Golang Expvar Example

main.go

package main

import (
  "expvar"
  "net/http"
  "time"
  "os"
  "io/ioutil"
  "strings"
  "strconv"
)

// Publish the port as soon as the program starts
func init() {
  go http.ListenAndServe(":8080", nil)
}

// Custom struct that will be exported
type Load struct {
  Load1  float64
  Load5  float64
  Load15 float64
}

// Function that will be called by expvar
// to export the information from the structure
// every time the endpoint is reached
func AllLoadAvg() interface{} {
  return Load{
     Load1:  loadAvg(0),
     Load5:  loadAvg(1),
     Load15: loadAvg(2),
  }
}

// Aux function to retrieve the load average
// in GNU/Linux systems
func loadAvg(position int) float64 {
  data, err := ioutil.ReadFile("/proc/loadavg")
  if err != nil {
     panic(err)
  }
  values := strings.Fields(string(data))

  load, err := strconv.ParseFloat(values[position], 64)
  if err != nil {
     panic(err)
  }

  return load
}

func main() {

  var (
     numberOfSecondsRunning = expvar.NewInt("system.numberOfSeconds")
     programName            = expvar.NewString("system.programName")
     lastLoad               = expvar.NewFloat("system.lastLoad")
     numberOfLoginsPerUser  = expvar.NewMap("system.numberOfLoginsPerUser")
  )

  // The contents returned by the function will be autoexported in JSON format
  expvar.Publish("system.allLoad", expvar.Func(AllLoadAvg))

  programName.Set(os.Args[0])

  // We will increment this metrics every second
  for {
     numberOfSecondsRunning.Add(1)
     lastLoad.Set(loadAvg(0))
     numberOfLoginsPerUser.Add("foo", 2)
     numberOfLoginsPerUser.Add("bar", 1)
     time.Sleep(1 * time.Second)
  }
}