Advanced Error Mocking

gRPC supports returning rich errors through the status package. Let's start by examining the type status.Status proto definition:

message Status {
  int32 code = 1;
  string message = 2;
  repeated google.protobuf.Any details = 3;
}

The code and message fields are straightforward. The code must be greater than 0 (0 is response OK = Success). See full list of error codes. The details field adds a bit of complexity. The repeated google.protobuf.Any details translates into func (s *Status) Details() []interface{} in Go. It means that it can contain any proto.Message.

The stubs management REST API allows the definition of rich error responses. All we need to do is provide the type the detail. Ex.:

{
    "fullMethod": "/carvalhorr.greeter.UserService/AddUser",
    "request": {
        "match": "exact",
        "content": {
            "name": "",
            "age": 0
        },
        "metadata": {}
    },
    "response": {
        "type": "error",
        "content": {},
        "error": {
            "code": 1,
            "message": "",
            "details": {
                "spec": {
                    "import": "google.golang.org/genproto/googleapis/rpc/errdetails",
                    "type": "BadRequest"
                },
                "values": [
                    {
                        "value": {
                            "field_violations": [
                                {
                                    "field": "Name",
                                    "description": "Name is mandatory"
                                },
                                {
                                    "field": "Age",
                                    "description": "Age must be a positive number"
                                }
                            ]
                        }
                    }
                ]
            }
        }
    }
}

The response.error.details.spec field is how we set the stub with any custom proto.Message. This field contains two fields: import and type. They correspond to the go import path and the type within that import respectively to the proto.Message. Note that this is not the proto definition, but the generated go type instead.

It is possible to have different values in each entry of response.error.details. To do so it is necessary to use the value's specOverride field, providing alternative import and type.