Hey all, Just catching up on the thread since I did the TestStream Go SDK implementation. The discussion about length prefixing behavior for known vs. unknown coders is interesting, since we ran into strings and byte slices getting extra length prefixes attached to them by Flink despite being known coders. Based on what's been said, that isn't expected behavior, right?
On Tue, Sep 7, 2021 at 2:46 PM Jan Lukavský <[email protected]> wrote: > On 9/7/21 6:02 PM, Reuven Lax wrote: > > Historically the DataflowRunner has been much more careful about not > breaking update, since this is a frequent operation by Dataflow users. I > think we've been less careful aboutt other runners, but as we see clearly > here Fllnk users do care about this as well, so we should probably test > upgrade compatibility for Flink. > > One strategy that Dataflow uses is to avoid embedding the Java serialized > form of a Coder in the graph, as this is a much higher risk of breakage (as > we see with the issue you llnked to). Possibly similar strategies should be > investigated for Fllink. > > +1, that would be great! > > > Reuven > > On Mon, Sep 6, 2021 at 1:29 AM Jan Lukavský <[email protected]> wrote: > >> > Unfortunately the most basic coders (e.g. bytes, string, kv, iterable) >> > care about Context because they predated this deprecation, and >> > changing coders is hard (due to no way to update the encoding for a >> > streaming pipeline). >> This is unrelated, but - regarding changing coders due to concerns about >> pipeline upgrades, we break this quite often, at least for some runners. >> Most recently [1]. >> >> > It is currently the latter for runners using this code (which not all >> > do, e.g. the ULR and Dataflow runners). I don't think we want to >> > ossify this decision as part of the spec. (Note that even what's >> > "known" and "unknown" can change from runner to runner.) >> This is interesting and unexpected for me. How do runners decide about >> how they encode elements between SDK harness and the runner? How do they >> inform the SDK harness about this decision? My impression was that this >> is well-defined at the model level. If not, then we have the reason for >> misunderstanding in this conversation. :-) >> >> Jan >> >> [1] >> >> https://lists.apache.org/thread.html/r51ee0bbaba2dcef13524a189c1f579f209483418a1568acff0e2c789%40%3Cdev.beam.apache.org%3E >> >> On 9/4/21 7:32 PM, Robert Bradshaw wrote: >> > On Sat, Sep 4, 2021 at 6:52 AM Jan Lukavský <[email protected]> wrote: >> >> On 9/3/21 9:50 PM, Robert Bradshaw wrote: >> >> >> >>> On Fri, Sep 3, 2021 at 11:42 AM Jan Lukavský<[email protected]> wrote: >> >>>> Hi Robert, >> >>>> >> >>>>> There's another hitch here for TestStream. For historical reasons, >> >>>>> coders actually represent two encodings: nested (aka self >> delimiting) >> >>>>> and unnested. TestStream elements are given as unnested encoded >> bytes, >> >>>>> but the nested encoding is required for sending data to the SDK. The >> >>>>> runner can't go from <nested encoding> to <unnested encoding> for an >> >>>>> arbitrary unknown coder. >> >>>>> >> >>>>> (Even if it weren't for this complication, to be able to send >> already >> >>>>> encoded bytes of an unknown coder to the SDK will also complicate >> the >> >>>>> logic in choosing the coder to be used for the channel and sending >> the >> >>>>> data, which is some of what you're running into (but can be solved >> >>>>> differently for inlined reads as the coder can always be known by >> the >> >>>>> runner).) >> >>>> It is hard for me to argue with "historical reasons". But - the >> "nested" >> >>>> and "unnested" coders look very similar to SDK-coder and runner-coder >> >>>> spaces. >> >>> Unfortunately, they're actually orthogonal to that. >> >> Hm, do you mean the Context passed to the encode/decode method? [1] >> That >> >> seems to be deprecated, I assume that most coders use the default >> >> implementation and simply ignore the Context? >> > Unfortunately the most basic coders (e.g. bytes, string, kv, iterable) >> > care about Context because they predated this deprecation, and >> > changing coders is hard (due to no way to update the encoding for a >> > streaming pipeline). >> > >> >> Even if not - whether or >> >> not the elements are encoded using NESTED Context or UNNESTED Context >> >> should be part of the contract of TestStream, right? Most likely it is >> >> the UNNESTED one, if I understand correctly what that does. Under what >> >> conditions is the deprecated encode/decode method used? >> > Yes, it's the UNNESTED one. >> > >> >> [1] >> >> >> https://github.com/apache/beam/blob/c4e0b4ac0777f37f5eb775a8a83c56f66b3baac3/sdks/java/core/src/main/java/org/apache/beam/sdk/coders/Coder.java#L134 >> >> >> >>>> The runner's