In general, language-neutral APIs and protocols are a key feature of portable
Beam.
Yes, sure, that is well understood. But - language neutral APIs requires
language neutral environment. That is why the portable Pipeline representation
is built around protocol buffers and gRPC. That is truly language-neutral. Once
we implement something around that - like in the case of ModelCoders.java - we
use a specific language for that and the language-neutral part is already gone.
The decision to include same-language-SDK coders into such language-specific
object plays no role in the fact it already is language-specific.
Not all runners are implemented using Java. For example, the portable
DirectRunner (FnAPI runner) is implemented using Python and Dataflow is
implemented using C++. Such runners will not be able to do this.
Yes, I'm aware of that and that is why I said "any Java native runner". It is
true, that non-Java runners *must* (as long as we don't include Read into SDK harness)
resort to expanding it to SDF. That is why use_deprecated_read is invalid setting for
such runner and should be handled accordingly.
Similarly, I think there were previous discussions related to using SDF as the
source framework for portable runners.
Don't get me wrong, I'm not trying to revoke this decision. On the other hand I
still think that the decision to use SDF implementation of Read or not should
be left to the runner.
I understand that there are some bugs related to SDF and portable Flink currently. How
much work do you think is needed here ? Will it be better to focus our efforts on fixing
remaining issues for SDF and portable runners instead of supporting
"use_deprecated_read" for that path ?
I'm not sure. I don't know portability and the SDK harness well enough to be
able to answer this. But we should really know why we do that. What exactly
does SDF bring to the Flink runner (and let's leave Flink aside of this - what
does it bring to runners that cannot make use of dynamic splitting, being it
admittedly a very cool feature)? Yes, supporting Java Read makes it impossible
to implement it in Python. But practically, I think that most of the Pipelines
will use x-lang for that. It makes very much sense to offload IOs to a more
performant environment.
Jan
On 7/25/21 6:54 PM, Chamikara Jayalath wrote:
On Sun, Jul 25, 2021 at 6:33 AM Jan Lukavský <[email protected]> wrote:
I'll start from the end.
I don't think we should be breaking language agnostic API layers (for example, definition
of model coders) just to support "use_deprecated_read".
"Breaking" and "fixing" can only be a matter of the definition of the object at hand. I
don't think, that Coder can be totally language agnostic - yes, the mapping between serialized form and
deserialized form can be _defined_ in a language agnostic way, but must be_implemented_ in a specific
language. If we choose the implementing language, what makes us treat SDK-specific coders defined by the SDK
of the same language as "unknown"? It is only our decision, that seems to have no practical
benefits.
In general, language-neutral APIs and protocols are a key feature of portable
Beam. See here: https://beam.apache.org/roadmap/portability/
(I did not look into all the old discussions and votes related to this but I'm
sure they are there)
Moreover, including SDK-specific coders into supported coders of the SDK runner construction
counterpart (that is, runner core-construction-java for Java SDK) is a necessary prerequisite for
unifying "classical" and "portable" runners, because the runner needs to
understand *all* SDK coders so that it can _inline_ the complete Pipeline (if the Pipeline SDK has
the same language as the runner), instead of running it through SDK harness. This need therefore is
not specific to supporting use_deprecated_read, but is a generic requirement, which only has the
first manifestation in the support of a transform not supported by SDK harness.
I think "use_deprecated_read" should be considered a stop-gap measure for Flink (and
Spark ?) till we have proper support for SDF. In fact I don't think an arbitrary portable runner
can support "use_deprecated_read" due to the following.
There seems to be nothing special about Flink regarding the support of
primitive Read. I think any Java native runner can implement it pretty much the
same way as Flink does. The question is if any other runner might want to do
that. The problem with Flink is that
Not all runners are implemented using Java. For example, the portable
DirectRunner (FnAPI runner) is implemented using Python and Dataflow is
implemented using C++. Such runners will not be able to do this.
1) portable SDF seems not to work [1]
2) even classical Flink runner has still issues with SDF - there are reports
of watermark being stuck when reading data via SDF, this gets resolved using
use_deprecated_read
3) Flink actually does not have any benefits from SDF, because it cannot make
use of the dynamic splitting, so this actually brings only implementation
burden without any practical benefit
Similarly, I think there were previous discussions related to using SDF as the
source framework for portable runners.
I understand that there are some bugs related to SDF and portable Flink currently. How much work do
you think is needed here ? Will it be better to focus our efforts on fixing remaining issues for
SDF and portable runners instead of supporting "use_deprecated_read" for that path ? Note
that I'm fine with fixing any issues related to "use_deprecated_read" for classic
(non-portable) Flink but I think you are trying to use x-lang hence probably need portable Flink.
Thanks,
Cham
I think that we should reiterate on the decision of deprecating Read - if we
can implement it via SDF, what is the reason to forbid a runner to make use of
a simpler implementation? The expansion of Read might be runner dependent, that
is something we do all the time, or am I missing something?
Jan
[1] https://issues.apache.org/jira/browse/BEAM-10940
On 7/25/21 1:38 AM, Chamikara Jayalath wrote:
I think we might be going down a bit of a rabbit hole with the support for
"use_deprecated_read" for portable Flink :)
I think "use_deprecated_read" should be considered a stop-gap measure for Flink (and
Spark ?) till we have proper support for SDF. In fact I don't think an arbitrary portable runner
can support "use_deprecated_read" due to the following.
(1) SDK Harness is not aware of BoundedSource/UnboundedSource. Only source
framework SDK Harness is aware of is SDF.
(2) Invoking BoundedSource/UnboundedSource is not a part of the Fn API
(3) A non-Java Beam portable runner will probably not be able to directly
invoke legacy Read transforms similar to the way Flink does today.
