apache/druid · error · ParseException

Fail to get protobuf schema because of invalid schema!

Error message

Fail to get protobuf schema because of invalid schema!

What it means

After fetching the schema by id, the decoder converts it to a protobuf Descriptor via schema.toDescriptor(). If that fails with an IOException, the stored schema is not a usable/valid protobuf schema, and Druid throws a ParseException with this message.

Solutions

Confirm the subject/schema for the consumed ids was registered with the PROTOBUF format, not Avro/JSON
Re-register the correct .proto schema under the subject and republish or use the correct schema id
Check that ProtobufSchemaProvider is configured in the decoder and registry compatibility settings allow the protobuf type

Example fix

// before: subject registered as AVRO
$ kafka-avro-console-producer --property schema.registry.url=...
// after: register protobuf schema
curl -X POST http://registry:8081/subjects/mytopic-value/versions -H 'Content-Type: application/vnd.schemaregistry.v1+json' -d '{"schemaType":"PROTOBUF","schema":"syntax=\"proto3\"; message Msg { ... }"}'

Defensive patterns

Strategy: validation

Validate before calling

Schema schema = registry.getSchemaById(id);
if (!(schema instanceof ProtobufSchema)) {
  throw new IllegalStateException("Schema id " + id + " is not PROTOBUF type: " + schema.schemaType());
}

Try / catch

try {
  byte[] parsed = decoder.parse(bytes);
} catch (ParseException e) {
  if (e.getMessage().contains("invalid schema")) {
    log.error("Registered schema for id is not a valid protobuf schema; re-register with schemaType=PROTOBUF", e.getCause());
  }
}

Prevention

Register subjects explicitly with schemaType=PROTOBUF
Reject Avro/JSON-schema producers writing to topics consumed by this decoder
Validate registry contents with a script that decodes each schema id as ProtobufSchema

When it happens

Trigger: registry.getSchemaById succeeds but ProtobufSchema.toDescriptor() throws IOException — the schema stored under the id is not a valid protobuf schema (e.g. registered as Avro/JSON in the same registry, or corrupt schema text).

Common situations: Using this decoder against a registry where subjects were registered with the wrong serialization format; manually edited or corrupted schema entries; protobufSchemaProvider misconfiguration.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/9e9b96b31b6d0386. Report an issue: GitHub.

Appendix: source

Thrown at extensions-core/protobuf-extensions/src/main/java/org/apache/druid/data/input/protobuf/SchemaRegistryBasedProtobufBytesDecoder.java:195

    int id = bytes.getInt(); // extract schema registry id
    bytes.get(); // ignore \0 byte before PB message
    int length = bytes.limit() - 2 - 4;
    Descriptors.Descriptor descriptor;
    try {
      ProtobufSchema schema = (ProtobufSchema) registry.getSchemaById(id);
      descriptor = schema.toDescriptor();
    }
    catch (RestClientException e) {
      LOGGER.error(e.getMessage());
      throw new ParseException(
          null,
          e,
          "Fail to get protobuf schema because of can not connect to registry or failed http request!"
      );
    }
    catch (IOException e) {
      LOGGER.error(e.getMessage());
      throw new ParseException(null, e, "Fail to get protobuf schema because of invalid schema!");
    }
    try {
      byte[] rawMessage = new byte[length];
      bytes.get(rawMessage, 0, length);
      return DynamicMessage.parseFrom(descriptor, rawMessage);
    }
    catch (Exception e) {
      LOGGER.error(e.getMessage());
      throw new ParseException(null, e, "Fail to decode protobuf message!");
    }
  }

  @Override
  public boolean equals(Object o)
  {
    if (this == o) {
      return true;
    }

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