Reuven
On Mon, Sep 6, 2021 at 1:29 AM Jan Lukavský <[email protected]
<mailto:[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
<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]
<mailto:[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]
<mailto:[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
<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
<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]
<mailto:[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] <mailto:[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).)