I don't think we should be breaking language agnostic API layers (for example, definition
of model coders) just to support "use_deprecated_read".
Thanks,
Cham
On Sat, Jul 24, 2021 at 11:50 AM Jan Lukavský <[email protected]> wrote:
On 7/24/21 12:34 AM, Robert Bradshaw wrote:
On Thu, Jul 22, 2021 at 10:20 AM Jan Lukavský <[email protected]> wrote:
Hi,
this was a ride. But I managed to get that working. I'd like to discuss two
points, though:
a) I had to push Java coders to ModelCoders for Java (which makes sense to
me, but is that correct?). See [1]. It is needed so that the Read transform
(executed directly in TaskManager) can correctly communicate with Java SDK
harness using custom coders (which is tested here [2]).
I think the intent was that ModelCoders represent the set of
language-agnostic in the model, though I have to admit I've always
been a bit fuzzy on when a coder must or must not be in that list.
I think that this definition works as long, as runner does not itself
interfere with the Pipeline. Once the runner starts (by itself, not via
SdkHarnessClient) producing data, it starts to be part of the
environment, and therefore it should understand its own Coders. I'd
propose the definition of "model coders" to be Coders that the SDK is
able to understand, which then works naturally for the ModelCoders
located in "core-construction-java", that it should understand Javs SDK
Coders.
b) I'd strongly prefer if we moved the handling of use_deprecated_read from
outside of the Read PTransform directly into expand method, see [3]. Though
this is not needed for the Read on Flink to work, it seems cleaner.
WDYT?
The default value of use_deprecated_read should depend on the runner
(e.g. some runners don't work well with it, others require it). As
such should not be visible to the PTransform's expand.
I think we should know what is the expected outcome. If a runner does
not support primitive Read (and therefore use_deprecated_read), what
should we do, if we have such experiment set? Should the Pipeline fail,
or should it be silently ignored? I think that we should fail, because
user expects something that cannot be fulfilled. Therefore, we have two
options - handling the experiment explicitly in runners that do not
support it, or handle it explicitly in all cases (both supported and
unsupported). The latter case is when we force runners to call explicit
conversion method (convertPrimitiveRead....). Every runner that does not
support primitive Read must handle the experiment either way, because
otherwise the experiment would be simply silently ignored, which is not
exactly user-friendly.
Jan
[1]
https://github.com/apache/beam/pull/15181/commits/394ddc3fdbaacc805d8f7ce02ad2698953f34375
[2]
https://github.com/apache/beam/pull/15181/files#diff-b1ec58edff6c096481ff336f6fc96e7ba5bcb740dff56c72606ff4f8f0bf85f3R201
[3]
https://github.com/apache/beam/pull/15181/commits/f1d3fd0217e5513995a72e92f68fe3d1d665c5bb
On 7/18/21 6:29 PM, Jan Lukavský wrote:
Hi,
I was debugging the issue and it relates to pipeline fusion - it seems that the
primitive Read transform gets fused and then is 'missing' as source. I'm a
little lost in the code, but the most strange parts are that:
a) I tried to reject fusion of primitive Read by adding
GreedyPCollectionFusers::cannotFuse for
PTransformTranslation.READ_TRANSFORM_URN to
GreedyPCollectionFusers.URN_FUSIBILITY_CHECKERS, but that didn't change the
exception
b) I tried adding Reshuffle.viaRandomKey between Read and PAssert, but that
didn't change it either
c) when I run portable Pipeline with use_deprecated_read on Flink it
actually runs (though it fails when it actually reads any data, but if the
input is empty, the job runs), so it does not hit the same issue, which is a
mystery to me
If anyone has any pointers that I can investigate, I'd be really grateful.
Thanks in advance,
Jan
On 7/16/21 2:00 PM, Jan Lukavský wrote:
Hi,
I hit another issue with the portable Flink runner. Long story short - reading
from Kafka is not working in portable Flink. After solving issues with
expansion service configuration (ability to add use_deprecated_read) option,
because flink portable runner has issues with SDF [1], [2]. After being able to
inject the use_deprecated_read into expansion service I was able to get an
execution DAG that has the UnboundedSource, but then more and more issues
appeared (probably related to missing LengthPrefixCoder somewhere - maybe at
the output from the primitive Read). I wanted to create a test for it and I
found out, that there actually is ReadSourcePortableTest in FlinkRunner, but
_it tests nothing_. The problem is that Read is transformed to SDF, so this
test tests the SDF, not the Read transform. As a result, the Read transform
does not work.
I tried using convertReadBasedSplittableDoFnsToPrimitiveReads so that I could
make the test fail and debug that, but I got into
java.lang.IllegalArgumentException: PCollectionNodes
[PCollectionNode{id=PAssert$0/GroupGlobally/ParDo(ToSingletonIterables)/ParMultiDo(ToSingletonIterables).output,
PCollection=unique_name:
"PAssert$0/GroupGlobally/ParDo(ToSingletonIterables)/ParMultiDo(ToSingletonIterables).output"
coder_id: "IterableCoder"
is_bounded: BOUNDED
windowing_strategy_id: "WindowingStrategy(GlobalWindows)"
}] were consumed but never produced
which gave me the last knock-out. :)
My current impression is that starting from Beam 2.25.0, portable FlinkRunner
is not able to read from Kafka. Could someone give me a hint about what is
wrong with using convertReadBasedSplittableDoFnsToPrimitiveReads in the test
[3]?
Jan
[1] https://issues.apache.org/jira/browse/BEAM-11991
[2] https://issues.apache.org/jira/browse/BEAM-11998
[3] https://github.com/apache/beam/pull/15181
