Grpc-web: How to use request type GET instead of POST?

[pvo13]

Is there a way to specify the request type for the calls grpc-web does? I only see POST requests, I would like to use GET.

I want to use GET requests because it would be a lot easier to cache the results in a PWA service worker since there are solutions for that readily available.

[johanbrandhorst]

The gRPC-Web protocol mandates POST requests for all requests. See https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-WEB.md (which references the HTTP/2 spec: https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#requests).

[pcj]

The spec actually does not specify behavior over HTTP 1.1 (did I miss it?). There is no specific mention of POST in the first document. The gRPC HTTP/2 spec, is IMHO completely orthogonal to grpc-web (otherwise grpc-web would not be needed in the first place!).

Personally I think GET should be supported on a per-request opt-in basis for exactly the reason that @pvo13 mentioned: service workers. In order to cache a response in a service worker for a POST, you have to intercept it and put it in something like IndexedDB (see https://w3c.github.io/ServiceWorker/#cache-put bullet point 4).

One issue with this however is how to encode the GET request payload.

[johanbrandhorst]

The second paragraph in the spec is:

This document lists the differences between the two protocols. To help tracking future revisions, this document describes a delta with the protocol details specified in the native gRPC protocol.

Thus, because the native gRPC protocol mandates POST, so does grpc-web.

[pcj]

OK you are right I did not see the part about the delta.

Nevertheless, I tried it and it works fine.

On the client side, rather than putting the framed binary request content as the xhr/fetch body, I base64-encoded it and put it into a new header x-grpc-web-request and used method GET (implies an ~8KB request size limit if what I read about header size limits are true).

On the server, it's a fairly minimal change:

// IsGrpcWebRequest determines if a request is a gRPC-Web request by checking that the "content-type" is
// "application/grpc-web" and that the method is POST.
func (w *WrappedGrpcServer) IsGrpcWebRequest(req *http.Request) bool {
-   return req.Method == http.MethodPost && strings.HasPrefix(req.Header.Get("content-type"), grpcWebContentType)
+   return strings.HasPrefix(req.Header.Get("content-type"), grpcWebContentType)
}


func hackIntoNormalGrpcRequest(req *http.Request) (*http.Request, bool) {
    // Hack, this should be a shallow copy, but let's see if this works
    req.ProtoMajor = 2
    req.ProtoMinor = 0

    contentType := req.Header.Get("content-type")
    incomingContentType := grpcWebContentType
    isTextFormat := strings.HasPrefix(contentType, grpcWebTextContentType)
    if isTextFormat {
        // body is base64-encoded: decode it; Wrap it in readerCloser so Body is still closed
        decoder := base64.NewDecoder(base64.StdEncoding, req.Body)
        req.Body = &readerCloser{reader: decoder, closer: req.Body}
        incomingContentType = grpcWebTextContentType
    }
+   if req.Method == http.MethodGet {
+       req.Method = http.MethodPost
+       in := strings.NewReader(req.Header.Get("x-grpc-web-request"))
+       decoder := base64.NewDecoder(base64.StdEncoding, in)
+       req.Body = &readerCloser{reader: decoder, closer: ioutil.NopCloser(in)}
+   }
    req.Header.Set("content-type", strings.Replace(contentType, incomingContentType, grpcContentType, 1))

    // Remove content-length header since it represents http1.1 payload size, not the sum of the h2
    // DATA frame payload lengths. https://http2.github.io/http2-spec/#malformed This effectively
    // switches to chunked encoding which is the default for h2
    req.Header.Del("content-length")

    return req, isTextFormat
}

Huzzah, progressive web apps with grpc-web 🍰 !

[johanbrandhorst]

Haha that's a cool hack. We're unlikely to merge something like that though as I'm sure you understand. If it works for you then feel free to fork the proxy 😀.

[pvo13]

Since gRPC web is an evolving protocol anyway, I think this feature should be added to it.
I created a feature request using this form: https://docs.google.com/forms/d/e/1FAIpQLSfpPkk3pM84-Bg_HFGQw8-YIM0sfapjcdPnjf16RgjuyejmbQ/viewform

[pcj]

Sounds good. I created a PR #935 in case other people want to experiment with this.