukavský <[email protected]> wrote: > > On 2/20/20 8:24 PM, Robert Bradshaw wrote: > > On Thu, Feb 13, 2020 at 12:42 PM Jan Lukavský <[email protected]> > <[email protected]> wrote: > > Hi, > > +1 for adding pipeline required features. I think being able to reject > pipeline with unknown requirement is pretty much needed, mostly because that > enables runners to completely decouple from SDKs, while being able to > recognize when a pipeline constructed with incomplatible version of SDK is > run. > > I'll add some observations I made when implementing the latest "requires time > sorted input" addition with regards to this discussion: > > a) the features of pipeline are not simple function of set of PTransforms > being present in the pipeline, but also depend on (type of) inputs. For > instance a PTransform might have a simple expansion to primitive PTransforms > in streaming case, but don't have such expansion in batch case. That is to > say, runner that doesn't actually know of a specific extension to some > PTransform _might_ actually execute it correctly under some conditions. But > _must_ fail in other cases. > > It sounds like what you're getting at here is a Statful ParDo that > requires "mostly" time sorted input (to keep the amount of state held > bounded) which is somewhat provided (with no bounds given) for > unbounded PCollections but not at all (in general) for batch. Rather > than phrase this as a conditional requirement, I would make a new > requirement "requires mostly time sorted input" (precise definition > TBD, it's hard to specify or guarantee upper bounds) which a runner > could then implement via exact time sorted input in batch and but more > cheaply as a no-op in streaming. > > +1, that makes sense. My example was a little incomplete, in the sense > that, for @RequiresTimeSortedInput does not have any requirements on runner > in streaming case, with one exception - the runner must be compiled with > the newest runners-core. That brings us to the fact, that runners > capabilities are actually not just function of the runner's code, but also > code that is imported from runners-core. There probably should be a way for > the core to export its capabilities (e.g. provides: > beam:requirement:pardo:time_sorted_input:streaming:v1), which should then > be united with capabilities of the runner itself. That way a runner which > uses runners-core (and StatefulDoFnRunner, that is a complication, not sure > how to deal with that), could be made able to satify > 'beam:requirement:pardo:time_sorted_input:streaming:v1' > simply by recompiling the runner with newest core. > > b) it would be good if this feature would work independently of portability > (for Java SDK). We still have (at least two) non-portable runners that are > IMO widely used in production and are likely to last for some time. > > Yes. As mentioned, we can still convert to portability to do such > analysis even if we don't use it for execution. > > > c) we can take advantage of these pipeline features to get rid of the > categories of @ValidatesRunner tests, because we could have just simply > @ValidatesRunner and each test would be matched against runner capabilities > (i.e. a runner would be tested with given test if and only if it would not > reject it) > > +1 > > > Jan > > On 2/13/20 8:42 PM, Robert Burke wrote: > > +1 to deferring for now. Since they should not be modified after adoption, it > makes sense not to get ahead of ourselves. > > On Thu, Feb 13, 2020, 10:59 AM Robert Bradshaw <[email protected]> > <[email protected]> wrote: > > On Thu, Feb 13, 2020 at 10:12 AM Robert Burke <[email protected]> > <[email protected]> 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 <[email protected]> <[email protected]> > wrote: > > On Wed, Feb 12, 2020 at 2:24 PM Kenneth Knowles <[email protected]> > <[email protected]> wrote: > > On Wed, Feb 12, 2020 at 12:04 PM Robert Bradshaw <[email protected]> > <[email protected]> wrote: > > On Wed, Feb 12, 2020 at 11:08 AM Luke Cwik <[email protected]> > <[email protected]> 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. > >
