How to implement rate limiting and throttling based on expvar metrics in Go?

To implement rate limiting and throttling based on expvar metrics in Go, you can follow these steps:

Import the required packages:

import (
	"fmt"
	"net/http"
	"time"

	"github.com/gorilla/mux"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
	"github.com/rs/xid"
)

Define a struct to keep track of metrics:

type Metrics struct {
	Counter  prometheus.Counter
	Registry *prometheus.Registry
}

Initialize the metrics:

func NewMetrics() *Metrics {
	m := &Metrics{
		Registry: prometheus.NewRegistry(),
	}
	m.Counter = prometheus.NewCounter(prometheus.CounterOpts{
		Namespace: "api",
		Name:      "requests_total",
		Help:      "Number of requests.",
	})
	m.Registry.MustRegister(m.Counter)

	return m
}

Implement a middleware to count the requests:

func (m *Metrics) Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		m.Counter.Inc()
		next.ServeHTTP(w, r)
	})
}

Implement rate limiting and throttling based on the metrics:

func (m *Metrics) RateLimitMiddleware(throttleInterval time.Duration, maxRequests int) mux.MiddlewareFunc {
	ticker := time.NewTicker(throttleInterval)
	connections := make(chan bool, maxRequests)

	for i := 0; i < maxRequests; i++ {
		connections <- true
	}

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			select {
			case <-ticker.C:
				connections <- true
			case <-connections:
				m.Counter.Inc()
				next.ServeHTTP(w, r)
				return
			}

			w.WriteHeader(http.StatusTooManyRequests)
			w.Write([]byte("Too many requests. Please try again later."))
		})
	}
}

Create a handler to handle the requests:

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

Create a router and add the middleware and handler:

func main() {
	r := mux.NewRouter()

	metrics := NewMetrics()
	r.Use(metrics.Middleware)

	rateLimiter := metrics.RateLimitMiddleware(1*time.Second, 100) // 100 requests per second
	r.HandleFunc("/api", Handler).Methods(http.MethodPost)
	r.Use(rateLimiter)

	// Expose the metrics
	r.Handle("/metrics", promhttp.HandlerFor(metrics.Registry, promhttp.HandlerOpts{}))

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

In this example, the Metrics struct is used to keep track of the number of requests. The NewMetrics function initializes the metrics, and the Middleware function counts the requests. The RateLimitMiddleware function implements rate limiting and throttling based on the metrics. The Handler function represents your actual request handler logic. Finally, you create a router using the mux package, add the middleware and handler, and expose the metrics using the /metrics path.

Note: This example uses the popular gorilla/mux package for routing and prometheus package for capturing the metrics. Make sure to import these packages by running the following command:

go get -u github.com/gorilla/mux github.com/prometheus/client_golang/prometheus github.com/prometheus/client_golang/prometheus/promhttp