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

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

Ben Chambers Ben Chambers · Accepted Answer · 2017-10-17T18:28:38

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?

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

2 Answers