Service definitions with protocol buffers

gRPC is based around defining a service: specifying the methods that can be called remotely with their parameters and return types. By default gRPC uses protocol buffers as the Interface Definition Language (IDL) for describing both the service interface and the structure of the payload messages, though other alternatives are possible. A service is declared in a .proto file, for example service HelloService { rpc SayHello(HelloRequest) returns (HelloResponse); }, with each message defined field by field. Starting from that definition, gRPC provides protocol buffer compiler plugins that generate client- and server-side code: on the server side you implement the methods declared by the service and run a gRPC server whose infrastructure decodes requests, executes the methods, and encodes responses; on the client side a local object known as a stub implements the same methods and wraps parameters in the appropriate protocol buffer message type. Treating the .proto definition as the single source of truth that both sides generate from is Koskamo editorial guidance. Scope: gRPC. Reviewed October 4, 2026.