A Go (golang) package providing high-performance asynchronous logging, message filtering by severity and category, and multiple message targets.



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ozzo-log is a Go package providing enhanced logging support for Go programs. It has the following features:

  • High performance through asynchronous logging;
  • Recording message severity levels;
  • Recording message categories;
  • Recording message call stacks;
  • Filtering via severity levels and categories;
  • Customizable message format;
  • Configurable and pluggable message handling through log targets;
  • Included console, file, network, and email log targets.


Go 1.2 or above.


Run the following command to install the package:

go get github.com/go-ozzo/ozzo-log

Getting Started

The following code snippet shows how you can use this package.

package main

import (

func main() {
    // creates the root logger
	logger := log.NewLogger()

	// adds a console target and a file target
	t1 := log.NewConsoleTarget()
	t2 := log.NewFileTarget()
	t2.FileName = "app.log"
	t2.MaxLevel = log.LevelError
	logger.Targets = append(logger.Targets, t1, t2)

	defer logger.Close()

	// calls log methods to log various log messages
	logger.Error("plain text error")
	logger.Error("error with format: %v", true)
	logger.Debug("some debug info")

	// customizes log category
	l := logger.GetLogger("app.services")
	l.Info("some info")
	l.Warning("some warning")


Loggers and Targets

A logger provides various log methods that can be called by application code to record messages of various severity levels.

A target filters log messages by their severity levels and message categories and processes the filtered messages in various ways, such as saving them in files, sending them in emails, etc.

A logger can be equipped with multiple targets each with different filtering conditions.

The following targets are included in the ozzo-log package.

  • ConsoleTarget: displays filtered messages to console window
  • FileTarget: saves filtered messages in a file (supporting file rotating)
  • NetworkTarget: sends filtered messages to an address on a network
  • MailTarget: sends filtered messages in emails

You can create a logger, configure its targets, and start to use logger with the following code:

// creates the root logger
logger := log.NewLogger()
logger.Targets = append(logger.Targets, target1, target2, ...)
...calling log methods...

Severity Levels

You can log a message of a particular severity level (following the RFC5424 standard) by calling one of the following methods of the Logger struct:

  • Emergency(): the system is unusable.
  • Alert(): action must be taken immediately.
  • Critical(): critical conditions.
  • Error(): error conditions.
  • Warning(): warning conditions.
  • Notice(): normal but significant conditions.
  • Info(): informational purpose.
  • Debug(): debugging purpose.

Message Categories

Each log message is associated with a category which can be used to group messages. For example, you may use the same category for messages logged by the same Go package. This will allow you to selectively send messages to different targets.

When you call log.NewLogger(), a root logger is returned which logs messages using the category named as app. To log messages with a different category, call the GetLogger() method of the root logger or a parent logger to get a child logger and then call its log methods:

logger := log.NewLogger()
// the message is of category "app"

l1 := logger.GetLogger("system")
// the message is of category "system"

l2 := l1.GetLogger("app.models")
// the message is of category "app.models"

Message Formatting

By default, each log message takes this format when being sent to different targets:

2015-10-22T08:39:28-04:00 [Error][app.models] something is wrong
...call stack (if enabled)...

You may customize the message format by specifying your own message formatter when calling Logger.GetLogger(). For example,

logger := log.NewLogger()
logger = logger.GetLogger("app", func (l *Logger, e *Entry) string {
    return fmt.Sprintf("%v [%v][%v] %v%v", e.Time.Format(time.RFC822Z), e.Level, e.Category, e.Message, e.CallStack)

Logging Call Stacks

By setting Logger.CallStackDepth as a positive number, it is possible to record call stack information for each log method call. You may further configure Logger.CallStackFilter so that only call stack frames containing the specified substring will be recorded. For example,

logger := log.NewLogger()
// record call stacks containing "myapp/src" up to 5 frames per log message
logger.CallStackDepth = 5
logger.CallStackFilter = "myapp/src"

Message Filtering

By default, messages of all severity levels will be recorded. You may customize Logger.MaxLevel to change this behavior. For example,

logger := log.NewLogger()
// only record messages between Emergency and Warning levels
logger.MaxLevel = log.LevelWarning

Besides filtering messages at the logger level, a finer grained message filtering can be done at target level. For each target, you can specify its MaxLevel similar to that with the logger; you can also specify which categories of the messages the target should handle. For example,

target := log.NewConsoleTarget()
// handle messages between Emergency and Info levels
target.MaxLevel = log.LevelInfo
// handle messages of categories which start with "system.db." or "app."
target.Categories = []string{"system.db.*", "app.*"}

Configuring Logger

When an application is deployed for production, a common need is to allow changing the logging configuration of the application without recompiling its source code. ozzo-log is designed with this in mind.

