Python - Apache Beam - Flink runner setup: ReadFromKafka returns error - RuntimeError: cannot encode a null byte[]

Question

I am trying to build a streaming beam pipeline in python which should capture messages from kafka and then execute further stages of data fetching from other sources and aggregation. The step-by-step process of what I have built till now is:

1. Running Kafka instance on localhost:9092

   ./bin/kafka-server-start.sh ./config/server.properties

2. Run beam-flink job server using docker

   docker run --net=host apache/beam_flink1.10_job_server:latest

3. Run beam-kafka pipeline

import apache_beam as beam
from apache_beam.io.external.kafka import ReadFromKafka, WriteToKafka
from apache_beam.options.pipeline_options import PipelineOptions, StandardOptions


if __name__ == '__main__':
    options = PipelineOptions([
        "--job_endpoint=localhost:8099",
        "--environment_type=LOOPBACK",
        "--streaming",
        "--environment_config={\"command\":\"/opt/apache/beam/boot\"}",
    ])

    options = options.view_as(StandardOptions)
    options.streaming = True

    pipeline = beam.Pipeline(options=options)

    result = (
        pipeline

        | "Read from kafka" >> ReadFromKafka(
            consumer_config={
                "bootstrap.servers": 'localhost:9092',
            }, 
            topics=['mytopic'],
            expansion_service='localhost:8097',
        )

        | beam.Map(print)
    )

    pipeline.run()

4. Publish new message using kafka-producer.sh

./bin/kafka-console-producer.sh --broker-list localhost:9092 --topic mytopic
>tryme

After publishing this trial message, the beam pipeline perceives the message but crashes giving this error:

RuntimeError: org.apache.beam.sdk.util.UserCodeException: org.apache.beam.sdk.coders.CoderException: cannot encode a null byte[]
    at org.apache.beam.sdk.util.UserCodeException.wrap(UserCodeException.java:36)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.outputTo(FnApiDoFnRunner.java:1014)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.access$1500(FnApiDoFnRunner.java:132)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$ProcessBundleContext.outputWithTimestamp(FnApiDoFnRunner.java:1483)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$ProcessBundleContext.output(FnApiDoFnRunner.java:1478)
    at org.apache.beam.sdk.io.kafka.KafkaIO$TypedWithoutMetadata$1.processElement(KafkaIO.java:1042)
    at org.apache.beam.sdk.io.kafka.KafkaIO$TypedWithoutMetadata$1$DoFnInvoker.invokeProcessElement(Unknown Source)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.processElementForParDo(FnApiDoFnRunner.java:740)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.access$700(FnApiDoFnRunner.java:132)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$Factory.lambda$createRunnerForPTransform$1(FnApiDoFnRunner.java:203)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:216)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:179)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.outputTo(FnApiDoFnRunner.java:1011)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.access$1500(FnApiDoFnRunner.java:132)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$ProcessBundleContext.outputWithTimestamp(FnApiDoFnRunner.java:1483)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$ProcessBundleContext.output(FnApiDoFnRunner.java:1478)
    at org.apache.beam.sdk.values.ValueWithRecordId$StripIdsDoFn.processElement(ValueWithRecordId.java:138)
    at org.apache.beam.sdk.values.ValueWithRecordId$StripIdsDoFn$DoFnInvoker.invokeProcessElement(Unknown Source)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.processElementForParDo(FnApiDoFnRunner.java:740)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.access$700(FnApiDoFnRunner.java:132)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$Factory.lambda$createRunnerForPTransform$1(FnApiDoFnRunner.java:203)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:216)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:179)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.outputTo(FnApiDoFnRunner.java:1011)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.access$1500(FnApiDoFnRunner.java:132)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$ProcessBundleContext.outputWithTimestamp(FnApiDoFnRunner.java:1483)
    at org.apache.beam.sdk.transforms.DoFnOutputReceivers$WindowedContextOutputReceiver.outputWithTimestamp(DoFnOutputReceivers.java:84)
    at org.apache.beam.sdk.io.Read$UnboundedSourceAsSDFWrapperFn.processElement(Read.java:516)
    at org.apache.beam.sdk.io.Read$UnboundedSourceAsSDFWrapperFn$DoFnInvoker.invokeProcessElement(Unknown Source)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.processElementForElementAndRestriction(FnApiDoFnRunner.java:838)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.processElementForSizedElementAndRestriction(FnApiDoFnRunner.java:808)
    at org.apache.beam.fn.harness.FnApiDoFnRunner.access$200(FnApiDoFnRunner.java:132)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$Factory$2.accept(FnApiDoFnRunner.java:226)
    at org.apache.beam.fn.harness.FnApiDoFnRunner$Factory$2.accept(FnApiDoFnRunner.java:223)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:216)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:179)
    at org.apache.beam.fn.harness.BeamFnDataReadRunner.forwardElementToConsumer(BeamFnDataReadRunner.java:204)
    at org.apache.beam.fn.harness.data.QueueingBeamFnDataClient.drainAndBlock(QueueingBeamFnDataClient.java:106)
    at org.apache.beam.fn.harness.control.ProcessBundleHandler.processBundle(ProcessBundleHandler.java:295)
    at org.apache.beam.fn.harness.control.BeamFnControlClient.delegateOnInstructionRequestType(BeamFnControlClient.java:173)
    at org.apache.beam.fn.harness.control.BeamFnControlClient.lambda$processInstructionRequests$0(BeamFnControlClient.java:157)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
Caused by: org.apache.beam.sdk.coders.CoderException: cannot encode a null byte[]
    at org.apache.beam.sdk.coders.ByteArrayCoder.encode(ByteArrayCoder.java:63)
    at org.apache.beam.sdk.coders.ByteArrayCoder.encode(ByteArrayCoder.java:56)
    at org.apache.beam.sdk.coders.ByteArrayCoder.encode(ByteArrayCoder.java:41)
    at org.apache.beam.sdk.coders.KvCoder.encode(KvCoder.java:70)
    at org.apache.beam.sdk.coders.KvCoder.encode(KvCoder.java:36)
    at org.apache.beam.sdk.util.WindowedValue$FullWindowedValueCoder.encode(WindowedValue.java:590)
    at org.apache.beam.sdk.util.WindowedValue$FullWindowedValueCoder.encode(WindowedValue.java:581)
    at org.apache.beam.sdk.util.WindowedValue$FullWindowedValueCoder.encode(WindowedValue.java:541)
    at org.apache.beam.sdk.fn.data.BeamFnDataSizeBasedBufferingOutboundObserver.accept(BeamFnDataSizeBasedBufferingOutboundObserver.java:109)
    at org.apache.beam.fn.harness.BeamFnDataWriteRunner.consume(BeamFnDataWriteRunner.java:155)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:216)
    at org.apache.beam.fn.harness.data.PCollectionConsumerRegistry$MetricTrackingFnDataReceiver.accept(PCollectionConsumerRegistry.java:179)

Answer 1

If you haven't solved it yet, you are publishing a message with a null key value which causes Kafka to throw the exception and results in your job failing.

You can publish messages as key-value pairs with the kafka command line tool like so:

./bin/kafka-console-producer.sh \
  --broker-list localhost:9092 \
  --topic my-topic \
  --property "parse.key=true" \
  --property "key.separator=:"
>my_key:my_value