Pull 与 Push
与监控系统对接方式有两种，一种是 Pull（拉取），另外一种 Push（推送）。
以 Prometheus 为例，应用程序通过暴露出 HTTP 接口，让 Prometheus 周期性地通过该接口抓取指标，这就是 Pull。而 Push 是应用程序主动将指标推送给 PushGateway， Prometheus 则去 PushGateway 抓取数据。
Go 标准库中有一个名为 expvar 的包，它的名字由 exp 和 var 两部分组合而成，意味着导出变量。
expvar 为公共变量提供了标准化的接口，并通过 HTTP 以 Json 的格式将这些变量暴露出去，很适合采用 Pull 的方式与监控系统进行对接。
使用 expvar 库
expvar 是标准库，意味着我们并不要额外的依赖，并且它还提供了一些开箱即用的指标。下面我们来学习一下该库的使用。
当引用了 expvar 库（import “expvar”），以下 init 函数将被自动调用。
func init() { http.HandleFunc("/debug/vars", expvarHandler) Publish("cmdline", Func(cmdline)) Publish("memstats", Func(memstats))}
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该函数为我们注册了 /debug/vars 路径的 HTTP 服务，访问该路径将得到 Json 格式的指标。
因此，我们还需要调用 ListenAndServe 绑定端口，并开始服务 HTTP 请求。
http.ListenAndServe(":8080", nil)
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完整代码如下
package mainimport ( _ "expvar" "net/http")func main() { http.ListenAndServe(":8080", nil)}
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将程序运行起来后，通过 curl 请求，得到以下结果
$ 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}]}}
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可以看到，expvar 默认已提供了两项指标，分别是程序执行命令（os.Args）和运行时内存分配（runtime.Memstats）信息。
expvar 库重点内容
expvar 中最重要的是 Publish 函数和 Var 接口。
func Publish(name string, v Var) {}
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Publish 函数签名中需要两个参数，name 是我们指定的指标名。例如在上文 expvar 的 init 函数下 Publish(“cmdline”, Func(cmdline))代码行，其中cmdline就是指标名，而Func(cmdline)则是实现了 Var 接口的 expvar.Func 类型变量。
Var 接口，它只定义了一个 String 方法。需要注意的是，该方法必须要返回一个有效的 Json字符串。
// 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}
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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
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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”)
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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}
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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}
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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, }}
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最后，通过 Publish 函数注册指标名即可。
expvar.Publish("metricName", expvar.Func(MyStructData))
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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 mainimport ( "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) }}
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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}}
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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。
但是， 在 https://go.dev/ 上统计的公共项目，该库的 import 数量还不足 1万。
相比于其他标准库的 import 数量而言，expvar 的存在感太低了。
以上就是使用 expvar 暴露 Go 程序运行指标的详细内容，更多请关注【创想鸟】其它相关文章！
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