How to implement rate limiting and monitoring with expvar in Go?

To implement rate limiting and monitoring with expvar in Go, you can use the expvar package in combination with other libraries like golang.org/x/time/rate and net/http. Here's an example code snippet:

Import the required packages:

import (
    "expvar"
    "net/http"
    "golang.org/x/time/rate"
)

Create a rate.Limiter object to define the rate limit:

var limiter = rate.NewLimiter(rate.Limit(10), 5)  // Allow 10 events per second with a burst of 5

Define an expvar.Int to keep track of the rate limit enforcement metrics:

var rateLimitExceeded = expvar.NewInt("rate_limit_exceeded")

Create an HTTP handler function to enforce rate limiting for a particular endpoint:

func rateLimitedHandler(h http.Handler) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        if !limiter.Allow() {
            rateLimitExceeded.Add(1)
            w.WriteHeader(http.StatusTooManyRequests)
            return
        }
        h.ServeHTTP(w, r)
    }
}

In your main program, wrap your original handler with the rate-limited handler:

func main() {
    // ...

    originalHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Your original handler logic goes here
    })

    rateLimitedHandler := rateLimitedHandler(originalHandler)

    http.Handle("/your-endpoint", rateLimitedHandler)

    // ...

    http.ListenAndServe(":8080", nil)
}

To monitor the rate limit enforcement, you can expose the metrics via an endpoint using expvar:

func main() {
    // ...

    http.Handle("/debug/vars", expvar.Handler())

    // ...

    http.ListenAndServe(":8080", nil)
}

This will expose the metrics at /debug/vars endpoint, and you can use tools like expvarmon or curl to monitor the metrics in real-time.

Remember to adjust the rate limit parameters (rate.Limit(10), 5) according to your desired limits and burst size.