Dead simple, super fast, zero allocation and modular logger for Golang
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Latest commit eb9cb12 Oct 18, 2018

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Onelog is a dead simple but very efficient JSON logger. It is one of the fastest JSON logger out there. Also, it is one of the logger with the lowest allocation.

It gives more control over log levels enabled by using bitwise operation for setting levels on a logger.

It is also modular as you can add a custom hook, define level text values, level and message keys.

Go 1.9 is required as it uses a type alias over gojay.Encoder.

It is named onelog as a reference to zerolog and because it sounds like One Love song from Bob Marley :)

Get Started

go get

Basic usage:

import ""

func main() {
    // create a new Logger
    // first argument is an io.Writer
    // second argument is the level, which is an integer
    logger := onelog.New(
        onelog.ALL, // shortcut for onelog.DEBUG|onelog.INFO|onelog.WARN|onelog.ERROR|onelog.FATAL,
    logger.Info("hello world !") // {"level":"info","message":"hello world"}


Levels are ints mapped to a string. The logger will check if level is enabled with an efficient bitwise &(AND), if disabled, it returns right away which makes onelog the fastest when running disabled logging with 0 allocs and less than 1ns/op. See benchmarks

When creating a logger you must use the | operator with different levels to toggle bytes.

Example if you want levels INFO and WARN:

logger := onelog.New(

This allows you to have a logger with different levels, for example you can do:

var logger *onelog.Logger

func init() {
    // if we are in debug mode, enable DEBUG lvl
    if os.Getenv("DEBUG") != "" {
        logger = onelog.New(
            onelog.ALL, // shortcut for onelog.DEBUG|onelog.INFO|onelog.WARN|onelog.ERROR|onelog.FATAL
    logger = onelog.New(

Available levels:

  • onelog.DEBUG
  • onelog.INFO
  • onelog.WARN
  • onelog.ERROR
  • onelog.FATAL

You can change their textual values by doing, do this only once at runtime as it is not thread safe:

onelog.LevelText(onelog.INFO, "INFO")


You can define a hook which will be run for every log message.


logger := onelog.New(
logger.Hook(func(e onelog.Entry) {
    e.String("time", time.Now().Format(time.RFC3339))
logger.Info("hello world !") // {"level":"info","message":"hello world","time":"2018-05-06T02:21:01+08:00"}


Without extra fields

Logging without extra fields is easy as:

logger := onelog.New(
logger.Debug("i'm not sure what's going on") // {"level":"debug","message":"i'm not sure what's going on"}
logger.Info("breaking news !") // {"level":"info","message":"breaking news !"}
logger.Warn("beware !") // {"level":"warn","message":"beware !"}
logger.Error("my printer is on fire") // {"level":"error","message":"my printer is on fire"}
logger.Fatal("oh my...") // {"level":"fatal","message":"oh my..."}

With extra fields

Logging with extra fields is quite simple, specially if you have used gojay:

logger := onelog.New(

logger.DebugWithFields("i'm not sure what's going on", func(e onelog.Entry) {
    e.String("string", "foobar")
    e.Int("int", 12345)
    e.Int64("int64", 12345)
    e.Float("float64", 0.15)
    e.Bool("bool", true)
    e.Err("err", errors.New("someError"))
    e.ObjectFunc("user", func(e Entry) {
        e.String("name", "somename")
// {"level":"debug","message":"i'm not sure what's going on","string":"foobar","int":12345,"int64":12345,"float64":0.15,"bool":true,"err":"someError","user":{"name":"somename"}}

logger.InfoWithFields("breaking news !", func(e onelog.Entry) {
    e.String("userID", "123455")
// {"level":"info","message":"breaking news !","userID":"123456"}

logger.WarnWithFields("beware !", func(e onelog.Entry) {
    e.String("userID", "123455")
// {"level":"warn","message":"beware !","userID":"123456"}

logger.ErrorWithFields("my printer is on fire", func(e onelog.Entry) {
    e.String("userID", "123455")
// {"level":"error","message":"my printer is on fire","userID":"123456"}

logger.FatalWithFields("oh my...", func(e onelog.Entry) {
    e.String("userID", "123455")
// {"level":"fatal","message":"oh my...","userID":"123456"}

Alternatively, you can use the chain syntax:

logger.InfoWith("foo bar").
    Int("testInt", 1).
    Int64("testInt64", 2).
    Float("testFloat", 1.15234).
    String("testString", "string").
    Bool("testBool", true).
    ObjectFunc("testObj", func(e Entry) {
        e.Int("testInt", 100)
    Object("testObj2", testObj). // implementation of gojay.MarshalerJSONObject
    Array("testArr", testArr). // implementation of gojay.MarshalerJSONArray
    Err("testErr", errors.New("my printer is on fire !")).
    Write() // don't forget to call this method! 

Accumulate context

You can create get a logger with some accumulated context that will be included on all logs created by this logger.

To do that, you must call the With method on a logger. Internally it creates a copy of the current logger and returns it.


logger := onelog.New(
).With(func(e onelog.Entry) {
    e.String("userID", "123456")

logger.Info("user logged in") // {"level":"info","message":"user logged in","userID","123456"}

logger.Debug("wtf?") // {"level":"debug","message":"wtf?","userID","123456"}

logger.ErrorWithFields("Oops", func(e onelog.Entry) {
    e.String("error_code", "ROFL")
}) // {"level":"error","message":"oops","userID","123456","error_code":"ROFL"}

Change levels txt values, message and/or level keys

You can change globally the levels values by calling the function:

onelog.LevelText(onelog.INFO, "INFO")

You can change the key of the message by calling the function:


You can change the key of the level by calling the function:


Beware, these changes are global (affects all instances of the logger). Also, these function should be called only once at runtime to avoid any data race issue.


For thorough benchmarks please see the results in the bench suite created by the author of zerolog here:

The benchmarks data presented below is the one from Uber's benchmark suite where we added onelog.

Benchmarks are here:

Disabled Logging

ns/op bytes/op allocs/op
Zap 8.73 0 0
zerolog 2.45 0 0
logrus 12.1 16 1
onelog 0.74 0 0

Disabled with fields

ns/op bytes/op allocs/op
Zap 208 768 5
zerolog 68.7 128 4
logrus 721 1493 12
onelog 1.31 0 0
onelog-chain 68.2 0 0

Logging basic message

ns/op bytes/op allocs/op
Zap 205 0 0
zerolog 135 0 0
logrus 1256 1554 24
onelog 84.8 0 0

Logging basic message and accumulated context

ns/op bytes/op allocs/op
Zap 276 0 0
zerolog 141 0 0
logrus 1256 1554 24
onelog 82.4 0 0

Logging message with extra fields

ns/op bytes/op allocs/op
Zap 1764 770 5
zerolog 1210 128 4
logrus 13211 13584 129
onelog 971 128 4
onelog-chain 1030 128 4