Getting websocket connection information in JSON-RPC method - go

Question

I am using JSON-RPC over Websocket. And, in an RPC method (say, Multiply in the example below), I need to know which connection called this method. The part below that says "// Need Websocket connection information here". How do I do so?
package main
import (
"code.google.com/p/go.net/websocket"
"net/http"
"net/rpc"
"net/rpc/jsonrpc"
)
type Args struct {
A int
B int
}
type Arith int
func (t *Arith) Multiply(args *Args, reply *int) error {
*reply = args.A * args.B
// Need Websocket connection information here
return nil
}
func main() {
rpc.Register(new(Arith))
http.Handle("/conn", websocket.Handler(serve))
http.ListenAndServe("localhost:7000", nil)
}
func serve(ws *websocket.Conn) {
jsonrpc.ServeConn(ws)
}

Answer 1

This will be challenging because it violates the abstraction that RPC provides. Here's a strategy suggestion:
Google uses a context object for lots of their APIs: https://blog.golang.org/context. Part of that interface is a Value method for arbitrary data:
Value(key interface{}) interface{}
That will give you something like thread-local storage, which is often used for this purpose in other programming languages.
So how do you add a context object to the request? One way would be to create a new, custom ServerCodec:
type ServerCodec interface {
ReadRequestHeader(*Request) error
ReadRequestBody(interface{}) error
// WriteResponse must be safe for concurrent use by multiple goroutines.
WriteResponse(*Response, interface{}) error
Close() error
}
Your implementation can mostly mirror jsonrpc's:
var params [1]interface{}
params[0] = x
return json.Unmarshal(*c.req.Params, &params)
But before returning, you can use a bit of reflection and look for a field in params with name/type Context and then fill it. Something like:
ctx := createContextSomehow()
v := reflect.ValueOf(x)
if v.Kind() == reflect.Ptr {
v = v.Elem()
if v.Kind() == reflect.Struct {
ctxv := v.FieldByName("Context")
ctxv.Set(ctx)
}
}
Then change your request:
type Args struct {
A int
B int
}
Change it to:
type Args struct {
A int
B int
Context context.Context
}
Kinda clumsy but I think you could make that work.