The whole notion of an absolute set of known coders is a misnomer. It would require all Runners and SDKs to be updated synchronously for every new coder they might want to share.
Instead, what we have are * Standard Coders which have well-defined, language-agnostic representations and encodings, which may be used for interoperability and efficiency, and * Required Coders which are the minimum needed to execute the pipeline. The latter consists only of bytes (for impulse), kv and iterable (for GBK), windowed value (for windowing information) and length prefix (to be able to handle anything else). On Wed, Sep 8, 2021 at 3:03 PM Robert Burke <[email protected]> wrote: > Is the claim that the Standard bytes and String_utf8 coders are not "known > coders"? > > What's the point of the standard coders if they are not the canonical > "known coders" that can generally be expected to be known by runners/other > SDKs? > > > https://github.com/apache/beam/blob/master/model/pipeline/src/main/proto/beam_runner_api.proto#L790 > > The Go SDK rather heavily biases towards using the standard coders for > their closes language equivalents rather than going into override/custom > specified soup. (It's not possible to globally override the coders for the > '[]byte' and 'string' types, nor is there often reason to.) > > On Wed, Sep 8, 2021, 2:56 PM Robert Bradshaw <[email protected]> wrote: > >> On Wed, Sep 8, 2021 at 1:48 PM Jack McCluskey <[email protected]> >> wrote: >> >>> 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. >>> >> >> Known to who? >> >> >>> Based on what's been said, that isn't expected behavior, right? >>> >> >> No, it's not. >> >> I would check to make sure the Go SDK is respecting the Coder (length >> prefixed or not) that's set on the channel, rather than guessing at what it >> expects it to be based on the Go type. >> >> >>> 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] >>> >>> >>>
