Use expvar to expose Go program running metrics

#Getting the running metrics of an application allows us to better understand its actual status. By connecting these indicators to monitoring systems such as prometheus and zabbix, applications can be continuously detected, and abnormalities can be alerted and handled in a timely manner.

Pull and Push

There are two ways to interface with the monitoring system, one is Pull , another kind of Push.

Taking Prometheus as an example, the application exposes the HTTP interface and allows Prometheus to periodically capture indicators through the interface. This is Pull. In Push, the application actively pushes indicators to PushGateway, and Prometheus goes to PushGateway to capture data.

There is a package called expvar in the Go standard library. Its name is a combination of exp and var, which means exporting variables.

expvar provides a standardized interface for public variables and exposes these variables in Json format through HTTP, which is very suitable for connecting with the monitoring system using Pull.

Using the expvar library

expvar is a standard library, which means we don’t need additional dependencies, and it also provides some out-of-the-box indicators used. Let's learn how to use this library.

When the expvar library is referenced (<span style="font-size: 15px;">import "expvar"</span>), the following init function will be automatically called.

func init() {
 http.HandleFunc("/debug/vars", expvarHandler)
 Publish("cmdline", Func(cmdline))
 Publish("memstats", Func(memstats))
}

This function registers the HTTP service of the /debug/vars path for us. Accessing this path will get indicators in Json format.

Therefore, we also need to call ListenAndServe to bind the port and start serving HTTP requests.

http.ListenAndServe(":8080", nil)

The complete code is as follows

package main

import (
 _ "expvar"
 "net/http"
)

func main() {
 http.ListenAndServe(":8080", nil)
}

After running the program, through curl request, the following results are obtained

$ curl localhost:8080/debug/vars
{
"cmdline": ["/var/folders/xk/gn46n46d503dsztbc6_9qb2h0000gn/T/go-build1657217338/b001/exe/main"],
"memstats": {"Alloc":278880,"TotalAlloc":278880,"Sys":8735760,"Lookups":0,"Mallocs":1169,"Frees":87,"HeapAlloc":278880,"HeapSys":3866624,"HeapIdle":2949120,"HeapInuse":917504,"HeapReleased":2899968,"HeapObjects":1082,"StackInuse":327680,"StackSys":327680,"MSpanInuse":28696,"MSpanSys":32640,"MCacheInuse":9600,"MCacheSys":15600,"BuckHashSys":3875,"GCSys":3826448,"OtherSys":662893,"NextGC":4194304,"LastGC":0,"PauseTotalNs":0,"PauseNs":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"PauseEnd":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"NumGC":0,"NumForcedGC":0,"GCCPUFraction":0,"EnableGC":true,"DebugGC":false,"BySize":[{"Size":0,"Mallocs":0,"Frees":0},{"Size":8,"Mallocs":41,"Frees":0},{"Size":16,"Mallocs":496,"Frees":0},{"Size":24,"Mallocs":63,"Frees":0},{"Size":32,"Mallocs":28,"Frees":0},{"Size":48,"Mallocs":134,"Frees":0},{"Size":64,"Mallocs":50,"Frees":0},{"Size":80,"Mallocs":17,"Frees":0},{"Size":96,"Mallocs":17,"Frees":0},{"Size":112,"Mallocs":6,"Frees":0},{"Size":128,"Mallocs":9,"Frees":0},{"Size":144,"Mallocs":9,"Frees":0},{"Size":160,"Mallocs":18,"Frees":0},{"Size":176,"Mallocs":6,"Frees":0},{"Size":192,"Mallocs":0,"Frees":0},{"Size":208,"Mallocs":37,"Frees":0},{"Size":224,"Mallocs":6,"Frees":0},{"Size":240,"Mallocs":0,"Frees":0},{"Size":256,"Mallocs":12,"Frees":0},{"Size":288,"Mallocs":7,"Frees":0},{"Size":320,"Mallocs":2,"Frees":0},{"Size":352,"Mallocs":13,"Frees":0},{"Size":384,"Mallocs":1,"Frees":0},{"Size":416,"Mallocs":30,"Frees":0},{"Size":448,"Mallocs":1,"Frees":0},{"Size":480,"Mallocs":2,"Frees":0},{"Size":512,"Mallocs":0,"Frees":0},{"Size":576,"Mallocs":5,"Frees":0},{"Size":640,"Mallocs":5,"Frees":0},{"Size":704,"Mallocs":3,"Frees":0},{"Size":768,"Mallocs":0,"Frees":0},{"Size":896,"Mallocs":6,"Frees":0},{"Size":1024,"Mallocs":8,"Frees":0},{"Size":1152,"Mallocs":10,"Frees":0},{"Size":1280,"Mallocs":3,"Frees":0},{"Size":1408,"Mallocs":1,"Frees":0},{"Size":1536,"Mallocs":0,"Frees":0},{"Size":1792,"Mallocs":7,"Frees":0},{"Size":2048,"Mallocs":2,"Frees":0},{"Size":2304,"Mallocs":3,"Frees":0},{"Size":2688,"Mallocs":2,"Frees":0},{"Size":3072,"Mallocs":0,"Frees":0},{"Size":3200,"Mallocs":0,"Frees":0},{"Size":3456,"Mallocs":0,"Frees":0},{"Size":4096,"Mallocs":8,"Frees":0},{"Size":4864,"Mallocs":1,"Frees":0},{"Size":5376,"Mallocs":1,"Frees":0},{"Size":6144,"Mallocs":2,"Frees":0},{"Size":6528,"Mallocs":0,"Frees":0},{"Size":6784,"Mallocs":0,"Frees":0},{"Size":6912,"Mallocs":0,"Frees":0},{"Size":8192,"Mallocs":2,"Frees":0},{"Size":9472,"Mallocs":8,"Frees":0},{"Size":9728,"Mallocs":0,"Frees":0},{"Size":10240,"Mallocs":0,"Frees":0},{"Size":10880,"Mallocs":0,"Frees":0},{"Size":12288,"Mallocs":0,"Frees":0},{"Size":13568,"Mallocs":0,"Frees":0},{"Size":14336,"Mallocs":0,"Frees":0},{"Size":16384,"Mallocs":0,"Frees":0},{"Size":18432,"Mallocs":0,"Frees":0}]}
}

