Apache Beam - Unable to infer a Coder on a DoFn with multiple output tags - java-8

Question

I am trying to execute a pipeline using Apache Beam but I get an error when trying to put some output tags:
import com.google.cloud.Tuple;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import org.apache.beam.sdk.Pipeline;
import org.apache.beam.sdk.io.gcp.pubsub.PubsubIO;
import org.apache.beam.sdk.io.gcp.pubsub.PubsubMessage;
import org.apache.beam.sdk.options.PipelineOptionsFactory;
import org.apache.beam.sdk.transforms.DoFn;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.transforms.windowing.FixedWindows;
import org.apache.beam.sdk.transforms.windowing.Window;
import org.apache.beam.sdk.values.TupleTag;
import org.apache.beam.sdk.values.TupleTagList;
import org.joda.time.Duration;
import java.lang.reflect.Type;
import java.util.Map;
import java.util.stream.Collectors;
/**
* The Transformer.
*/
class Transformer {
final static TupleTag<Map<String, String>> successfulTransformation = new TupleTag<>();
final static TupleTag<Tuple<String, String>> failedTransformation = new TupleTag<>();
/**
* The entry point of the application.
*
* #param args the input arguments
*/
public static void main(String... args) {
TransformerOptions options = PipelineOptionsFactory.fromArgs(args)
.withValidation()
.as(TransformerOptions.class);
Pipeline p = Pipeline.create(options);
p.apply("Input", PubsubIO
.readMessagesWithAttributes()
.withIdAttribute("id")
.fromTopic(options.getTopicName()))
.apply(Window.<PubsubMessage>into(FixedWindows
.of(Duration.standardSeconds(60))))
.apply("Transform",
ParDo.of(new JsonTransformer())
.withOutputTags(successfulTransformation,
TupleTagList.of(failedTransformation)));
p.run().waitUntilFinish();
}
/**
* Deserialize the input and convert it to a key-value pairs map.
*/
static class JsonTransformer extends DoFn<PubsubMessage, Map<String, String>> {
/**
* Process each element.
*
* #param c the processing context
*/
#ProcessElement
public void processElement(ProcessContext c) {
String messagePayload = new String(c.element().getPayload());
try {
Type type = new TypeToken<Map<String, String>>() {
}.getType();
Gson gson = new Gson();
Map<String, String> map = gson.fromJson(messagePayload, type);
c.output(map);
} catch (Exception e) {
LOG.error("Failed to process input {} -- adding to dead letter file", c.element(), e);
String attributes = c.element()
.getAttributeMap()
.entrySet().stream().map((entry) ->
String.format("%s -> %s\n", entry.getKey(), entry.getValue()))
.collect(Collectors.joining());
c.output(failedTransformation, Tuple.of(attributes, messagePayload));
}
}
}
}
The error shown is:
Exception in thread "main" java.lang.IllegalStateException: Unable to
return a default Coder for Transform.out1 [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 Coder for V. Building a
Coder using a registered CoderProvider failed. See suppressed
exceptions for detailed failures. Using the default output Coder from
the producing PTransform failed: Unable to provide a Coder for V.
Building a Coder using a registered CoderProvider failed.
I tried different ways to fix the issue but I think I just do not understand what is the problem. I know that these lines cause the error to happen:
.withOutputTags(successfulTransformation,TupleTagList.of(failedTransformation))
but I do not get which part of it, what part needs a specific Coder and what is "V" in the error (from "Unable to provide a Coder for V").
Why is the error happening? I also tried to look at Apache Beam's docs but they do not seems to explain such a usage nor I understand much from the section discussing about coders.
Thanks

Answer 1

First, I would suggest the following -- change:
final static TupleTag<Map<String, String>> successfulTransformation =
new TupleTag<>();
final static TupleTag<Tuple<String, String>> failedTransformation =
new TupleTag<>();
into this:
final static TupleTag<Map<String, String>> successfulTransformation =
new TupleTag<Map<String, String>>() {};
final static TupleTag<Tuple<String, String>> failedTransformation =
new TupleTag<Tuple<String, String>>() {};
That should help the coder inference determine the type of the side output. Also, have you properly registered a CoderProvider for Tuple?

Answer 2

Thanks to #Ben Chambers' answer, Kotlin is:
val successTag = object : TupleTag<MyObj>() {}
val deadLetterTag = object : TupleTag<String>() {}