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).)