As you can see, expvar has provided two indicators by default, namely program execution command (os.Args) and runtime memory allocation (runtime.Memstats) information.

Key contents of expvar library

The most important things in expvar are the Publish function and Var interface.

func Publish(name string, v Var) {}

Publish function signature requires two parameters, name is the indicator name we specified. For example, under the init function of expvar above, <span style="font-size: 15px;">Publish("cmdline", Func(cmdline))</span> line of code, where <span style="font-size: 15px;">cmdline</span> is the indicator name, and <span style="font-size: 15px;">Func(cmdline)</span> is the expvar.Func type variable that implements the Var interface .

Var interface, which only defines a String method. It should be noted that this method must return a valid Json string.

// Var is an abstract type for all exported variables.
type Var interface {
 // String returns a valid JSON value for the variable.
 // Types with String methods that do not return valid JSON
 // (such as time.Time) must not be used as a Var.
 String() string
}

为了方便使用，expvar 库中提供了五种导出变量类型，它们均实现了 Var 接口。

type Int struct {
 i int64
}

type Float struct {
 f uint64
}

type Map struct {
 m      sync.Map // map[string]Var
 keysMu sync.RWMutex
 keys   []string // sorted
}

type String struct {
 s atomic.Value // string
}

type Func func() any

我们分别通过调用 expvar.NewXXX 函数即可完成前四种类型的变量创建与指标注册。

intVar = expvar.NewInt(“metricName”)
floatVar = expvar.NewFloat(“metricName”)
mapVar = expvar.NewMap(“metricName”)
stringVar = expvar.NewString(“metricName”)