responsibility is not to go from "<nested >> >>>> encoding>" (SDK coder) to "<unnested encoding>" for arbitrary coder. >> >>>> That is really impossible. But a coder is a function, right? Function >> >>>> maps from universe A to universe B (in general). TestStream provides >> a >> >>>> set of elements, and these elements are the "universe". For those >> >>>> elements it also provides the encoded form, which can be interpreted >> as >> >>>> the definition of the coder. >> >>> The problem here is that there is not "the encoded form" for a Coder >> >>> but two encoded forms, and we have the wrong one. Things could be made >> >>> to work if we had the other. >> >> Which two encoded forms do you refer to? Elements encoded by both the >> >> SDK-coder and runner-coder (and I ignore the Context here once again) >> >> have the same binary representation (which they must have, otherwise it >> >> would be impossible to decode elements coming from the runner to the >> >> SDK-harness or vice-versa). >> >>>> Therefore - technically (and formally) - >> >>>> the SDK coder for the TestStream is known to the runner, regardless >> of >> >>>> the language the runner is written in. >> >>>> >> >>>> To move this discussion forward, I think we should look for answers >> to >> >>>> the following questions: >> >>>> >> >>>> a) do we have any clues that show, that the proposed "in runner" >> >>>> solution will not work? >> >>> OK, thinking about it some more, in the TestStream, we can use the >> >>> happy coincidence that >> >>> >> >>> LengthPrefixed(C).encode(x, nested=True) == >> >>> VarLong.encode(len(C.encode(x, nested=False))) || C.encode(x, >> >>> nested=False) >> >>> >> >>> (where || denotes concatenation) and the fact that we have >> >>> >> >>> C.encode(x, nested=False) >> >>> >> >>> in hand. >> >>> >> >>> A possible fix here for the OP's question is that when rehydrating the >> >>> TestStream transform it must behave differently according to the coder >> >>> used in the subsequent channel (e.g. for known coders, it decodes the >> >>> elements and emits them directly, but for unknown coders, it prefixes >> >>> them with their length and emits byte strings. It gets more >> >>> complicated for nested coders, e.g. for a KV<known-coder, >> >>> unknown-coder> the channel might be LP(KV<known-coder, unknown-coder)) >> >>> or KV<known-coder, LP(unknown-coder)) which have different encodings >> >>> (and the latter, which is the default, requires transcoding the bytes >> >>> to inject the length in the middle which is found by decoding the >> >>> first component). As well as getting more complex, this really seems >> >>> to violate the spirit of separation of concerns. >> >> How do we make the decision if the channel is LP<KV<..>> or >> >> KV<LP<unknown>, known>? From my understanding it is always the latter, >> >> because of [2]. >> >> >> >> [2] >> >> >> https://github.com/apache/beam/blob/c4e0b4ac0777f37f5eb775a8a83c56f66b3baac3/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/wire/LengthPrefixUnknownCoders.java#L48 >> > It is currently the latter for runners using this code (which not all >> > do, e.g. the ULR and Dataflow runners). I don't think we want to >> > ossify this decision as part of the spec. (Note that even what's >> > "known" and "unknown" can change from runner to runner.) >> > >> >>>> b) do we think, that it will not be robust enough to incorporate >> the >> >>>> other use-cases (line generic transform inlining, taking into account >> >>>> that this applies only to runners that are written in the same >> language >> >>>> as the submitting SDK, because otherwise, there is nothing to >> inline)? >> >>> Being in the same language is not a prerequisite to "inlining," e.g. >> >>> the PubSub source on Dataflow is recognized as such and not executed >> >>> as SDK code but natively. >> >> Agree, that is actually exactly what happens with the TestStream. The >> >> transform need not be in the same language, as long as it is completely >> >> understood by the runner, including the SDK-coder (either explicitly - >> >> which might be due to the PCollection coder being composed of >> well-known >> >> coders only, or implicitly like in the case of TestStream, where the >> >> elements are encoded using the SDK coder. >> >>> It is more likely that inlining occurs in the same language if there >> >>> are UDFs involved. >> >>> >> >>>> I'm convinced, that the TestStream-decode expansion solution is an >> >>>> ad-hoc solution to a generic problem, which is why I'm still >> bothering >> >>>> this mailing list with my emails on this. :-) >> >>>> >> >>>> WDYT? >> >>> While not a solution to the general problem, I think the >> >>> TestStream-only-does-bytes simplifies its definition (primitives >> >>> should have as simple/easy