Add generic JSON-over-GRPC wrapper to examples/rest-grpc-multiplex

[muusbolla]

Resolves #1068 and #1069.

Add a function to examples/rest-grpc-multiplex that provides a generic wrapper to expose a HTTP/JSON frontend for any gRPC call. For the provided example code, this provides a HTTP endpoint with the following behavior:

POST localhost:3000/Hello
Header: content-type "application/json"
Body: { "name": "muusbolla" }

RESPONSE 200 OK
Header: content-type "application/json"
Body: { "message": "Hello muusbolla, my name is HAL 9000." }

Motivation

Make it easy to provide an API that is compatible with both gRPC and JSON, that uses a single backing implementation, with minimal additional boilerplate required for each new endpoint. This can be very helpful for users to create backwards/forwards-compatible APIs.

Solution

For context - the following is a specific implementation of a JSON wrapper around the GrpcServiceImpl::say_hello function that is compatible with the axum::Handler trait:

async fn json_wrap_hello(Json(body): Json<HelloRequest>) -> Result<Json<HelloReply>, GrpcErrorAsJson> {
    let handler = GRPC_SERVICE.get().unwrap();
    let r = handler.say_hello(tonic::Request::new(body)).await;
    match r {
        Ok(r) => Ok(Json(r.into_inner())),
        Err(e) => Err(GrpcErrorAsJson(e))
    }
}

#[tokio::main]
async fn main() {
    let rest = Router::new()
        .route("/Hello", axum::routing::any(json_wrap_hello);
}

The json_wrap_grpc function included in this PR is a genericization of the above code, making it possible to wrap any GRPC handler into an axum::Handler with a single function call.

It was quite a hassle to make the generic version compile (I was butting heads with rust-lang/rust#36582 and/or rust-lang/rust#41078), so I am hoping that this can be helpful to others who might want to do something similar.

[davidpdrsn]

I appreciate what you're trying to do but we cannot merge something this complicated as an example. That is just gonna confuse people. So unless you're able to find a simpler solution I don't think we can merge this.

[muusbolla]

Yeah, I understand. Now that I've made the PR at least people who are looking to do this can find it from the discussion.

[takkuumi]

To clarify here, I integrated tonic in axum, which is mainly used to solve the internal communication of the cluster. But I think, your example is helpful for wanting to access rpc through rest interface. Before, I referred to some spring projects, which can implement an interface to receive http, grpc and websocket access at the same time through annotations, but I don't have time to study these details now. When I have time, I'd like to try if I can do it based on axum and tonic.

[muusbolla]

To clarify here, I integrated tonic in axum, which is mainly used to solve the internal communication of the cluster. But I think, your example is helpful for wanting to access rpc through rest interface. Before, I referred to some spring projects, which can implement an interface to receive http, grpc and websocket access at the same time through annotations, but I don't have time to study these details now. When I have time, I'd like to try if I can do it based on axum and tonic.

If there was an implementation that was codegen-based, or somehow had access to the routing of tonic and could automatically generate routes based on that, that would be really nice.