fiber/docs/api/log.md

---
id: log
title: 📃 Log
description: Fiber's built-in log package
sidebar_position: 6
---

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

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

## Custom Log

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

```go
type CommonLogger interface {
    Logger
    FormatLogger
    WithLogger
}

type AllLogger interface {
    CommonLogger
    ConfigurableLogger
    WithLogger
}
```

## Print Log

**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 logs will be entered into `messageKey`, with the default key being `msg`.

```go
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 Fish.")
log.Panic("The system is down.")
```

### Formatted Logging

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

```go
log.Debugf("Hello %s", "boy")
log.Infof("%d is the answer to life, the universe, and everything", 42)
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.", "fish")
```

### 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`.

```go
log.Debugw("", "greeting", "Hello", "target", "boy")
log.Infow("", "number", 42)
log.Warnw("", "job", "boss")
log.Errorw("", "name", "Master Shifu")
log.Fatalw("", "fruit", "fish")
```

## Global Log

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

```go
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](https://github.com/gofiber/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:

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

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

type customLogger struct {
    stdlog *log.Logger
}

// Implement required methods for the AllLogger interface...

// Inject your custom logger
fiberlog.SetLogger(&customLogger{
    stdlog: log.New(os.Stdout, "CUSTOM ", log.LstdFlags),
})
```

## Set Level

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

**Note:** This method is not **concurrent-safe**.

```go
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

```go
var logger fiberlog.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 a file, in this case `test.log`:

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

### Writing Logs to Both Console and File

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

```go
// Output to ./test.log file
file, err := os.OpenFile("test.log", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
if err != nil {
    log.Fatal("Failed to open log file:", err)
}
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.

```go
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.

## Logger

You can use Logger to retrieve the logger instance. It is useful when you need to access underlying methods of the logger.
To retrieve the Logger instance, use the following method:

```go
logger := fiberlog.DefaultLogger() // Call DefaultLogger to get the default logger instance

stdlogger, ok := logger.Logger().(*log.Logger) // Get the logger instance and assert it to *log.Logger
if !ok {
    panic("logger is not *log.Logger")
}

stdlogger.SetFlags(0) // Hide timestamp by setting flags to 0
```