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gRPC API Configuration

In some situations annotating the proto file of service is not an option. For example, you might not have control over the proto file, or you might want to expose the same gRPC API multiple times in completely different ways.

gRPC-Gateway supports 2 ways of dealing with these situations:


Providing this parameter to the protoc plugin will make it produce the HTTP mapping even for methods without any HttpRule annotation. This is similar to how Cloud Endpoints behaves and uses the way gRPC itself maps to HTTP/2:

  • HTTP method is POST
  • URI path is built from the service’s name and method: /<fully qualified service name>/<method name> (e.g.: /my.package.EchoService/Echo)
  • HTTP body is the serialized protobuf message.

NOTE: the same option is also supported by the gen-openapiv2 plugin.

Using an external configuration file

Google Cloud Platform offers a way to do this for services hosted with them called “gRPC API Configuration”. It can be used to define the behavior of a gRPC API service without modifications to the service itself in the form of YAML configuration files.

gRPC-Gateway generators implement the HTTP rules part of this specification. This allows you to take a completely unannotated service proto file, add a YAML file describing its HTTP endpoints and use them together like an annotated proto file with the gRPC-Gateway generators.

OpenAPI options may also be configured via “OpenAPI Configuration” in the form of YAML configuration files.

Usage of gRPC API Configuration YAML files

The following is equivalent to the basic example but without direct annotation for gRPC-Gateway in the proto file. Only some steps require minor changes to use a gRPC API Configuration YAML file instead:

  1. Define your service in gRPC as usual


    syntax = "proto3";
    package your.service.v1;
    option go_package = "";
    message StringMessage {
      string value = 1;
    service YourService {
      rpc Echo(StringMessage) returns (StringMessage) {}
  2. Instead of annotating the proto file in this step leave it untouched and create a your_service.yaml with the following content:

    type: google.api.Service
    config_version: 3
        - selector: your.service.v1.YourService.Echo
          post: /v1/example/echo
          body: "*"

    Use a linter to validate your YAML.

  3. Generate gRPC stub as before

    protoc -I . \
      --go_out ./gen/go/ \
      --go_opt paths=source_relative \
      --go-grpc_out ./gen/go/ \
      --go-grpc_opt paths=source_relative \

It will generate a stub file with path ./gen/go/your/service/v1/your_service.pb.go.

  1. Implement your service in gRPC as usual

  2. Generate the reverse-proxy. Here we have to pass the path to the your_service.yaml in addition to the proto file:

    protoc -I . \
      --grpc-gateway_out ./gen/go \
      --grpc-gateway_opt logtostderr=true \
      --grpc-gateway_opt paths=source_relative \
      --grpc-gateway_opt grpc_api_configuration=path/to/your_service.yaml \

    This will generate a reverse proxy gen/go/your/service/v1/ that is identical to the one produced for the annotated proto.

  3. Generate the optional your_service.swagger.json

    protoc -I . --openapiv2_out ./gen/go \
      --openapiv2_opt grpc_api_configuration=path/to/your_service.yaml \

    or using an OpenAPI configuration file

    protoc -I . --openapiv2_out ./gen/go \
      --openapiv2_opt grpc_api_configuration=path/to/your_service.yaml \
      --openapiv2_opt openapi_configuration=path/to/your_service_swagger.yaml \

    For an example of an OpenAPI configuration file, see unannotated_echo_service.swagger.yaml, which adds OpenAPI options to unannotated_echo_service.proto.

    protoc -I . --openapiv2_out ./gen/go \
      --openapiv2_opt grpc_api_configuration=path/to/your_service.yaml \

All other steps work as before. If you want you can remove the googleapis include path in step 3 and 4 as the unannotated proto no longer requires them.