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[英]How do I set the coder for a PCollection<List<String>> in Apache Beam?
[英]Coder issues with Apache Beam and CombineFn
我们正在使用Apache Beam和DirectRunner
作为跑步者来构建管道。 我们目前正在尝试一个简单的管道,我们:
CombineFn
组合这些窗口,将它们从事件转换为事件列表。 管道代码:
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()));
ParseEvent函数:
static class ParseEventFn extends DoFn<String, Event> {
@ProcessElement
public void processElement(ProcessContext c) {
String json = c.element();
c.output(gson.fromJson(json, Event.class));
}
}
CombineEvents功能:
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;
}
}
尝试使用Maven和DirectRunner
在本地运行时,我们收到以下错误:
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
为巨大的代码转储道歉 - 想要提供所有上下文。
我很好奇为什么它抱怨没有java.lang.Object
和org.apache.beam.sdk.values.KV<K, OutputT>
默认编码器 - 据我所知,我们的管道正在改变类型之间String
, Event
和EventListWrapper
- 后两个类在类本身上设置了默认编码器(两种情况下都是AvroCoder
)。
错误发生在我们应用CombineFn的行上 - 可以确认没有这个转换,管道工作。
我怀疑我们在某种程度上错误地设置了联合变换,但是迄今为止还没有在Beam文档中找到任何指示我们正确方向的东西。
任何见解将不胜感激 - 提前感谢!
你看到java.lang.Object
的可能原因是因为Beam试图推断出一个未解析的类型变量的编码器,它将被解析为Object
。 这可能是如何在Combine
完成编码器推断的错误。
另外,我希望Accum
类也会导致编码器推断失败。 您可以覆盖getAccumulatorCoder
中的CombineFn
以直接提供一个。
您是否检查过将Serializable添加到累加器是否可以正常工作?
所以在Accum类中添加“implements Serializable”...
public static class Accum implements Serializable {
EventListWrapper eventListWrapper = new EventListWrapper();
}
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