How to expose and track request/response sizes with expvar in Go?

To expose and track request/response sizes with expvar in Go, you can follow these steps:

Import the necessary packages:

import (
	"expvar"
	"net/http"
)

Define global variables to track the request and response sizes:

var (
    requestsSize  = expvar.NewInt("request_size")
    responseSize  = expvar.NewInt("response_size")
)

Create middleware to measure the size of the incoming request. This middleware will update the requestsSize variable with the size of each incoming request:

func measureRequestSize(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        next.ServeHTTP(w, r)
        requestsSize.Add(int64(r.ContentLength))
    })
}

Create middleware to measure the size of the outgoing response. This middleware will update the responseSize variable with the size of each outgoing response:

func measureResponseSize(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        next.ServeHTTP(w, r)
        proxyWriter := w.(*httputil.ReverseProxy).BufferPool.(*httputil.BufferPool).ProxyWriter
        responseSize.Add(int64(proxyWriter.BytesWritten()))
    })
}

Note: Make sure to adjust the cast to httputil.ReverseProxy and httputil.BufferPool based on your specific use case. The example above assumes you are using a reverse proxy in your application.

Use the middleware in your main request handler:

func mainHandler(w http.ResponseWriter, r *http.Request) {
    // Your main request handler logic here
}

func main() {
    http.Handle("/", measureRequestSize(measureResponseSize(http.HandlerFunc(mainHandler))))
    http.ListenAndServe(":8080", nil)
}

Now, you can access the request and response sizes by visiting the /debug/vars endpoint of your application:

http://localhost:8080/debug/vars

The output will include the request size (request_size) and the response size (response_size) under the expvar section.

Note: Remember to import the required packages and customize the middleware and handlers based on your application's needs.

By following these steps, you can expose and track the request/response sizes using expvar in your Go application.