Expose application metrics with expvar

To determine whether your application meets its performance objectives and to help identify bottlenecks, you need to measure your program’s performance and collect metrics. They tend to be response time, throughput, and resource utilization (how much CPU, memory, disk I/O, and network bandwidth your application consumes while performing its tasks).


Metrics provide information about how close your program is to your performance goals. In addition, they also help you identify problem areas and bottlenecks within your application. Metric types could be grouped under the following categories:

  • Network metric related to network bandwidth usage.
  • System metrics are related to processor, memory, disk I/O, and network I/O.
  • Platform metrics are related to execution runtime.
  • Application metrics include custom performance counters.

How Measuring Applies to Life Cycle

You should start to measure as soon as you have a defined set of performance objectives for your program. This should be early in the application design phase.

You must continue to measure application performance throughout the life cycle to determine whether your application is trending toward or away from its performance objectives.

Exposing and collecting metrics in Golang

Golang provides a variety of packages for exposing metrics, application monitoring and performance analysis.

In the article, we will explore expvar package that can provide command line arguments, allocation stats, heap stats and garbage collection metrics. In addition, it allows you to define variables to export and publish over http.

The package should be imported to register http handler into the default http mux:

import _ "expvar"

Also it publishes ‘cmdline’ and ‘memstats’ variables for the current process. If you don’t run an HTTP server, you should the following code snippet to make available an HTTP endpoint:

http.ListenAndServe(":8080", http.DefaultServeMux)

You could access the exported variables on

  "cmdline": [
  "memstats": {
    "Alloc": 204728,
    "TotalAlloc": 204728,
    "Sys": 4720888,
    "Lookups": 6,
    "Mallocs": 887,
    "Frees": 0,
    "HeapAlloc": 204728,
    "HeapSys": 1671168,
    "HeapIdle": 966656,
    "HeapInuse": 704512,
    "HeapReleased": 0,
    "HeapObjects": 887,
    "StackInuse": 425984,
    "StackSys": 425984,
    "MSpanInuse": 9520,
    "MSpanSys": 16384,
    "MCacheInuse": 9664,
    "MCacheSys": 16384,
    "BuckHashSys": 1443053,
    "GCSys": 65536,
    "OtherSys": 1082379,
    "NextGC": 4194304,
    "LastGC": 0,
    "PauseTotalNs": 0,
    "PauseNs": [],
    "PauseEnd": [],
    "NumGC": 0,
    "GCCPUFraction": 0,
    "EnableGC": true,
    "DebugGC": false,
    "BySize": [
        "Size": 17664,
        "Mallocs": 0,
        "Frees": 0

In addtion exported variables could be accessed by using expvar.Get function:

memstatsFunc := expvar.Get("memstats").(expvar.Func)
memstats := memstatsFunc().(runtime.MemStats)

If you want to collect all exported variables you should consider using expvar.Do function which invokes the provided callback function for every variable in thread safe way. The expvar.KeyValue type has Key and Value field that returns the variable name and variable value.

The sample below prints out all exported variables:

expvar.Do(func(variable expvar.KeyValue) {
	fmt.Printf("expvar.Key: %s expvar.Value: %s", variable.Key, variable.Value)

The package allows exporting of integers, floats and string variables.

var (
	orderCounter      *expvar.Int
	balanceCounter    *expvar.Float
	transactionMetric *expvar.String

func init() {
	orderCounter = expvar.NewInt("counter")
	balanceCounter = expvar.NewFloat("balance")
	transactionMetrics = expvar.NewString("transaction")

Note that it’s recommended to register the new exported variables in the init function of your package.

Then you should the variables in the similar way as their counterparts:

// Adds an integer value to expvar.Int counter
// Sets a float value to expvar.Float metrics
// Sets a string to expvar.String metrics
transactionMetrics.Set("this is my transaction")

If you want to do something more complex you should use the expvar.Publish function which register any type that obeys expvar.Var interface:

type Var interface {
  String() string

Lets define our own metrics that exports time.Time type:

type TimeVar struct {
	value time.Time

// Sets a time.Time as time metrics value
func (v *TimeVar) Set(date time.Time) {
	v.value = date

// Adds a time.Duration to current time metrics value
func (v *TimeVar) Add(duration time.Duration) {
	v.value = v.value.Add(duration)

// Converts the TimeVar metrics to string
func (v *TimeVar) String() string {
	return v.value.Format(time.UnixDate)

Then we should use expvar.Publish function to export this type:

var (
	timeMetrics        *TimeVar

func init() {
	timeMetrics = &TimeVar{value: time.Now()}
	expvar.Publish("Time", timeMetrics)

We can use the exported variable in the following way:

// Adds an hour to the initial time metrics value
timeMetrics.Add(1 * time.Hour)


Having set metrics objects early in your application’s design phase, you begin to measure by collecting them. You continue to measure throughout the application life cycle to determine whether your application’s performance is trending toward or away from its performance goals.

In Golang this goal is considered as important part of every application. The ease of use and out of the box support give us confidence to build scalable and performan programs.

Published by in programming languages and tagged #expvar, #go and #metrics using 759 words.