log

Log is a simple, highly configurable, Structured Logging library

Why another logging library?

There's a lot of great stuff out there, but this library contains a number of unique features noted below using *.

Features

• Logger is simple, only logic to create the log entry and send it off to the handlers, they take it from there.
• Handlers are simple to write + easy to register + easy to remove
• *Ability to specify which log levels get sent to each handler
• *Handlers & Log Levels are configurable at runtime.
• *WithError automatically extracts and adds file, line and package in error output.
• *Convenient context helpers GetContext & SetContext
• *Works with go-playground/errors extracting wrapped errors, types and tags when used with the WithError interface. This is the default but configurable to support more or other error libraries using SetWithErrorFn.
• *Default logger for quick prototyping and cli applications. It is automatically removed when you register one of your own.

Installation

Use go get

go get github.com/go-playground/log/v8@latest

Usage

import the log package.

package main

import (
	"io"
	stdlog "log"

	"github.com/go-playground/errors/v5"
	"github.com/go-playground/log/v8"
)

func main() {
	log.RedirectGoStdLog(true)

	// Trace
	defer log.WithTrace().Info("time to run")

	log.Debug("debug")
	log.Info("info")
	log.Notice("notice")
	log.Warn("warn")
	log.Error("error")
	// log.Panic("panic") // this will panic
	log.Alert("alert")
	// log.Fatal("fatal") // this will call os.Exit(1)

	err := errors.New("this is the inner error").AddTags(errors.T("inner", "tag"))
	err = errors.Wrap(err, "this is the wrapping error").AddTags(errors.T("outer", "tag"))

	// logging with fields can be used with any of the above
	log.WithError(err).WithFields(log.F("key", "value")).Info("test info")

	// log unwrapped error
	log.WithError(io.EOF).Error("unwrapped error")

	// predefined global fields
	log.WithDefaultFields(log.Fields{
		log.F("program", "test"),
		log.F("version", "0.1.3"),
	}...)

	log.WithField("key", "value").Info("testing default fields")

	// or request scoped default fields
	logger := log.WithFields(
		log.F("request", "req"),
		log.F("scoped", "sco"),
	)

	logger.WithField("key", "value").Info("test")

	stdlog.Println("This was redirected from Go STD output!")
	log.RedirectGoStdLog(false)
	stdlog.Println("This was NOT redirected from Go STD output!")
}

Adding your own Handler

package main

import (
	"bytes"
	"fmt"

	"github.com/go-playground/log/v8"
)

// CustomHandler is your custom handler
type CustomHandler struct {
	// whatever properties you need
}

// Log accepts log entries to be processed
func (c *CustomHandler) Log(e log.Entry) {

	// below prints to os.Stderr but could marshal to JSON
	// and send to central logging server
	//																						       ---------
	// 				                                                                 |----------> | console |
	//                                                                               |             ---------
	// i.e. -----------------               -----------------     Unmarshal    -------------       --------
	//     | app log handler | -- json --> | central log app | --    to    -> | log handler | --> | syslog |
	//      -----------------               -----------------       Entry      -------------       --------
	//      																         |             ---------
	//                                  									         |----------> | DataDog |
	//          																	        	   ---------
	b := new(bytes.Buffer)
	b.Reset()
	b.WriteString(e.Message)

	for _, f := range e.Fields {
		_, _ = fmt.Fprintf(b, " %s=%v", f.Key, f.Value)
	}
	fmt.Println(b.String())
}

func main() {

	cLog := new(CustomHandler)
	log.AddHandler(cLog, log.AllLevels...)

	// Trace
	defer log.WithTrace().Info("took this long")

	log.Debug("debug")
	log.Info("info")
	log.Notice("notice")
	log.Warn("warn")
	log.Error("error")
	// log.Panic("panic") // this will panic
	log.Alert("alert")
	// log.Fatal("fatal") // this will call os.Exit(1)

	// logging with fields can be used with any of the above
	log.WithField("key", "value").Info("test info")
}

Go 1.21+ slog compatibility

There is a compatibility layer for slog, which allows redirecting slog to this logger and ability to output to an slog.Handler+.

Log Level Definitions
---------------------

| Level  | Description                                                                                                                                                                                               |
|--------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Debug  | Info useful to developers for debugging the application, not useful during normal operations.                                                                                                             |
| Info   | Normal operational messages which may be harvested for reporting, measuring throughput, etc. no action required.                                                                                          |
| Notice | Normal but significant condition. Events that are unusual but not error conditions eg. might be summarized in an email to developers or admins to spot potential problems - no immediate action required. |
| Warn   | Warning messages, not an error, but indication that an error will occur if action is not taken, e.g. file system 85% full. Each item must be resolved within a given time.                                |
| Error  | Non-urgent failures, these should be relayed to developers or admins; each item must be resolved within a given time.                                                                                     |
| Panic  | A "panic" condition usually affecting multiple apps/servers/sites. At this level it would usually notify all tech staff on call.                                                                          |
| Alert  | Action must be taken immediately. Should be corrected immediately, therefore notify staff who can fix the problem. An example would be the loss of a primary ISP connection.                              |
| Fatal  | Should be corrected immediately, but indicates failure in a primary system, an example is a loss of a backup ISP connection. ( same as SYSLOG CRITICAL )                                                  |

Handlers
-------------
Pull requests for new handlers are welcome when they don't pull in dependencies, it is preferred to have a dedicated package in this case.

| Handler | Description                                                                                                                              | Docs                                                                                                                                                              |
| ------- | ---------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| json    | Allows for log messages to be sent to any wrtier in json format.                                                                         | [![GoDoc](https://godoc.org/github.com/go-playground/log/handlers/json?status.svg)](https://godoc.org/github.com/go-playground/log/handlers/json)                 |

Package Versioning
----------
This package strictly adheres to semantic versioning guidelines.