Proto3

Proto3 is the third version of Protocol Buffers, a language-neutral, platform-neutral data serialization format and interface description language developed by Google. It defines a compact binary encoding for structured data and is widely used for communication protocols, data storage, and service definitions, including gRPC APIs.

In Proto3, a .proto file uses syntax = "proto3" to describe messages and services. Data structures are defined with fields assigned numeric tags. Supported scalar field types include double, float, int32, int64, uint32, uint64, sint32, sint64, fixed32, fixed64, sfixed32, sfixed64, bool, string, and bytes, as well as enumerations, nested messages, repeated fields, and map fields. Oneof is supported, allowing mutual exclusivity among fields. In addition, there are well-known wrapper types (for example, google.protobuf.StringValue) used when presence information for scalars is needed.

Proto3 simplifies and changes several aspects of Proto2. It removes required fields, extensions, and default values for scalar types. Scalar fields do not carry presence information by default; presence can be detected via wrappers or, in newer versions, the optional keyword. Unknown fields encountered during parsing are typically discarded, which supports forward and backward compatibility as schemas evolve.

The binary encoding is designed to be compact and efficient, with a separate JSON mapping for interoperability. Proto3 is widely supported across programming languages and platforms, with official implementations in several languages and strong integration with gRPC, which uses Proto3 as its IDL by default. The ecosystem also includes tools for code generation, validation, and versioning to manage evolving data schemas.