Protobuf compatibility issue with Kafka Connect 8

[brunomendola]

I'm extending the Kafka Connect 8.1.0 image to include the connector libs (v2.10.0). Here is the Dockerfile:

FROM confluentinc/cp-kafka-connect:8.1.0

USER root

RUN microdnf -y install tar

USER 1000

RUN mkdir /usr/share/java/kafka-connectors && curl -s https://api.github.com/repos/Aiven-Open/bigquery-connector-for-apache-kafka/releases/latest | awk -F\" '/browser_download_url.*.tar/{print $(NF-1)}' | xargs curl -s -L | tar -xf - -C /usr/share/java/kafka-connectors

I already have Kafka and Confluent Schema Registry in Kubernetes, and I want to consume Protobuf messages with the connector and (of course) sending them to BigQuery.

So I create the connector with the following config JSON:

{
  "name": "bigquery-sink",
  "config": {
    "connector.class": "com.wepay.kafka.connect.bigquery.BigQuerySinkConnector",
    "tasks.max": "3",

    "topics": "mytopic",

    "project": "myproject",
    "datasets": ".*=mydataset",

    "key.converter": "org.apache.kafka.connect.storage.StringConverter",

    "value.converter": "io.confluent.connect.protobuf.ProtobufConverter",
    "value.converter.schema.registry.url": "http://schema-registry-service.schema-registry.svc.cluster.local:8081",

    "autoCreateTables": "true",
    "sanitizeTopics": "true",

    "defaultDataset": "mydataset",
    "keyfile": "/var/secrets/google/GOOGLE_APPLICATION_CREDENTIALS",
    "allBQFieldsNullable": "true",

    "bigQueryRetry": "3",
    "maxWriteSize": "10000",
    "allowNewBigQueryFields": "true",
    "allowBigQueryRequiredFieldRelaxation": "true"
  }
}

When I receive the first message, Kafka Connect fails to deserialize it and logs the following error:

[2025-10-24 10:25:58,200] ERROR WorkerSinkTask{id=bigquery-sink-1} Task threw an uncaught and unrecoverable exception. Task is being killed and will not recover until manually restarted (org.apache.kafka.connect.runtime.WorkerTask)
com.google.common.util.concurrent.ExecutionError: java.lang.VerifyError: Bad type on operand stack
Exception Details:
  Location:
    io/confluent/kafka/schemaregistry/protobuf/ProtobufSchema.toProtoFile(Lcom/google/protobuf/DescriptorProtos$FileDescriptorProto;)Lcom/squareup/wire/schema/internal/parser/ProtoFileElement; @1248: invokestatic
  Reason:
    Type 'com/google/protobuf/DescriptorProtos$FileOptions' (current frame, stack[1]) is not assignable to 'com/google/protobuf/GeneratedMessage$ExtendableMessage'

Looks like an incompatibility of the Protobuf library shipped with the connector with the one already in Kafka Connect: since version 8.0.1, Kafka Connect upgraded to Protobuf 4, but the connector v2.10.0 still includes Protobuf 3 libraries.

When downgrading Kafka Connect to version 7.9.4 (the one before the Protobuf upgrade) the connector starts working perfectly.