For example, you can use a JSON file to specify how the application and its logger should be configured:

    "Logger": {
        "Targets": [
                "type": "ConsoleTarget",
                "type": "FileTarget",
                "FileName": "app.log",
                "MaxLevel": 4   // Warning or above

Assuming the JSON file is app.json, in your application code you can use the ozzo-config package to load the JSON file and configure the logger used by the application:

package main

import (

func main() {
    c := config.New()
    // register the target types to allow configuring Logger.Targets.
    c.Register("ConsoleTarget", log.NewConsoleTarget)
    c.Register("FileTarget", log.NewFileTarget)

    logger := log.NewLogger()
    if err := c.Configure(logger, "Logger"); err != nil {

To change the logger configuration, simply modify the JSON file without recompiling the Go source files.

  • Logger is not working

    Logger is not working

    Hi! I try to use Logger as shown in example. The application builds without errors. The file is created, but it is always empty ( And In console nothing to.

    opened by kolkov 11
  • Add Chinese Translation

    Add Chinese Translation

    Hi Xue Qiang, please have a look and do not merge it at this moment. As in I am about to polish it and remove all the original English words in a while.

    opened by qiansen1386 4
  • 持续运行几个小时后日志调用会阻塞。



    环境是 windows 10 和 ubuntu x32 1404 , 语言版本:go version go1.6.2 windows/amd64


    package logger
    import (
    var logger = log.NewLogger()
    // main 函数调用这个函数初始化 log
    func Open(fileName string) {
        ct := log.NewConsoleTarget()
        ct.MaxLevel = log.LevelDebug
        ft := log.NewFileTarget()
        ft.FileName = fileName
        ft.MaxLevel = log.LevelInfo
        ft.MaxBytes = 1 * 1024 * 1024 //2M
        ft.BackupCount = 10
        ft.Rotate = true //自动切割旋转日志文件
        logger.Targets = append(logger.Targets, ct, ft)
    // 其他模块调用这个函数获得自己的 Logger 并使用
    func GetLogger(category string, formatter ...log.Formatter) *log.Logger {
        return logger.GetLogger(category, formatter...)
    func Close() {



    持续运行几个小时(最近这次是2个小时)后,日志调用阻塞,任何涉及到日志的流程全部阻塞,同时控制台、日志文件不会再输出新内容。 完全没有调用 ozzo 的代码能正常运行:我同时使用了 xorm ,打开了打印sql语句功能。有一个定时调用正常时不会调用 ozzo 打印日志, ozzo 完全阻塞后能看到这个流程还在继续运行,能够看到 xorm 定时打印到控制台的sql语句。


    2016-06-30T15:50:02+08:00 [Debug][yfforward] .......
    2016-06-30T15:50:02+08:00 [Info][yfforward] .......
    2016-06-30T15:50:03+08:00 [Info][yfforward] ......
    2016-06-30T15:50:03+08:00 [Debug][yfforward] ......
    2016-06-30T15:50:03+08:00 [Info][yfforward] ......
    2016-06-30T15:50:04+08:00 [Info][yfforward] ......
    [xorm] [info]  2016/06/30 15:50:19.176599 [sql] SELECT........
    [xorm] [info]  2016/06/30 15:50:19.177598 [sql] UPDATE.......
    [xorm] [info]  2016/06/30 15:50:40.648707 [sql] SELECT ........


    2016-06-30T15:50:00+08:00 [Info][yfforward] .....
    2016-06-30T15:50:01+08:00 [Info][yfforward] ........
    2016-06-30T15:50:01+08:00 [Info][yfforward] ......
    2016-06-30T15:50:02+08:00 [Info][yfforward] ......
    2016-06-30T15:50:02+08:00 [Info][yfforward] .....
    2016-06-30T15:50:03+08:00 [Info][yfforward] ......
    2016-06-30T15:50:03+08:00 [Info][yfforward] ........

    猜测可能是多线程竟态问题,但是使用 tace 选项编译的程序运行中并没有报任何错误...