to implement definitions as possible) and >> >>> brings it closer to the other root we have: Impulse. (We could go a >> >>> step further and rather than emitting encoded elements, with the data >> >>> in the proto itself, it emits sequence numbers, and a subsequent ParDo >> >>> maps those to concrete elements (e.g. via an in-memory map), but that >> >>> further step doesn't buy much...) >> >>> >> >>> Only runners that want to do inlining would have to take on the >> >>> complexity of a fully generic solution. >> >> I think that if the simplification brings something, we can do that, >> but >> >> I'd like to understand why we cannot (or should not) use the generic >> >> solution. I think it definitely *should* be possible to use a generic >> >> solution, because otherwise the solution would not be generic. And it >> >> would imply, that we are unable to do generic transform inlining, which >> >> I would find really strange. That would immediately mean, that we are >> >> unable to construct classical runner as a special case of the portable >> >> one, which would be bad I think. >> >> >> >> The elements in the TestStreamPayload are encoded with pure SDK-coder, >> >> or does this go through the LengthPrefixUnknownCoders logic? If not, >> >> then the problem would be there, because that means, that the SDK-coder >> >> cannot be (implicitly) defined in the runner. If the elements would be >> >> encoded using LP, then it would be possible to decode them using >> >> runner-coder and the problem should be solved, or am I still missing >> >> some key parts? >> > Yes, the problem is precisely that there are (unspecified) constraints >> > on the coder used by the TestStreamPayload. Just requiring that it be >> > length prefixed is not enough, you have to make constraints on >> > sometimes pushing down the length prefixing if it's a composite (like >> > a KV) that depend on what the runner is expected to support in terms >> > of composites and/or the choices it chooses for the channel (and the >> > runner, not knowing the coder, can't transcode between these choices). >> > >> > The simpler solution is to constrain this coder to just be byte[] >> > rather than let it be a little bit flexible (but not wholly flexible). >> > >> > As for a fully generic solution, I think the issue encountered with >> > inlining Read vs. TestStream are related to this, but not really the >> > same. With TestStream one has an encoded representation of the >> > elements provided by the SDK that the Runner and has no SDK >> > representation/execution whereas with the Reads one has unencoded >> > elements in hand and a Coder that is understood by both (so long as >> > the channel can be negotiated correctly). FWIW, I think the proper >> > solution to inlining a Read (or other Transform that would typically >> > be executed in the SDK) is to treat it as a special environment (where >> > we know logically it can work) and then elide, as possible, the >> > various encodings, grpc calls, etc. that are unneeded as everything is >> > in process. >> > >> >>>> On 9/3/21 7:03 PM, Robert Bradshaw wrote: >> >>>>> On Fri, Sep 3, 2021 at 2:40 AM Jan Lukavský<[email protected]> >> wrote: >> >>>>>> On 9/3/21 1:06 AM, Robert Bradshaw wrote: >> >>>>>>> On Thu, Sep 2, 2021 at 1:03 AM Jan Lukavský<[email protected]> >> wrote: >> >>>>>>>> Hi, >> >>>>>>>> >> >>>>>>>> I had some more time thinking about this and I'll try to recap >> that. >> >>>>>>>> First some invariants: >> >>>>>>>> >> >>>>>>>> a) each PCollection<T> has actually two coders - an _SDK >> coder_ and a >> >>>>>>>> _runner coder_. These coders have the property, that each one can >> >>>>>>>> _decode_ what the other encoded, but the opposite is not true, >> the >> >>>>>>>> coders cannot _encode_ what the other _decoded_ (in general). >> >>>>>>>> >> >>>>>>>> b) when is a PCollection<T> computed inside an >> environment, the >> >>>>>>>> elements are encoded using SDK coder on the side of SDK-harness >> and >> >>>>>>>> decoded using runner coder after receiving in the runner >> >>>>>>>> >> >>>>>>>> c) under specific circumstances, the encode-decode step >> can be >> >>>>>>>> optimized out, that is the case where the SDK coder and all its >> >>>>>>>> subcoders are all well-known to the runner (in the present, that >> means >> >>>>>>>> that all the parts present in the model coders set). The reason >> for that >> >>>>>>>> is that in this specific situation runner_decode(sdk_encode(X)) >> = X. >> >>>>>>>> This property is essential. >> >>>>>>> However, in general, X can only pass from the SDK to the runner >> (or >> >>>>>>> vice versa) in encoded form. >> >>>>>> In general yes, but we are (mostly) talking transform inlining >> here, so >> >>>>>> it that particular situation, the elements might