例如 expvar.NewInt 函数，它会内部调用 Publish 方法完成指标名与 expvar.Int 类型变量的绑定。

func NewInt(name string) *Int {
 v := new(Int)
 Publish(name, v)
 return v
}

而 expvar.Func 类型，其实是为了让我们可以自定义导出类型。

例如，假如我们想要暴露以下定义的结构体

type MyStruct struct {
 Field1 string
 Field2 int
 Field3 float64
}

首先需要创建一个数据生成函数。它用于在每次调用 HTTP 服务路径时，通过该函数导出这里面的数据。

func MyStructData() interface{} {
 return MyStruct{
  Field1: "Gopher",
  Field2: 22,
  Field3: 19.99,
 }
}

最后，通过 Publish 函数注册指标名即可。

expvar.Publish("metricName", expvar.Func(MyStructData))

完整示例

下面，我们给出一个覆盖五种导出变量类型的完整示例。

package main

import (
 "expvar"
 "github.com/shirou/gopsutil/v3/host"
 "github.com/shirou/gopsutil/v3/load"
 "github.com/shirou/gopsutil/v3/mem"
 "net/http"
 "time"
)

type Load struct {
 Load1  float64
 Load5  float64
 Load15 float64
}

func AllLoadAvg() interface{} {
 return Load{
  Load1:  LoadAvg(1),
  Load5:  LoadAvg(5),
  Load15: LoadAvg(15),
 }
}

func LoadAvg(loadNumber int) float64 {
 avg, _ := load.Avg()
 switch loadNumber {
 case 5:
  return avg.Load5
 case 15:
  return avg.Load15
 default:
  return avg.Load1
 }
}

func main() {
 var (
  aliveOfSeconds = expvar.NewInt("aliveOfSeconds")
  hostID         = expvar.NewString("hostID")
  lastLoad       = expvar.NewFloat("lastLoad")
  virtualMemory  = expvar.NewMap("virtualMemory")
 )
 expvar.Publish("allLoadAvg", expvar.Func(AllLoadAvg))
 h, _ := host.HostID()
 hostID.Set(h)

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

 for {
  aliveOfSeconds.Add(1)
  vm, _ := mem.VirtualMemory()
  lastLoad.Set(LoadAvg(1))
  virtualMemory.Add("active", int64(vm.Active))
  virtualMemory.Add("buffer", int64(vm.Buffers))
  time.Sleep(1 * time.Second)
 }
}

在上述示例中，我们通过 gopsutil 库（介绍可见还在自己写 Go 系统监控函数吗一文）获取了一些系统信息，并展示了如何通过 expvar 中的各种变量类型将这些信息进行导出。