    为了确认故障位置,针对 http api 服务 handle 函数增加了 fmt.Print ,代码如下:

        fmt.Printf("fmt.Print [ping] %v %v ping %v", r.RemoteAddr, rtype, raddr)
        log.Info("[ping] %v %v ping %v", r.RemoteAddr, rtype, raddr)
        fmt.Println("fmt.Print end")

    然后访问这个 api,控制台输出如下:

    ··· [xorm] [info] 2016/06/30 15:58:57.089083 [sql] UPDATEXXXXXXXXXXXXXX fmt.Print [ping] TCP ping www.163.com:80[xorm] [info] 2016/06/30 15:59:07.088631 [sql] SELECT XXXXXXXXXXXXXXXXXXXXXXX [xorm] [info] XXXXXXXXXXXXXXXXXXXXXX [xorm] [info] 2016/06/30 15:59:17.104377 XXXXXXXXXXXXXXXXXXXXXXXXX ···

    可以看到是ozzo日志调用阻塞了。xorm 的输出是上面那个不涉及 ozzo 的定时调用的输出。



    opened by GameXG 3
  • Catching standard panics

    Catching standard panics


    Sorry for a newbie question, but how could I catch standard panics with this? For example I got this panic from Go:

    2016/02/17 11:22:29 http: panic serving [::1]:58256: runtime error: index out of rang
    goroutine 17 [running]:
    net/http.(*conn).serve.func1(0xc0820b2000, 0x14110f8, 0xc0820ac018)
            c:/go/src/net/http/server.go:1287 +0xbc
    main.whoisHandler(0x1411278, 0xc0820b20b0, 0xc0820b01c0)
            C:/Projects/test/main.go:121 +0xeb7
    net/http.HandlerFunc.ServeHTTP(0x7d1de0, 0x1411278, 0xc0820b20b0, 0xc0820b01c0)
            c:/go/src/net/http/server.go:1422 +0x41
    net/http.(*ServeMux).ServeHTTP(0xc08205a3f0, 0x1411278, 0xc0820b20b0, 0xc0820b01c0)
            c:/go/src/net/http/server.go:1699 +0x184
    net/http.serverHandler.ServeHTTP(0xc0820a8000, 0x1411278, 0xc0820b20b0, 0xc0820b01c0)
            c:/go/src/net/http/server.go:1862 +0x1a5
            c:/go/src/net/http/server.go:1361 +0xbf5
    created by net/http.(*Server).Serve
            c:/go/src/net/http/server.go:1910 +0x3fd

    How could I log it properly and possibly redirect the output as well? Using the standard log library it's as simple as setting the log.SetOutput(logFile).

    P.S. I know this has to be a bad error handling, but what if I missed something and such a panic arises.

    opened by importnil 3
  • Debugging ozzo-config.Config

    Debugging ozzo-config.Config

    I builded a windows version of dalve and plugin that support it. But when I debugging my App, i don't see any values loaded into this struct...

    func LoadConfig() *config.Config{
        // Load config
        c := config.New()
        return c
      "Version": "1.0",
      "StaticServer": {
        "Path": "./static"
      "WebServer": {
        "PortHTTP": "8080",
        "PortHTTPS": "8088",
        "Domain": "localhost"
      "Logger": {
        "Filename": "app.log",
        "Email": "****@gmail.com"
        "Type": "mysql",
        "Host": "localhost",
        "DbName": "inframe",
        "Login": "root",
        "Password": ""

    image Are you see the same in Linux or more?

    opened by kolkov 0
  • Error of wrong Target

    Error of wrong Target

            target := log.NewFileTarget()
    	target.FileName = "\"qweqw\""
    	l.logger.Targets = append(l.logger.Targets, target)
            err = logger.Open()
    	if err != nil {
            // run my code

    After running this code I get the message only:

    Failed to open target: FileTarget was unable to create a log file: open "qweqw": The filename, directory name, or volume label syntax is incorrect.

    And the code is executed further silent! (no panic)

    IMHO it's wrong behavior

    opened by githubjeka 0
  • create directories from config file

    create directories from config file

    I want to ask to add it creating folders from config file if it is not exist. for example: "type": "FileTarget", "FileName": "c:/sed/1.log" the library creates c:\sed

    To rewrite localy the library is bad decision.

    opened by annakozyreva1 0
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