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. 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. 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?
b) do we think, that it will not be robust enough to incorporate the
other use-cases (line generic transform inlining, taking into accoung
that this applies only to runners that are written in the same language
as the submitting SDK, because otherwise, there is nothing to inline)?
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?
Jan
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).)
