Проблеми с кодера с Apache Beam и CombineFn

Ние изграждаме тръбопровод, използвайки Apache Beam и DirectRunner като бегач. В момента се опитваме да създадем прост тръбопровод, чрез който:

1. Изтеглете данни от Google Cloud Pub/Sub (в момента използвате емулатора за локално изпълнение)
2. Десериализирайте в Java обект
3. Прозоречни събития с фиксирани прозорци от 1 минута
4. Комбинирайте тези прозорци с помощта на персонализиран CombineFn, който ги трансформира от събития в списък със събития.

Код на тръбопровода:

pipeline
.apply(PubsubIO.<String>read().topic(options.getTopic()).withCoder(StringUtf8Coder.of()))

.apply("ParseEvent", ParDo.of(new ParseEventFn()))

.apply("WindowOneMinute",Window.<Event>into(FixedWindows.of(Duration.standardMinutes(1))))              

.apply("CombineEvents", Combine.globally(new CombineEventsFn()));

Функция ParseEvent:

    static class ParseEventFn extends DoFn<String, Event> {
        @ProcessElement
        public void processElement(ProcessContext c) {
            String json = c.element();
            c.output(gson.fromJson(json, Event.class));
        }
    }

Функция CombineEvents:

public static class CombineEventsFn extends CombineFn<Event, CombineEventsFn.Accum, EventListWrapper> {
        public static class Accum {
            EventListWrapper eventListWrapper = new EventListWrapper();
        }

        @Override
        public Accum createAccumulator() {
            return new Accum();
        }

        @Override
        public Accum addInput(Accum accumulator, Event event) {
            accumulator.eventListWrapper.events.add(event);
            return accumulator;
        }

        @Override
        public Accum mergeAccumulators(Iterable<Accum> accumulators) {
            Accum merged = createAccumulator();
            for (Accum accum : accumulators) {
                merged.eventListWrapper.events.addAll(accum.eventListWrapper.events);
            }
            return merged;
        }

        @Override
        public EventListWrapper extractOutput(Accum accumulator) {
            return accumulator.eventListWrapper;
        }

    }

Когато се опитваме да стартираме това локално с помощта на Maven и DirectRunner, получаваме следната грешка:

java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:293)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalStateException: Unable to return a default Coder for CombineEvents/Combine.perKey(CombineEvents)/Combine.GroupedValues/ParDo(Anonymous).out [PCollection]. Correct one of the following root causes:
  No Coder has been manually specified;  you may do so using .setCoder().
  Inferring a Coder from the CoderRegistry failed: Unable to provide a default Coder for org.apache.beam.sdk.values.KV<K, OutputT>. Correct one of the following root causes:
  Building a Coder using a registered CoderFactory failed: Cannot provide coder for parameterized type org.apache.beam.sdk.values.KV<K, OutputT>: Unable to provide a default Coder for java.lang.Object. Correct one of the following root causes:
  Building a Coder using a registered CoderFactory failed: Cannot provide coder based on value with class java.lang.Object: No CoderFactory has been registered for the class.
  Building a Coder from the @DefaultCoder annotation failed: Class java.lang.Object does not have a @DefaultCoder annotation.
  Building a Coder from the fallback CoderProvider failed: Cannot provide coder for type java.lang.Object: org.apache.beam.sdk.coders.protobuf.ProtoCoder$2@6e610150 could not provide a Coder for type java.lang.Object: Cannot provide ProtoCoder because java.lang.Object is not a subclass of com.google.protobuf.Message; org.apache.beam.sdk.coders.SerializableCoder$1@7adc59c8 could not provide a Coder for type java.lang.Object: Cannot provide SerializableCoder because java.lang.Object does not implement Serializable.
  Building a Coder from the @DefaultCoder annotation failed: Class org.apache.beam.sdk.values.KV does not have a @DefaultCoder annotation.
  Using the default output Coder from the producing PTransform failed: Unable to provide a default Coder for org.apache.beam.sdk.values.KV<K, OutputT>. Correct one of the following root causes:
  Building a Coder using a registered CoderFactory failed: Cannot provide coder for parameterized type org.apache.beam.sdk.values.KV<K, OutputT>: Unable to provide a default Coder for java.lang.Object. Correct one of the following root causes:
  Building a Coder using a registered CoderFactory failed: Cannot provide coder based on value with class java.lang.Object: No CoderFactory has been registered for the class.
  Building a Coder from the @DefaultCoder annotation failed: Class java.lang.Object does not have a @DefaultCoder annotation.
  Building a Coder from the fallback CoderProvider failed: Cannot provide coder for type java.lang.Object: org.apache.beam.sdk.coders.protobuf.ProtoCoder$2@6e610150 could not provide a Coder for type java.lang.Object: Cannot provide ProtoCoder because java.lang.Object is not a subclass of com.google.protobuf.Message; org.apache.beam.sdk.coders.SerializableCoder$1@7adc59c8 could not provide a Coder for type java.lang.Object: Cannot provide SerializableCoder because java.lang.Object does not implement Serializable.
  Building a Coder from the @DefaultCoder annotation failed: Class org.apache.beam.sdk.values.KV does not have a @DefaultCoder annotation.
    at org.apache.beam.sdk.repackaged.com.google.common.base.Preconditions.checkState(Preconditions.java:444)
    at org.apache.beam.sdk.values.TypedPValue.getCoder(TypedPValue.java:51)
    at org.apache.beam.sdk.values.PCollection.getCoder(PCollection.java:130)
    at org.apache.beam.sdk.values.TypedPValue.finishSpecifying(TypedPValue.java:90)
    at org.apache.beam.sdk.runners.TransformHierarchy.finishSpecifyingInput(TransformHierarchy.java:143)
    at org.apache.beam.sdk.Pipeline.applyInternal(Pipeline.java:418)
    at org.apache.beam.sdk.Pipeline.applyTransform(Pipeline.java:334)
    at org.apache.beam.sdk.values.PCollection.apply(PCollection.java:154)
    at org.apache.beam.sdk.transforms.Combine$Globally.expand(Combine.java:1459)
    at org.apache.beam.sdk.transforms.Combine$Globally.expand(Combine.java:1336)
    at org.apache.beam.sdk.Pipeline.applyInternal(Pipeline.java:420)
    at org.apache.beam.sdk.Pipeline.applyTransform(Pipeline.java:350)
    at org.apache.beam.sdk.values.PCollection.apply(PCollection.java:167)
at ***************************.main(***************.java:231)
... 6 more

Извинения за огромното изхвърляне на код - исках да осигуря целия контекст.

Любопитен съм защо се оплаква, че няма кодер по подразбиране както за java.lang.Object, така и за org.apache.beam.sdk.values.KV<K, OutputT> - доколкото мога да разбера, нашият конвейер променя типовете между String, Event и EventListWrapper - последните два класа имат своите кодери по подразбиране, зададени на класа себе си (AvroCoder и в двата случая).

Грешката възниква на реда, където прилагаме CombineFn - може да потвърди, че без тази трансформация конвейерът работи.

Подозирам, че сме настроили комбинираната трансформация по някакъв начин неправилно, но досега не сме намерили нищо в документацията на Beam, което да ни насочи в правилната посока.

Всяко прозрение ще бъде оценено - благодаря предварително!