Original post

go.dev goreport build coverage stability-stable


  • No Dependencies
  • Intuitive Interfaces
  • Consistent Writers
    • FileWriter, rotating & effective
    • ConsoleWriter, colorful & templating
    • MultiWriter, multiple level dispatch
    • AsyncWriter, asynchronously writing
    • JournalWriter, systemd logging
    • EventlogWriter, windows system event
  • Third-party(StdLog/Grpc/Logr) Logger Interceptor
  • High Performance



// DefaultLogger is the global logger.
var DefaultLogger = Logger{
        Level:      DebugLevel,
        Caller:     0,
        TimeField:  "",
        TimeFormat: "",
        Writer:     os.Stderr,

// A Logger represents an active logging object that generates lines of JSON output to an io.Writer.
type Logger struct {
        // Level defines log levels.
        Level Level

        // Caller determines if adds the file:line of the "caller" key.
        Caller int

        // TimeField defines the time filed name in output.  It uses "time" in if empty.
        TimeField string

        // TimeFormat specifies the time format in output. It uses time.RFC3389 in if empty.
        // If set with `TimeFormatUnix`, `TimeFormatUnixMs`, times are formated as UNIX timestamp.
        TimeFormat string

        // Writer specifies the writer of output. It uses os.Stderr in if empty.
        Writer io.Writer

FileWriter & ConsoleWriter

// FileWriter is an io.WriteCloser that writes to the specified filename.
type FileWriter struct {
        // Filename is the file to write logs to.  Backup log files will be retained
        // in the same directory.
        Filename string

        // FileMode represents the file's mode and permission bits.  The default
        // mode is 0644
        FileMode os.FileMode

        // MaxSize is the maximum size in bytes of the log file before it gets rotated.
        MaxSize int64

        // MaxBackups is the maximum number of old log files to retain.  The default
        // is to retain all old log files
        MaxBackups int

        // LocalTime determines if the time used for formatting the timestamps in
        // log files is the computer's local time.  The default is to use UTC time.
        LocalTime bool

        // HostName determines if the hostname used for formatting in log files.
        HostName bool

        // ProcessID determines if the pid used for formatting in log files.
        ProcessID bool

// ConsoleWriter parses the JSON input and writes it in an
// (optionally) colorized, human-friendly format to output Writer.
// Default output format:
//     {Time} {Level} {Goid} {Caller} > {Message} {Key}={Value} {Key}={Value}
// Note: ConsoleWriter performance is not good, it will parses JSON input into
// structured records, then outputs them in a specific order.
type ConsoleWriter struct {
        // ColorOutput determines if used colorized output.
        ColorOutput bool

        // QuoteString determines if quoting string values.
        QuoteString bool

        // EndWithMessage determines if output message in the end.
        EndWithMessage bool

        // Template specifies an optional text/template for creating a
        // user-defined output format, available arguments are:
        //    type . struct {
        //        Time     string    // "2019-07-10T05:35:54.277Z"
        //        Level    Level     // log.InfoLevel
        //        Caller   string    // "prog.go:42"
        //        Goid     string    // "123"
        //        Message  string    // "a structure message"
        //        Stack    string    // "<stack string>"
        //        KeyValue []struct {
        //            Key   string       // "foo"
        //            Value string       // "bar"
        //        }
        //    }
        // See https://github.com/phuslu/log#template-console-writer for example.
        // If Template is not nil, ColorOutput, QuoteString and EndWithMessage are override.
        Template *template.Template

        // Writer is the output destination. using os.Stderr if empty.
        Writer io.Writer

Getting Started

Simple Logging Example

A out of box example. playground

package main

import (

func main() {
        log.Printf("Hello, %s", "世界")
        log.Info().Str("foo", "bar").Int("number", 42).Msg("hi, phuslog")
        log.Error().Str("foo", "bar").Int("number", 42).Msgf("oops, %s", "phuslog")

// Output:
//   {"time":"2020-03-22T09:58:41.828Z","message":"Hello, 世界"}
//   {"time":"2020-03-22T09:58:41.828Z","level":"info","foo":"bar","number":42,"message":"hi, phuslog"}
//   {"time":"2020-03-22T09:58:41.828Z","level":"error","foo":"bar","number":42,"message":"oops, phuslog"}

Note: By default log writes to os.Stderr

Customize the configuration and formatting:

