This looks (and likely has the same cause) similar to what I have
experienced when making primitive Read supported by Flink. The final
solution would be to make SDK coders known to the runner of the same SDK
(already present in various different threads). But until then, the
solution seems to be something like [1]. The root cause is that the
executable stage expects its input to be encoded by the SDK harness, and
that part is missing when the transform is inlined (like Read in my
case, or TestStream in your case). The intoWireTypes method simulates
precisely this part - it encodes the PCollection via coder defined in
the SDK harness and then decodes it by coder defined by the runner
(which match on binary level, but produce different types).
Jan
[1]
https://github.com/apache/beam/blob/dd7945f9f259a2989f9396d1d7a8dcb122711a52/runners/flink/src/main/java/org/apache/beam/runners/flink/FlinkStreamingPortablePipelineTranslator.java#L657
On 8/31/21 7:27 PM, Luke Cwik wrote:
I originally wasn't for making it a composite because it changes the
"graph" structure but the more I thought about it the more I like it.
On Tue, Aug 31, 2021 at 10:06 AM Robert Bradshaw <[email protected]
<mailto:[email protected]>> wrote:
On Tue, Aug 31, 2021 at 9:18 AM Luke Cwik <[email protected]
<mailto:[email protected]>> wrote:
>
> On Mon, Aug 30, 2021 at 7:07 PM Ke Wu <[email protected]
<mailto:[email protected]>> wrote:
>>
>> Hello everyone,
>>
>> This is Ke. I am working on enable TestStream support for Samza
Runner in portable mode and discovers something unexpected.
>>
>> In my implementation for Samza Runner, couple of tests are
failing with errors like
>>
>>
>> java.lang.ClassCastException: java.lang.Integer cannot be cast
to [B
>>
>> I noticed these tests have the same symptom on Flink Runner as
well, which are currently excluded:
>>
>> https://issues.apache.org/jira/browse/BEAM-12048
<https://issues.apache.org/jira/browse/BEAM-12048>
>> https://issues.apache.org/jira/browse/BEAM-12050
<https://issues.apache.org/jira/browse/BEAM-12050>
>>
>>
>> After some more digging, I realized that it is because the
combination of following facts:
>>
>> TestStream is a primitive transform, therefore, Runners are
supposed to translate directly, the most intuitive implementation
for each runner to do is to parse the payload to decode
TestStream.Event [1] on the Runner process to be handed over to
subsequent stages.
>> When TestStream used with Integers, i.e. VarIntCoder to
initialize, since VarIntCoder is NOT a registered ModelCoder [2],
it will be treated as custom coder during conversion to protobuf
pipeline [3] and will be replaced with byte array coder [4] when
runner sends data to SDK worker.
>> Therefore an error occurs because the decoded TestStream.Event
has Integer as its value but the remote input receiver is
expecting byte array, causing java.lang.ClassCastException:
java.lang.Integer cannot be cast to [B
>>
>>
>> In addition, I tried to update all these failed tests to use
Long instead of Integer, and all tests will pass since
VarLongCoder is a known coder. I do understand that runner process
does not have user artifacts staged so it can only use coders in
beam model when communicating with SDK worker process.
>>
>> Couple of questions on this:
>>
>> 1. Is it expected that VarIntegerCoder is not a known coder?
>
>
> Yes since no one has worked to make it a well known coder.
The notion of "integer" vs. "long" is also language-specific detail as
well, so not sure it makes sense as a well-known coder.
> It can be made a well known coder and this would solve the
immediate problem but not the long term issue of portable
TestStream not supporting arbitrary types.
+1. Rather than making coder a property of TestStream, I would be in
favor of the TestStream primitive always producing bytes (basically,
by definition), and providing a composite that consists of this
followed by a decoding to give us a typed TestStream.
>> 2. Is TestStream always supposed to be translated the payload
as raw bytes in order that runner process can always send it to
SDK worker with the default byte array coder and asks SDK worker
to decode accordingly?
>
>
> Having the runner treat it always as bytes and not T is likely
the best solution but isn't necessary.
>
>> 3. If Yes to 2), then does it mean, TestStream needs to be
translated in a completely different way in portable mode from
classic mode since in classic mode, translator can directly
translates the payload to its final format.
>>
>
> There are a few ways to fix the current implementation to work
for all types. One way would be if we required the encoded_element
to be the "nested" encoding and then ensured that the runner uses
a WindowedValue<ByteArrayCoder in outer context> and the SDK used
WindowedValue<T> (note that this isn't
WindowedValue<LengthPrefix<T>>) for the wire coders. This is quite
annoying cause the runner inserts length prefixing in a lot of
places (effectively every time it sees an unknown type) so we
would need to special case this and propagate this correction
through any runner native transforms (e.g. GBK) until the SDK
consumes it.
>
> Another way would be to ensure that the SDK always uses
LengthPrefix<T> as the PCollection encoding and the
encoded_element format. This would mean that the runner can
translate it to a T if it so chooses and won't have the annoying
special case propagation logic. This leaks the length prefixing
into the SDK at graph construction time which is not what it was
meant for.
>
> Swapping to use an existing well known type is by far the
easiest approach as you had discovered and won't impact the
correctness of the tests.
>
>>
>> Best,
>> Ke
>>
>>
>> [1]
https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/TestStreamTranslation.java#L52
<https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/TestStreamTranslation.java#L52>
>> [2]
https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/ModelCoderRegistrar.java#L65
<https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/ModelCoderRegistrar.java#L65>
>> [3]
https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/CoderTranslation.java#L99
<https://github.com/apache/beam/blob/master/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/CoderTranslation.java#L99>
>> [4]
https://github.com/apache/beam/blob/master/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/wire/WireCoders.java#L93
<https://github.com/apache/beam/blob/master/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/wire/WireCoders.java#L93>
