Gokit: Validate request/payload in transport layer

Question

I am using go-kit to create an RPC endpoint. I am creating an endpoint like this

httptransport.NewServer(
    endPoint.MakeGetBlogEndPoint(blogService),
    transport.DecodeGetBlogRequest,
    transport.EncodeGetBlogResponse

Below is my DecodeGetBlogRequest function

func DecodeGetBlogRequest(c context.Context, r *http.Request) (interface{}, error) {
        vars := mux.Vars(r)
        id, err := strconv.Atoi(vars["id"])
        if err != nil {
            return nil, err
        }
        req := endPoint.GetBlogRequest{
            ID: id,
        }
        return req, nil
    }

What I want to do is validate the HTTP request in this function and if found invalid, send a response with a valid error code from here only, without passing it to the service layer. i.e. If ID is not a valid number, return 400 Bad Request response from here.

But as I don't have a ResponseWriter reference in this function, I am not sure how to do it.

I am following this example from go-kit docs https://gokit.io/examples/stringsvc.html

Is it a valid assumption that request/payload should be validated in the transport layer only and the service layer should only be called if the request/payload is valid? If yes, how to do so in this example?

Answer 1

You could use ServerErrorEncoder which returns Server options (can be found in github.com/go-kit/kit/transport/server.go). Basically in your transport layer, apart from the Decode and Encode functions, you can define an YourErrorEncoderFunc() function which could look like the following. This will catch any error thrown in the transport layer.
YourErrorEncoderFunc(_ context.Context, err error, w http.ResponseWriter).

You will need to attach this function as an option in your endpoint registration like:

ABCOpts := []httptransport.ServerOption{
            httptransport.ServerErrorEncoder(YourErrorEncoderFunc),
            }

r.Methods("GET").Path("/api/v1/abc/def").Handler(httptransport.NewServer(
    endpoints.GetDataEndpoint,
    DecodeGetRequest,
    EncodeGetResponse,
    ABCOpts...,
))

This will stop at transport layer if your request validation is invalid and throw and error in the http response based of whatever format you've written in YourErrorEncoderFunc().