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📃 Log

Logs serve as an essential tool for observing program behavior, diagnosing issues, and setting up corresponding alerts. Well-structured logs can significantly enhance search efficiency and streamline the troubleshooting process.

Fiber offers a default mechanism for logging to standard output. Additionally, it provides several global functions, including log.Info, log.Errorf, log.Warnw, among others, to facilitate comprehensive logging capabilities.

Log levels​

const (
    LevelTrace Level = iota
    LevelDebug
    LevelInfo
    LevelWarn
    LevelError
    LevelFatal
    LevelPanic
)

Custom log​

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

type CommonLogger interface {
    Logger
    FormatLogger
    WithLogger
}

type AllLogger interface {
    CommonLogger
    ControlLogger
    WithLogger
}

Note: The Fatal level method will terminate the program after printing the log message. Please use it with caution.

Basic Logging​

Logs of different levels can be directly printed. These will be entered into messageKey, with the default key being 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.")

Formatted Logging​

Logs of different levels can be formatted before printing. All such methods end with an 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")

Key-Value Logging​

Print a message with key-value pairs. If the key and value are not paired correctly, the log will output KEYVALS UNPAIRED.

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

Global log​

For projects that require a simple, global logging function to print messages at any time, Fiber provides a global log.

import "github.com/gofiber/fiber/v3/log"

log.Info("info")
log.Warn("warn")

These global log functions allow you to log messages conveniently throughout your project.

The above example uses the default log.DefaultLogger for standard output. You can also find various pre-implemented adapters under the contrib package such as fiberzap and fiberzerolog, or you can implement your own logger and set it as the global logger using log.SetLogger.This flexibility allows you to tailor the logging behavior to suit your project's needs.

Here's an example using a custom logger:

import (
    "log"
    fiberlog "github.com/gofiber/fiber/v3/log"
)

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

type customLogger struct {
    stdlog *log.Logger
}

// ...
// inject your custom logger
fiberlog.SetLogger(customLogger)

Set Level​

log.SetLevel sets the minimum level of logs that will be output. The default log level is LevelTrace.

Note that this method is not concurrent-safe.

import "github.com/gofiber/fiber/v3/log"

log.SetLevel(log.LevelInfo)

Setting the log level allows you to control the verbosity of the logs, filtering out messages below the specified level.

Set output​

log.SetOutput sets the output destination of the logger. By default, the logger outputs logs to the console.

Writing logs to stderr​

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

This allows you to customize where the logs are written, such as to a file, an external logging service, or any other desired destination.

Writing logs to a file​

Set the output destination to the file, in this case test.log:

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

Writing logs to both console and file​

The following example will write the logs to both test.log and stdout:

// 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)
log.SetOutput(iw)

Bind context​

To bind a logger to a specific context, use the following method. This will return a CommonLogger instance that is bound to the specified context.

commonLogger := log.WithContext(ctx)
commonLogger.Info("info")

Binding the logger to a context allows you to include context-specific information in your logs, improving traceability and debugging.