Hi Jan, Here is my understanding,
Runner is being brought up by job server driver, which is up and running before the job submission, i.e. it is job agnostic. Therefore, the runner it brought up does not have any SDK coder available and artifact staging only happens for SDK workers. You are right that Read and TestStream are sources, however the one thing that distinguish them is that Read transform is a composite transform and the decoding happens in ParDo/ExecutableStage, i.e. on SDK worker. The proposal here is also to make the public facing TestStream transform a composite transform instead of primitive now, so that the decoding would occur on the SDK worker side where SDK coder is available, and the primitive that powers TestStream, which will be directly translated by runner to always produce raw bytes, and these raw bytes will be decoded on the SDK worker side. Best, Ke > On Aug 31, 2021, at 2:56 PM, Jan Lukavský <[email protected]> wrote: > > Sorry if I'm missing something obvious, but I don't quite see the difference > between Read and TestStream regarding the discussed issue with coders. Couple > of thoughts: > > a) both Read and TestStream are _sources_ - they produce elements that are > consumed by downstream transforms > > b) the coder of a particular PCollection is defined by the Pipeline proto - > it is the (client side) SDK that owns the Pipeline and that defines all the > coders > > c) runners must adhere to these coders, because otherwise there is risk of > coder mismatch, most probably on edges like x-lang transforms or inlined > transforms > > I tried the approach of encoding the output of Read into byte array as well, > but that turns out to have the problem that once there is a (partially) known > coder in play, this does not work, because the consuming transform > (executable stage) expects to see the wire coder - that is not simply byte > array, because the type of elements might be for instance KV<K, V>, where > KvCoder is one of ModelCoders. That does not encode using LengthPrefixCoder > and as such will be incompatible with LengthPrefixCoder(ByteArrayCoder). The > TestStream needs to know the coder of elements, because that defines where > exactly must or must not be inserted length-prefixing. The logic in > LengthPrefixUnknownCoders [1] is recursive for ModelCoders. > > [1] > https://github.com/apache/beam/blob/ff70e740a2155592dfcb302ff6303cc19660a268/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/wire/LengthPrefixUnknownCoders.java#L48 > > <https://github.com/apache/beam/blob/ff70e740a2155592dfcb302ff6303cc19660a268/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/wire/LengthPrefixUnknownCoders.java#L48> > On 8/31/21 11:29 PM, Ke Wu wrote: >> Awesome! Thank you Luke and Robert. >> >> Also created https://issues.apache.org/jira/browse/BEAM-12828 >> <https://issues.apache.org/jira/browse/BEAM-12828> to track unit test >> conversion. I could take it after I updated Samza runner to support >> TestStream in portable mode. >> >>> On Aug 31, 2021, at 2:05 PM, Robert Bradshaw <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> Created https://issues.apache.org/jira/browse/BEAM-12827 >>> <https://issues.apache.org/jira/browse/BEAM-12827> to track this. >>> >>> +1 to converting tests to just use longs for better coverage for now. >>> >>> Also, yes, this is very similar to the issues encountered by Reads, >>> but the solution is a bit simpler as there's no need for the >>> TestStream primitive to interact with the decoded version of the >>> elements (unlike Reads, where the sources often give elements in >>> un-encoded form) and no user code to run. >>> >>> - Robert >>> >>> >>> >>> On Tue, Aug 31, 2021 at 11:00 AM Jan Lukavský <[email protected] >>> <mailto:[email protected]>> wrote: >>>> >>>> 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 >>>> >>>> <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> >>
