.proto File — Protocol Buffers Schema

Protocol Buffers (protobuf) schema files define the structure of data for Google's language-neutral, platform-neutral serialization mechanism. PROTO files describe message types with typed fields (int32, string, bool, bytes, enums, nested messages), service definitions for gRPC APIs, and options for customizing code generation. The protoc compiler generates serialization/deserialization code for multiple languages including C++, Java, Python, Go, C#, Ruby, and JavaScript. Protocol Buffers produce binary-encoded data that is significantly smaller and faster to parse than JSON or XML — typically 3-10x smaller and 20-100x faster to serialize. The format supports schema evolution through field numbering, allowing new fields to be added and old fields deprecated without breaking existing code. Protocol Buffers are used extensively at Google and across the industry for inter-service communication, data storage, configuration, and API definitions. gRPC, Google's high-performance RPC framework, uses Protocol Buffers as its interface definition language and serialization format. Proto3, the current version, simplifies the language while maintaining backward compatibility with proto2 data.