curl 访问 localhost:8080/debug/vars 结果如下

$ curl localhost:8080/debug/vars
{
"aliveOfSeconds": 1,
"allLoadAvg": {"Load1":1.69580078125,"Load5":1.97412109375,"Load15":1.90283203125},
"cmdline": ["/var/folders/xk/gn46n46d503dsztbc6_9qb2h0000gn/T/go-build3566019824/b001/exe/main"],
"hostID": "7a1a74f2-30fc-5bc1-b439-6b7aef22e58d",
"lastLoad": 1.69580078125,
"memstats": {"Alloc":256208,"TotalAlloc":256208,"Sys":8735760,"Lookups":0,"Mallocs":1089,"Frees":48,"HeapAlloc":256208,"HeapSys":3866624,"HeapIdle":2891776,"HeapInuse":974848,"HeapReleased":2859008,"HeapObjects":1041,"StackInuse":327680,"StackSys":327680,"MSpanInuse":19992,"MSpanSys":32640,"MCacheInuse":9600,"MCacheSys":15600,"BuckHashSys":3905,"GCSys":3851120,"OtherSys":638191,"NextGC":4194304,"LastGC":0,"PauseTotalNs":0,"PauseNs":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"PauseEnd":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"NumGC":0,"NumForcedGC":0,"GCCPUFraction":0,"EnableGC":true,"DebugGC":false,"BySize":[{"Size":0,"Mallocs":0,"Frees":0},{"Size":8,"Mallocs":35,"Frees":0},{"Size":16,"Mallocs":415,"Frees":0},{"Size":24,"Mallocs":71,"Frees":0},{"Size":32,"Mallocs":37,"Frees":0},{"Size":48,"Mallocs":141,"Frees":0},{"Size":64,"Mallocs":52,"Frees":0},{"Size":80,"Mallocs":20,"Frees":0},{"Size":96,"Mallocs":23,"Frees":0},{"Size":112,"Mallocs":14,"Frees":0},{"Size":128,"Mallocs":7,"Frees":0},{"Size":144,"Mallocs":7,"Frees":0},{"Size":160,"Mallocs":18,"Frees":0},{"Size":176,"Mallocs":6,"Frees":0},{"Size":192,"Mallocs":1,"Frees":0},{"Size":208,"Mallocs":42,"Frees":0},{"Size":224,"Mallocs":3,"Frees":0},{"Size":240,"Mallocs":0,"Frees":0},{"Size":256,"Mallocs":9,"Frees":0},{"Size":288,"Mallocs":8,"Frees":0},{"Size":320,"Mallocs":5,"Frees":0},{"Size":352,"Mallocs":13,"Frees":0},{"Size":384,"Mallocs":3,"Frees":0},{"Size":416,"Mallocs":33,"Frees":0},{"Size":448,"Mallocs":0,"Frees":0},{"Size":480,"Mallocs":2,"Frees":0},{"Size":512,"Mallocs":1,"Frees":0},{"Size":576,"Mallocs":4,"Frees":0},{"Size":640,"Mallocs":8,"Frees":0},{"Size":704,"Mallocs":3,"Frees":0},{"Size":768,"Mallocs":1,"Frees":0},{"Size":896,"Mallocs":6,"Frees":0},{"Size":1024,"Mallocs":8,"Frees":0},{"Size":1152,"Mallocs":9,"Frees":0},{"Size":1280,"Mallocs":3,"Frees":0},{"Size":1408,"Mallocs":1,"Frees":0},{"Size":1536,"Mallocs":1,"Frees":0},{"Size":1792,"Mallocs":9,"Frees":0},{"Size":2048,"Mallocs":1,"Frees":0},{"Size":2304,"Mallocs":2,"Frees":0},{"Size":2688,"Mallocs":2,"Frees":0},{"Size":3072,"Mallocs":0,"Frees":0},{"Size":3200,"Mallocs":1,"Frees":0},{"Size":3456,"Mallocs":0,"Frees":0},{"Size":4096,"Mallocs":5,"Frees":0},{"Size":4864,"Mallocs":0,"Frees":0},{"Size":5376,"Mallocs":1,"Frees":0},{"Size":6144,"Mallocs":1,"Frees":0},{"Size":6528,"Mallocs":0,"Frees":0},{"Size":6784,"Mallocs":0,"Frees":0},{"Size":6912,"Mallocs":0,"Frees":0},{"Size":8192,"Mallocs":1,"Frees":0},{"Size":9472,"Mallocs":8,"Frees":0},{"Size":9728,"Mallocs":0,"Frees":0},{"Size":10240,"Mallocs":0,"Frees":0},{"Size":10880,"Mallocs":0,"Frees":0},{"Size":12288,"Mallocs":0,"Frees":0},{"Size":13568,"Mallocs":0,"Frees":0},{"Size":14336,"Mallocs":0,"Frees":0},{"Size":16384,"Mallocs":0,"Frees":0},{"Size":18432,"Mallocs":0,"Frees":0}]},
"virtualMemory": {"active": 1957449728, "buffer": 0}
}

总结

标准库 expvar 为需要导出的公共变量提供了一个标准化的接口，使用比较简单。

expvar 包内部定义的几种基础类型都相应给出了并发安全的操作方法，我们不需要去重复实现一遍，能够开箱即用。

However, according to the statistics of public projects on https://go.dev/, the number of imports of this library is less than 10,000.

Compared with the number of imports of other standard libraries, expvar’s The sense of presence is too low.

The above is the detailed content of Use expvar to expose Go program running metrics. For more information, please follow other related articles on the PHP Chinese website!