How to Duplicate HTTP Requests to Multiple Servers in Go for Seamless Server Transitions?

Duplicating HTTP Requests to Multiple Servers in Go

When transitioning from one server version to another, it may be desirable to duplicate incoming HTTP requests to both versions for a seamless handover. However, the naive approach of modifying the request directly is met with an error.

To address this, the solution lies in creating a new HTTP request and copying necessary parts from the incoming request. This involves:

1. Buffering the request body: As the same body may need to be used in both servers, it must be buffered for reading.
2. Rebuilding the request: A new HTTP request is created with the original method and a URL created from the RequestURI. Relevant headers are copied over.

Here's a code example:

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 ensures that both the legacy and new server versions receive duplicate requests without modifying the original request.

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