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I'm trying to learn Go and figured a nice little project would be an A/B testing proxy to put in front of a web server. Little did I know Go essentially offers a reverse proxy out of the box, so the setup was easy. I've got it to the point where I'm proxying traffic, but here's the thing, I have trouble implementing the actual functionality because wherever I have access to the response, I don't have access to assigned A/B test variations:

The problem is that I can't figure out how to share the assigned variations between the handleFunc and modifyResponse , without changing the upstream server. I'd like to be able to share this context (basically a map[string]string somehow.

Code sample:

Here's a distilled version of my code, where my question basically is, how can modifyRequest know about random assignments that happened in handleFunc ?

package main
import (
    config2 "ab-proxy/config"
    "bytes"
    "fmt"
    "io/ioutil"
    "net/http"
    "net/http/httputil"
    "net/url"
    "strconv"
    "strings"
var config config2.ProxyConfig
var reverseProxy *httputil.ReverseProxy
var tests config2.Tests
func overwriteCookie(req *http.Request, cookie *http.Cookie) {
    // omitted for brevity, will replace a cookie header, instead of adding a second value
func parseRequestCookiesToAssignedTests(req *http.Request) map[string]string {
    // omitted for brevity, builds a map where the key is the identifier of the test, the value the assigned variant
func renderCookieForAssignedTests(assignedTests map[string]string) string {
    // omitted for brevity, builds a cookie string
func main () {
    var err error
    if  config, err = config2.LoadConfig(); err != nil {
        fmt.Println(err)
        return
    if tests, err = config2.LoadTests(); err != nil {
        fmt.Println(err)
        return
    upstreamUrl, _ := url.Parse("0.0.0.0:80")
    reverseProxy = httputil.NewSingleHostReverseProxy(upstreamUrl)
    reverseProxy.ModifyResponse = modifyResponse
    http.HandleFunc("/", handleRequest)
    if err := http.ListenAndServe("0.0.0.0:80", nil); err != nil {
        fmt.Println("Could not start proxy")
func handleRequest(res http.ResponseWriter, req *http.Request) {
    assigned := parseRequestCookiesToAssignedTests(req)
    newCookies := make(map[string]string)
    for _, test := range tests.Entries {
        val, ok := assigned[test.Identifier]
        if ok {
            newCookies[test.Identifier] = val
        } else {
            newCookies[test.Identifier] = "not-assigned-yet" // this will be replaced by random variation assignment
    testCookie := http.Cookie{Name: config.Cookie.Name, Value: renderCookieForAssignedTests(newCookies)}
    // Add cookie to request to be sent to upstream
    overwriteCookie(req, &testCookie)
    // Add cookie to response to be returned to client
    http.SetCookie(res, &testCookie)
    reverseProxy.ServeHTTP(res, req)
func modifyResponse (response *http.Response) error {
    body, err := ioutil.ReadAll(response.Body)
    if err != nil {
        return  err
    err = response.Body.Close()
    if err != nil {
        return err
    response.Body = ioutil.NopCloser(bytes.NewReader(body))
    response.ContentLength = int64(len(body))
    response.Header.Set("Content-Length", strconv.Itoa(len(body)))
    return nil
Use a standard context.Context.  This is accessible in your handler via the *http.Request.  And the request is also accessible via the *http.Response argument to modifyResponse.
In your handler:
ctx := req.Context()
// Set values, deadlines, etc.
req = req.WithContext(ctx)
reverseProxy.ServeHTTP(res, req)
Then in modifyResponse:
ctx := response.Request.Context()
// fetch values, check for cancellation, etc
                Ah, amazing! Not even a fancy IDE could help me find out that there's even a .Request property on the response and that there's in fact a generic context.Context class that does exactly what I'm looking for. That helps a lot, thanks!
– Erik Booij
                Mar 17, 2019 at 19:30
        
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