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📃 Log

We can use logs to observe program behavior, diagnose problems, or configure corresponding alarms. And defining a well structured log can improve search efficiency and facilitate handling of problems.

Fiber provides a default way to print logs in the standard output. It also provides several global functions, such as log.Info, log.Errorf, log.Warnw, etc.

Log levels

const (
LevelTrace Level = iota

Custom log

Fiber provides the AllLogger interface for adapting the various log libraries.

type CommonLogger interface {

type AllLogger interface {

Note: The method of calling the Fatal level will interrupt the program running after printing the log, please use it with caution. Directly print logs of different levels, which will be entered into messageKey, the default is msg.

log.Info("Hello, World!")
log.Debug("Are you OK?")
log.Info("42 is the answer to life, the universe, and everything")
log.Warn("We are under attack!")
log.Error("Houston, we have a problem.")
log.Fatal("So Long, and Thanks for All the Fislog.")
log.Panic("The system is down.")

Format and print logs of different levels, all methods end with f

log.Debugf("Hello %s", "boy")
log.Infof("%d is the answer to life, the universe, and everything", 233)
log.Warnf("We are under attack %s!", "boss")
log.Errorf("%s, we have a problem.", "Master Shifu")
log.Fatalf("So Long, and Thanks for All the %s.", "banana")

Print a message with the key and value, or KEYVALS UNPAIRED if the key and value are not a pair.

log.Debugw("", "Hello", "boy")
log.Infow("", "number", 233)
log.Warnw("", "job", "boss")
log.Errorw("", "name", "Master Shifu")
log.Fatalw("", "fruit", "banana")

Global log

If you are in a project and just want to use a simple log function that can be printed at any time in the global, we provide a global log.

import ""


The above is using the default log.DefaultLogger standard output. You can also find an already implemented adaptation under contrib, or use your own implemented Logger and use log.SetLogger to set the global log logger.

import (
fiberlog ""

var _ log.AllLogger = (*customLogger)(nil)

type customLogger struct {
stdlog *log.Logger

// ...
// inject your custom logger

Set Level

log.SetLevel sets the level of logs below which logs will not be output. The default logger is LevelTrace.

Note that this method is not concurrent-safe.

import ""


Set output

log.SetOutput sets the output destination of the logger. The default logger types the log in the console.

var logger AllLogger = &defaultLogger{
stdlog: log.New(os.Stderr, "", log.LstdFlags|log.Lshortfile|log.Lmicroseconds),
depth: 4,

Set the output destination to the file.

// Output to ./test.log file
f, err := os.OpenFile("test.log", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
if err != nil {

Set the output destination to the console and file.

// Output to ./test.log file
file, _ := os.OpenFile("test.log", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
iw := io.MultiWriter(os.Stdout, file)

Bind context

Set the context, using the following method will return a CommonLogger instance bound to the specified context

commonLogger := log.WithContext(ctx)