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crflynn / pbspark

protobuf pyspark conversion
MIT License
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Confluent SR #44

Open michaelandrepearce opened 1 year ago

michaelandrepearce commented 1 year ago

Any plans to support Confluent SR and using dynamic message rather than having concrete message types akin to the avro SR support with Generic message

crflynn commented 1 year ago

Can you elaborate on how this would work? Would you expect the output column to be StringType() with json string?

crflynn commented 1 year ago

I tried implementing this and it's doesn't quite work because of how protobuf is implemented in the schema registry. The confluent package doesn't support this either, presumably for the same reason: https://github.com/confluentinc/confluent-kafka-python/issues/1174

What I found was that when registering a schema, confluent serializes the FileDescriptor from the message type, and registers it. I assume schema registry then does some FileDescriptor validation. But when you fetch the schema from the registry it returns a .proto file render of the passed FileDescriptor, rather than the serialized FileDescriptor itself.

The python protobuf lib doesn't have any tooling or interface for parsing .proto files directly; it expects FileDescriptor instances throughout. The parsing of .proto files happens in the protoc tool which is in C++. It will parse the .proto files and then pass FileDescriptors into the codegen plugins.

Here is the _schema_to_str function which encodes the serialized FileDescriptor (file_descriptor.serialized_pb) to submit to schema registry: https://github.com/confluentinc/confluent-kafka-python/blob/842e2df13b3eebc5ae5562f050008c1932c8332d/src/confluent_kafka/schema_registry/protobuf.py#L117

In order to have a dynamic ProtobufDeserializer and dynamically create message types, we would need to either be able to fetch the serialized_pb for a FileDescriptor from the registry, or parse the .proto file into a FileDescriptor.

As far as I can tell the protoc entrypoint is here: https://github.com/protocolbuffers/protobuf/blob/cc83427ee3cf14de4c0308e46ff54381aa6f647a/src/google/protobuf/compiler/command_line_interface.cc#L1187 and the parsing happens in this file: https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/compiler/parser.cc so we would have to have some way to access this functionality or port it to python.