To customize logger filed name and format. playground

log.DefaultLogger = log.Logger{
        Level:      log.InfoLevel,
        Caller:     1,
        TimeField:  "date",
        TimeFormat: "2006-01-02",
        Writer:     os.Stderr,
log.Info().Str("foo", "bar").Msgf("hello %s", "world")

// Output: {"date":"2019-07-04","level":"info","caller":"prog.go:16","foo":"bar","message":"hello world"}

Rotating File Writer

To log to a rotating file, use FileWriter. playground

package main

import (


func main() {
        logger := log.Logger{
                Level:      log.ParseLevel("info"),
                Writer:     &log.FileWriter{
                        Filename:   "main.log",
                        FileMode:   0600,
                        MaxSize:    50*1024*1024,
                        MaxBackups: 7,
                        LocalTime:  false,

        runner := cron.New(cron.WithSeconds(), cron.WithLocation(time.UTC))
        runner.AddFunc("0 0 * * * *", func() { logger.Writer.(*log.FileWriter).Rotate() })
        go runner.Run()

        for {
                logger.Info().Msg("hello world")

Pretty Console Writer

To log a human-friendly, colorized output, use ConsoleWriter. playground

if log.IsTerminal(os.Stderr.Fd()) {
        log.DefaultLogger = log.Logger{
                Caller: 1,
                Writer: &log.ConsoleWriter{
                        ColorOutput:    true,
                        QuoteString:    true,
                        EndWithMessage: true,

log.Debug().Int("everything", 42).Str("foo", "bar").Msg("hello world")
log.Info().Int("everything", 42).Str("foo", "bar").Msg("hello world")
log.Warn().Int("everything", 42).Str("foo", "bar").Msg("hello world")
log.Error().Err(errors.New("an error")).Msg("hello world")

Pretty logging

Note: pretty logging also works on windows console

Template Console Writer

To log with user-defined format(e.g. glog), using ConsoleWriter.Template. playground

package main

import (

var glog = (&log.Logger{
        Level:      log.InfoLevel,
        Caller:     1,
        TimeFormat: "0102 15:04:05.999999",
        Writer: &log.ConsoleWriter{
                Template: template.Must(template.New("").Parse(
                        `{{.Level.First}}{{.Time}} {{.Goid}} {{.Caller}}] {{.Message}}`)),

func main() {
        glog.Infof("hello glog %s", "Info")
        glog.Warnf("hello glog %s", "Earn")
        glog.Errorf("hello glog %s", "Error")
        glog.Fatalf("hello glog %s", "Fatal")

// Output:
// I0725 09:59:57.503246 19 console_test.go:183] hello glog Info
// W0725 09:59:57.504247 19 console_test.go:184] hello glog Earn
// E0725 09:59:57.504247 19 console_test.go:185] hello glog Error
// F0725 09:59:57.504247 19 console_test.go:186] hello glog Fatal

Note: refer to ColorTemplate and sprig to make it functional.

MultiWriter & AsyncWriter & JournalWriter & EventlogWriter

To log to different writers by different levels, use MultiWriter.

log.DefaultLogger.Writer = &log.MultiWriter{
        InfoWriter:   &log.FileWriter{Filename: "main.INFO"},
        WarnWriter:   &log.FileWriter{Filename: "main.WARNING"},
        ErrorWriter:  &log.FileWriter{Filename: "main.ERROR"},
        StderrWriter: &log.ConsoleWriter{ColorOutput: true},
        StderrLevel:  log.ErrorLevel,
log.Info().Int("number", 42).Str("foo", "bar").Msg("a info log")
log.Warn().Int("number", 42).Str("foo", "bar").Msg("a warn log")
log.Error().Int("number", 42).Str("foo", "bar").Msg("a error log")

To log to file asynchronously for performance stability, use AsyncWriter.

log.DefaultLogger.Writer = &log.AsyncWriter{
        BufferSize:   32 * 1024,
        ChannelSize:  100,
        SyncDuration: 5 * time.Second,
        Writer:       &log.FileWriter{Filename: "main.log"},
log.Info().Int("number", 42).Str("foo", "bar").Msg("a async info log")
log.Warn().Int("number", 42).Str("foo", "bar").Msg("a async warn log")

To log to linux systemd journald, using JournalWriter.

log.DefaultLogger.Writer = &log.JournalWriter{}
log.Info().Int("number", 42).Str("foo", "bar").Msg("hello world")

To log to windows system event, using EventlogWriter.

