# 应用程序的日志管理

程序中记录日志的首要目的：Troubleshooting。通过记录程序中对外部系统与模块的依赖调用、重要状态信息的变化、关键变量、关键逻辑等，显示基于时间轴的程序运行轨迹，显示业务是否正常、是否存在非预期执行，在出问题时方便还原现场，推断程序运行过程、理清问题的方向。

本文将讨论在实现日志功能过程中常见的一些问题，包括基础API、格式化、日志转发及可视化等方面，代码采用Go语言描述。

## 一、Basic

**1、后台输出**

```go
package main

import (
  "fmt"
)

func main(){
  fmt.Println("------hello world-----")
}
```

**2、There are no exceptions in Golang, only errors.**

Go语言不支持传统的 try…catch…finally 这种异常，因为Go语言的设计者们认为，将异常与控制结构混在一起会很容易使得代码变得混乱。因为开发者很容易滥用异常，甚至一个小小的错误都抛出一个异常，替代方案是使用多值返回来返回错误。当然Go并不是全面否定异常的存在，或者用recover+panic语法实现，只是极力不鼓励多用异常。

```go
package main

import (
    "log"
    "errors"
    "fmt"
)

func main() {
   /* local variable definition */
  ...

   /* function for division which return an error if divide by 0 */
   ret,err = div(a, b)
   if err != nil {
      log.Fatal(err)
    }
    fmt.Println(ret)
}
```

**3、写入日志文件:**

```go
package main

import (
  "log"
  "os"
)

func main(){
  f,err :=os.OpenFile("test.log",os.O_WRONLY|os.O_CREATE|os.O_APPEND,0644)
  if err !=nil{
    log.Fatal(err)
  }
  defer f.Close()
  log.SetOutput(f)
  log.Println("==========works==============")
}
```

```bash
YRMacBook-Pro:go-log yanrui$ more test.log
2017/05/24 21:46:25 ==========works==============
```

## 二、格式化

推荐日志工具库：logrus

```go
$ go get github.com/Sirupsen/logrus
```

**1、JSON format**

```go
package main

import (
  log "github.com/Sirupsen/logrus"
  "github.com/logmatic/logmatic-go"
)

func main() {
    // use JSONFormatter
    log.SetFormatter(&logmatic.JSONFormatter{})
    // log an event as usual with logrus
    log.WithFields(log.Fields{"string": "foo", "int": 1, "float": 1.1 }).Info("My first ssl event from golang")
}
```

日志输出样式：

```go
{
  "@marker":["sourcecode","golang"],
  "date":"2017-05-24T15:27:40+08:00",
  "float":1.1,"int":1,"level":"info",
  "message":"My first ssl event from golang",
  "string":"foo"
}
```

## 三、附加上下文

通过logrus库可以加入一些上下文信息，例如：主机名称，程序名称或者会话参数等。

```go
contextLogger := log.WithFields(log.Fields{
  "common": "XXX common content XXX",
  "other": "YYY special context YYY",
})

contextLogger.Info("AAAAAAAAAAAA")
contextLogger.Info("BBBBBBBBBBBB")
```

日志输出样式：

```bash
YRMacBook-Pro:go-log yanrui$ go run LogMatic.go
{"@marker":["sourcecode","golang"],"common":"XXX common content XXX","date":"2017-05-24T17:00:08+08:00","level":"info","message":"AAAAAAAAAAAA","other":"YYY special context YYY"}
{"@marker":["sourcecode","golang"],"common":"XXX common content XXX","date":"2017-05-24T17:00:08+08:00","level":"info","message":"BBBBBBBBBBBB","other":"YYY special context YYY"}
YRMacBook-Pro:go-log yanrui$
```

## 四、Hooks

我们还可以利用Hook机制实现日志功能扩展，例如Syslog hook，将输出的日志发送到指定的Syslog服务。

```go
package main

import (
  log "github.com/sirupsen/logrus"
  "gopkg.in/gemnasium/logrus-airbrake-hook.v2" // the package is named "aibrake"
  logrus_syslog "github.com/sirupsen/logrus/hooks/syslog"
  "log/syslog"
)

func main(){
    hook, err := logrus_syslog.NewSyslogHook("udp", "59.37.0.1:514", syslog.LOG_INFO, "")
    if err != nil {
      log.Error("Unable to connect to local syslog daemon")
    } else {
      log.AddHook(hook)
    }
}
```

验证是否发送Syslog：

```go
$ sudo tcpdump | grep 59.37.0.1
tcpdump: data link type PKTAP
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on pktap, link-type PKTAP (Apple DLT_PKTAP), capture size 262144 bytes
18:51:05.663612 IP 192.168.199.15.58819 > 59.37.0.1.syslog: SYSLOG kernel.info, length: 314
18:51:05.663657 IP 192.168.199.15.58819 > 59.37.0.1.syslog: SYSLOG kernel.info, length: 314
```

## 五、可视化

在真实场景中日志数据体量非常庞大，日志存储只是第一步，更多的情况是需要查看特定指标或者能够快速检索信息，此时日志分析平台就发挥作用了。以logmatic为例，可以在它的官网注册<https://logmatic.io/，免费体验。>

在使用logmatic之前，需要下载它的hook支持：

```go
$ go get github.com/logmatic/logmatic-go
```

```go
func main() {
    // instantiate a new Logger with your Logmatic APIKey
    // 国内访问比较慢
      log.AddHook(logmatic.NewLogmaticHook("p53uTkOhSEqI3-116DynkQ"))

    // ..........
}
```

效果如下： ![](http://riboseyim-qiniu.riboseyim.com/logmatic_demo_1.png)

![](http://riboseyim-qiniu.riboseyim.com/logmatic_demo_2.png)


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