be passed in >> decoded form. >> >>>>>>>> d) from b) immediately follows, that when a PTransform >> does not run in >> >>>>>>>> an environment (and this might be due to the transform being >> runner >> >>>>>>>> native, inlined, source (e.g. Impulse or TestStream)) the >> elements have >> >>>>>>>> to be encoded by SDK coder, immediately following decode by >> runner >> >>>>>>>> coder. That (surprisingly) applies even to situations when >> runner is >> >>>>>>>> implemented using different language than the client SDK, >> because it >> >>>>>>>> implies that the type of produced elements must be one of types >> encoded >> >>>>>>>> using model coders (well-known to the runner, otherwise the SDK >> will not >> >>>>>>>> be able to consume it). But - due to property c) - this means >> that this >> >>>>>>>> encode-decode step can be optimized out. This does not mean that >> it is >> >>>>>>>> not (logically) present, though. This is exactly the case of >> native >> >>>>>>>> Impulse transform. >> >>>>>>>> >> >>>>>>>> Now, from that we can conclude that on the boundary between >> executable >> >>>>>>>> stages, or between runner (inlined) transform and executable >> stage, each >> >>>>>>>> PCollection has to be encoded using SDK coder and immediately >> decoded by >> >>>>>>>> runner coder, *unless this can be optimized out* by property c). >> >>>>>>>> >> >>>>>>>> This gives us two options where to implement this encode/decode >> step: >> >>>>>>>> >> >>>>>>>> 1) completely inside runner with the possibility to >> optimize the >> >>>>>>>> encode/decode step by identity under right circumstances >> >>>>>>>> >> >>>>>>>> 2) partly in the runner and partly in the SDK - that is we >> encode >> >>>>>>>> elements of PCollection using SDK coder into bytes, pass those >> to the >> >>>>>>>> SDK harness and apply a custom decode step there. This works >> because SDK >> >>>>>>>> coder encoded elements are in byte[], and that is well-known >> coder type. >> >>>>>>>> We again only leverage property c) and optimize the SDK coder >> encode, >> >>>>>>>> runner decode step out. >> >>>>>>>> >> >>>>>>>> The option 2) is exactly the proposal of TestStream producing >> byte[] and >> >>>>>>>> decoding inside SDK-harness, the TestStream is actually inlined >> >>>>>>>> transform, the elements are produced directly in runner (the SDK >> coder >> >>>>>>>> is not known to the runner, but that does not matter, because the >> >>>>>>>> elements are already encoded by client). >> >>>>>>>> >> >>>>>>>> From the above it seems to me, that option 1) should be >> preferred, because: >> >>>>>>>> >> >>>>>>>> i) it is generic, applicable to all inlined transforms, >> any sources >> >>>>>>>> >> >>>>>>>> ii) it is consistent with how things logically work >> underneath >> >>>>>>>> >> >>>>>>>> iii) it offers better room for optimization - option 2) >> might result >> >>>>>>>> in cases when the elements are passed from the runner to the >> SDK-harness >> >>>>>>>> only for the sake of the decoding from SDK coder and immediately >> >>>>>>>> encoding back using SDK-coder and returned back to the runner. >> This >> >>>>>>>> would be the case when TestStream would be directly consumed by >> inlined >> >>>>>>>> (or external) transform. >> >>>>>>> (1) is not possible if the Coder in question is not known to the >> >>>>>>> Runner, which is why I proposed (2). >> >>>>>> There is no particular need for the coder to be known. If >> transform is >> >>>>>> to be inlined, what *has* to be known is the SDK-encoded form of >> the >> >>>>>> elements. That holds true if: >> >>>>>> >> >>>>>> a) either the SDK coder is known, or >> >>>>>> >> >>>>>> b) encoded form of the produced elements is known in advance >> >>>>>> >> >>>>>> For TestStream it is the case b). For inlined primitive Read (or >> any >> >>>>>> other transform which executes code) it is a). >> >>>>> There's another hitch here for TestStream. For historical reasons, >> >>>>> coders actually represent two encodings: nested (aka self >> delimiting) >> >>>>> and unnested. TestStream elements are given as unnested encoded >> bytes, >> >>>>> but the nested encoding is required for sending data to the SDK. The >> >>>>> runner can't go from <nested encoding> to <unnested encoding> for an >> >>>>> arbitrary unknown coder. >> >>>>> >> >>>>> (Even if it weren't for this complication, to be able to send >> already >> >>>>> encoded bytes of an unknown coder to the SDK will also complicate >> the >> >>>>> logic in choosing the coder to be used for the channel and sending >> the >> >>>>> data, which is some of what you're running into (but can be solved >> >>>>> differently for inlined reads as the coder can always be known by >> the >> >>>>> runner).) >> > -- Jack McCluskey SWE - DataPLS PLAT/ Beam Go RDU [email protected]