log.DefaultLogger.Writer = &log.EventlogWriter{
        Source: ".NET Runtime",
        ID:     1000,
log.Info().Int("number", 42).Str("foo", "bar").Msg("hello world")

Sugar Logger

In contexts where performance is nice, but not critical, use the SugaredLogger. It’s 20% slower than Logger but still faster than other structured logging packages playground

package main

import (

func main() {
        sugar := log.DefaultLogger.Sugar(log.NewContext(nil).Str("tag", "hi suagr").Value())
        sugar.Infof("hello %s", "世界")
        sugar.Infow("i am a leading message", "foo", "bar", "number", 42)

        sugar = sugar.Level(log.ErrorLevel)
        sugar.Printf("hello %s", "世界")
        sugar.Log("number", 42, "a_key", "a_value", "message", "a suagr message")

StdLog & Logr & Grpc Interceptor

Using wrapped loggers for stdlog/grpc/logr. playground

package main

import (
        stdLog "log"

func main() {
        ctx := log.NewContext(nil).Str("tag", "hi log").Value()

        var stdlog *stdLog.Logger = log.DefaultLogger.Std(log.InfoLevel, ctx, "prefix ", stdLog.LstdFlags)
        stdlog.Print("hello from stdlog Print")
        stdlog.Println("hello from stdlog Println")
        stdlog.Printf("hello from stdlog %s", "Printf")

        var grpclog grpclog.LoggerV2 = log.DefaultLogger.Grpc(ctx)
        grpclog.Infof("hello %s", "grpclog Infof message")
        grpclog.Errorf("hello %s", "grpclog Errorf message")

        var logrLog logr.Logger = log.DefaultLogger.Logr(ctx)
        logrLog = logrLog.WithName("a_named_logger").WithValues("a_key", "a_value")
        logrLog.Info("hello", "foo", "bar", "number", 42)
        logrLog.Error(errors.New("this is a error"), "hello", "foo", "bar", "number", 42)

Contextual Fields

To add preserved key:value pairs to each event, use NewContext. playground

ctx := log.NewContext(nil).Str("ctx_str", "a ctx str").Value()

logger := log.Logger{Level: log.InfoLevel}
logger.Debug().Context(ctx).Int("no0", 0).Msg("zero")
logger.Info().Context(ctx).Int("no1", 1).Msg("first")
logger.Info().Context(ctx).Int("no2", 2).Msg("second")

// Output:
//   {"time":"2020-07-12T05:03:43.949Z","level":"info","ctx_str":"a ctx str","no1":1,"message":"first"}
//   {"time":"2020-07-12T05:03:43.949Z","level":"info","ctx_str":"a ctx str","no2":2,"message":"second"}

High Performance

A quick and simple benchmark with logrus/zap/zerolog, which runs on github actions:

// go test -v -run=none -bench=. -benchtime=10s -benchmem log_test.go
package main

import (


var fakeMessage = "Test logging, but use a somewhat realistic message length. "

func BenchmarkLogrus(b *testing.B) {
        logger := logrus.New()
        for i := 0; i < b.N; i++ {
                logger.WithFields(logrus.Fields{"foo": "bar", "int": 42}).Info(fakeMessage)

func BenchmarkZap(b *testing.B) {
        logger := zap.New(zapcore.NewCore(
        for i := 0; i < b.N; i++ {
                logger.Info(fakeMessage, zap.String("foo", "bar"), zap.Int("int", 42))

func BenchmarkZeroLog(b *testing.B) {
        logger := zerolog.New(ioutil.Discard).With().Timestamp().Logger()
        for i := 0; i < b.N; i++ {
                logger.Info().Str("foo", "bar").Int("int", 42).Msg(fakeMessage)

func BenchmarkPhusLog(b *testing.B) {
        logger := log.Logger{Writer: ioutil.Discard}
        for i := 0; i < b.N; i++ {
                logger.Info().Str("foo", "bar").Int("int", 42).Msg(fakeMessage)

Performance results on 2020-08-20 test

BenchmarkLogrus-2             2548255              4737 ns/op            1832 B/op         31 allocs/op
BenchmarkZap-2          11387668              1074 ns/op             128 B/op          1 allocs/op
BenchmarkZeroLog-2      21846039               548 ns/op               0 B/op          0 allocs/op
BenchmarkPhusLog-2      51170454               233 ns/op               0 B/op          0 allocs/op


This log is heavily inspired by zerolog, glog, quicktemplate, gjson, zap and lumberjack.