How Can I Forward Go HTTP Requests to Another Server While Avoiding 'http: Request.RequestURI can't be set in client requests'?

Go HTTP: Forwarding Incoming Requests to Another Server

Suppose you have legacy and versioned services ("legacy" and "version_2_of_doom") running alongside each other. You want all POST requests to be handled by both services simultaneously for a smooth transition.

Initially, you attempted to modify the legacy service's handler to duplicate the request to "version_2_of_doom" directly. However, this approach failed with the error "http: Request.RequestURI can't be set in client requests."

The preferred solution is to create a new request and copy only the necessary parts from the incoming request. This process resembles what a reverse proxy does, which you can examine in the "net/http/httputil" package.

Here's an example implementation:

func handler(w http.ResponseWriter, req *http.Request) {
    body, err := ioutil.ReadAll(req.Body)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    req.Body = ioutil.NopCloser(bytes.NewReader(body))

    url := fmt.Sprintf("%s://%s%s", proxyScheme, proxyHost, req.RequestURI)

    proxyReq, err := http.NewRequest(req.Method, url, bytes.NewReader(body))

    proxyReq.Header = make(http.Header)
    for h, val := range req.Header {
        proxyReq.Header[h] = val
    }

    resp, err := httpClient.Do(proxyReq)
    if err != nil {
        http.Error(w, err.Error(), http.StatusBadGateway)
        return
    }
    defer resp.Body.Close()

    // legacy code
}

This approach allows you to selectively copy headers and create a new request URI to send to the other server. By following this pattern, you can seamlessly forward incoming HTTP requests to multiple services without altering the request details.

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