Golang : Get expvar(export variables) to work with multiplexer

Golang has the expvar package that allows your web application to display debugging information during run time. Basically, what it does is to expose these variables(debug information or metrics) via HTTP at /debug/vars in JSON format.

Run this code example

 package main

 import (
 "expvar"
 "net/http"
 "strconv"
 "time"
 )

 type ServerUpTime struct {
 StartTime time.Time
 }

 func (t *ServerUpTime) String() string {
 return strconv.FormatFloat(time.Since(t.StartTime).Seconds(), 'f', 2, 64)
 }

 func init() {
 // add one counter to display server up time
 expvar.Publish("server uptime", &ServerUpTime{time.Now()})
 }

 func SayHelloWorld(w http.ResponseWriter, r *http.Request) {
 html := "Hello"
 html = html + " World"

 w.Write([]byte(html))
 }

 func main() {
 http.HandleFunc("/", SayHelloWorld)

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

and point your web browser to let say :

localhost:8080/debug/vars

will produce information such as :

{ "cmdline": ["/var/folders/nd/5m9x1vj4338yt1rg02zbph0000gn/T/go-build469965791/command-line-arguments/_obj/exe/expvar-no-result"], "memstats": {"Alloc":148448,"TotalAlloc":148448,"Sys":3377400,"Lookups":6,"Mallocs":678,"Frees":0,"HeapAlloc":148448,"HeapSys":753664,

...

{"Size":10496,"Mallocs":0,"Frees":0},{"Size":12288,"Mallocs":0,"Frees":0},{"Size":13568,"Mallocs":0,"Frees":0},{"Size":14080,"Mallocs":0,"Frees":0},{"Size":16384,"Mallocs":0,"Frees":0},{"Size":16640,"Mallocs":0,"Frees":0},{"Size":17664,"Mallocs":0,"Frees":0}]}, "server uptime": 4.16 }

However, the expvar package will not show any information at /debug/vars when the above code is converted to use http.NewServeMux (or any multiplexers like Gin or Gorilla). To get the expvar package to work again with multiplexer, we will need to borrow some codes from https://golang.org/src/expvar/expvar.go and plug the codes into the previous code example.

 package main

 import (
 "expvar"
 "net/http"
 "runtime"
 "strconv"
 "time"
 "fmt"
 "os"
 )

 type ServerUpTime struct {
 StartTime time.Time
 }

 func (t *ServerUpTime) String() string {
 return strconv.FormatFloat(time.Since(t.StartTime).Seconds(), 'f', 2, 64)
 }

 func init() {
 expvar.Publish("server uptime", &ServerUpTime{time.Now()})
 }

 func expvarHandler(w http.ResponseWriter, r *http.Request) {
 w.Header().Set("Content-Type", "application/json; charset=utf-8")
 fmt.Fprintf(w, "{\n")
 first := true
 expvar.Do(func(kv expvar.KeyValue) {
 if !first {
 fmt.Fprintf(w, ",\n")
 }
 first = false
 fmt.Fprintf(w, "%q: %s", kv.Key, kv.Value)
 })
 fmt.Fprintf(w, "\n}\n")
 }

 func cmdline() interface{} {
 return os.Args
 }

 func memstats() interface{} {
 stats := new(runtime.MemStats)
 runtime.ReadMemStats(stats)
 return *stats
 }

 func SayHelloWorld(w http.ResponseWriter, r *http.Request) {
 html := "Hello"
 html = html + " World"

 w.Write([]byte(html))
 }

 func main() {
 mux := http.NewServeMux()
 mux.HandleFunc("/", SayHelloWorld)
 mux.HandleFunc("/debug/vars", expvarHandler)

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

Run this modified code example and pointing your browser to localhost:8080/debug/vars should work now.

NOTES : You can configure another application to eat/parse/store the debugging data produced by your web application in crontab at certain time interval. The debugging data exported by expvar package can be very useful in analysing performance and resource/memory consumption of your web application.

Happy coding!

References :

https://golang.org/src/expvar/expvar.go

https://golang.org/pkg/expvar

https://www.socketloop.com/references/golang-expvar-publish-function-example