How to expose and track custom application-specific error rates using expvar in Go?

To expose and track custom application-specific error rates using expvar in Go, you can follow these steps:

Import the necessary packages:

import (
	"expvar"
	"net/http"
	"sync/atomic"
)

Define a struct to hold the error statistics:

type ErrorStats struct {
	Total   uint64 `json:"total"`
	Success uint64 `json:"success"`
	Failure uint64 `json:"failure"`
}

Create an instance of the error statistics struct:

var errorStats = &ErrorStats{}

Initialize the expvar variables with the appropriate labels:

func init() {
	expvar.Publish("error_total", expvar.Func(func() interface{} {
		return atomic.LoadUint64(&errorStats.Total)
	}))

	expvar.Publish("error_success", expvar.Func(func() interface{} {
		return atomic.LoadUint64(&errorStats.Success)
	}))

	expvar.Publish("error_failure", expvar.Func(func() interface{} {
		return atomic.LoadUint64(&errorStats.Failure)
	}))
}

Create a middleware function that tracks the error rate:

func ErrorRateMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Your application-specific error handling code here

		// Increment the appropriate error count
		atomic.AddUint64(&errorStats.Total, 1)

		if err != nil {
			atomic.AddUint64(&errorStats.Failure, 1)
		} else {
			atomic.AddUint64(&errorStats.Success, 1)
		}

		// Call the next handler
		next.ServeHTTP(w, r)
	})
}

Use the middleware in your main handler:

func main() {
	// Your setup code here

	// Create a router and add the middleware
	router := http.NewServeMux()
	router.Handle("/", ErrorRateMiddleware(http.HandlerFunc(handler)))

	// Your server configuration and start call here

}

With these steps in place, you can access the error rates by navigating to the /debug/vars endpoint of your running Go application. The exposed variables, such as error_total, error_success, and error_failure, will reflect the current error statistics.

Remember to adapt the middleware code with your application-specific error handling code.