# Logrus <img src="http://i.imgur.com/hTeVwmJ.png" width="40" height="40" alt=":walrus:" class="emoji" title=":walrus:"/> [![Build Status](https://travis-ci.org/Sirupsen/logrus.svg?branch=master)](https://travis-ci.org/Sirupsen/logrus) Logrus is a structured logger for Go (golang), completely API compatible with the standard library logger. [Godoc][godoc]. Nicely color-coded in development (when a TTY is attached, otherwise just plain text): ![Colored](http://i.imgur.com/PY7qMwd.png) With `log.Formatter = new(logrus.JSONFormatter)`, for easy parsing by logstash or Splunk: ```json {"animal":"walrus","level":"info","msg":"A group of walrus emerges from the ocean","size":10,"time":"2014-03-10 19:57:38.562264131 -0400 EDT"} {"level":"warning","msg":"The group's number increased tremendously!", "number":122,"omg":true,"time":"2014-03-10 19:57:38.562471297 -0400 EDT"} {"animal":"walrus","level":"info","msg":"A giant walrus appears!", "size":10,"time":"2014-03-10 19:57:38.562500591 -0400 EDT"} {"animal":"walrus","level":"info","msg":"Tremendously sized cow enters the ocean.", "size":9,"time":"2014-03-10 19:57:38.562527896 -0400 EDT"} {"level":"fatal","msg":"The ice breaks!","number":100,"omg":true, "time":"2014-03-10 19:57:38.562543128 -0400 EDT"} ``` With the default `log.Formatter = new(logrus.TextFormatter)` when a TTY is not attached, the output is compatible with the [l2met](http://r.32k.io/l2met-introduction) format: ```text time="2014-04-20 15:36:23.830442383 -0400 EDT" level="info" msg="A group of walrus emerges from the ocean" animal="walrus" size=10 time="2014-04-20 15:36:23.830584199 -0400 EDT" level="warning" msg="The group's number increased tremendously!" omg=true number=122 time="2014-04-20 15:36:23.830596521 -0400 EDT" level="info" msg="A giant walrus appears!" animal="walrus" size=10 time="2014-04-20 15:36:23.830611837 -0400 EDT" level="info" msg="Tremendously sized cow enters the ocean." animal="walrus" size=9 time="2014-04-20 15:36:23.830626464 -0400 EDT" level="fatal" msg="The ice breaks!" omg=true number=100 ``` #### Example Note again that Logrus is API compatible with the stdlib logger, so if you remove the `log` import and create a global `log` variable as below it will just work. ```go package main import ( "github.com/Sirupsen/logrus" ) var log = logrus.New() func init() { log.Formatter = new(logrus.JSONFormatter) log.Formatter = new(logrus.TextFormatter) // default } func main() { log.WithFields(logrus.Fields{ "animal": "walrus", "size": 10, }).Info("A group of walrus emerges from the ocean") log.WithFields(logrus.Fields{ "omg": true, "number": 122, }).Warn("The group's number increased tremendously!") log.WithFields(logrus.Fields{ "omg": true, "number": 100, }).Fatal("The ice breaks!") } ``` #### Fields Logrus encourages careful, structured logging though logging fields instead of long, unparseable error messages. For example, instead of: `log.Fatalf("Failed to send event %s to topic %s with key %d")`, you should log the much more discoverable: ```go log = logrus.New() log.WithFields(logrus.Fields{ "event": event, "topic": topic, "key": key }).Fatal("Failed to send event") ``` We've found this API forces you to think about logging in a way that produces much more useful logging messages. We've been in countless situations where just a single added field to a log statement that was already there would've saved us hours. The `WithFields` call is optional. In general, with Logrus using any of the `printf`-family functions should be seen as a hint you should add a field, however, you can still use the `printf`-family functions with Logrus. #### Hooks You can add hooks for logging levels. For example to send errors to an exception tracking service on `Error`, `Fatal` and `Panic`, info to StatsD or log to multiple places simultaneously, e.g. syslog. ```go // Not the real implementation of the Airbrake hook. Just a simple sample. var log = logrus.New() func init() { log.Hooks.Add(new(AirbrakeHook)) } type AirbrakeHook struct{} // `Fire()` takes the entry that the hook is fired for. `entry.Data[]` contains // the fields for the entry. See the Fields section of the README. func (hook *AirbrakeHook) Fire(entry *logrus.Entry) error { err := airbrake.Notify(entry.Data["error"].(error)) if err != nil { log.WithFields(logrus.Fields{ "source": "airbrake", "endpoint": airbrake.Endpoint, }).Info("Failed to send error to Airbrake") } return nil } // `Levels()` returns a slice of `Levels` the hook is fired for. func (hook *AirbrakeHook) Levels() []logrus.Level { return []logrus.Level{ logrus.Error, logrus.Fatal, logrus.Panic, } } ``` Logrus comes with built-in hooks. Add those, or your custom hook, in `init`: ```go import ( "github.com/Sirupsen/logrus" "github.com/Sirupsen/logrus/hooks/airbrake" ) func init() { log.Hooks.Add(new(logrus_airbrake.AirbrakeHook)) } ``` * [`github.com/Sirupsen/logrus/hooks/airbrake`](https://github.com/Sirupsen/logrus/blob/master/hooks/airbrake/airbrake.go). Send errors to an exception tracking service compatible with the Airbrake API. Uses [`airbrake-go`](https://github.com/tobi/airbrake-go) behind the scenes. #### Level logging Logrus has six logging levels: Debug, Info, Warning, Error, Fatal and Panic. ```go log.Debug("Useful debugging information.") log.Info("Something noteworthy happened!") log.Warn("You should probably take a look at this.") log.Error("Something failed but I'm not quitting.") // Calls os.Exit(1) after logging log.Fatal("Bye.") // Calls panic() after logging log.Panic("I'm bailing.") ``` You can set the logging level on a `Logger`, then it will only log entries with that severity or anything above it: ```go // Will log anything that is info or above (warn, error, fatal, panic). Default. log.Level = logrus.Info ``` It may be useful to set `log.Level = logrus.Debug` in a debug or verbose environment if your application has that. #### Entries Besides the fields added with `WithField` or `WithFields` some fields are automatically added to all logging events: 1. `time`. The timestamp when the entry was created. 2. `msg`. The logging message passed to `{Info,Warn,Error,Fatal,Panic}` after the `AddFields` call. E.g. `Failed to send event.` 3. `level`. The logging level. E.g. `info`. #### Environments Logrus has no notion of environment. If you wish for hooks and formatters to only be used in specific environments, you should handle that yourself. For example, if your application has a global variable `Environment`, which is a string representation of the environment you could do: ```go init() { // do something here to set environment depending on an environment variable // or command-line flag log := logrus.New() if Environment == "production" { log.Formatter = new(logrus.JSONFormatter) } else { // The TextFormatter is default, you don't actually have to do this. log.Formatter = new(logrus.TextFormatter) } } ``` This configuration is how `logrus` was intended to be used, but JSON in production is mostly only useful if you do log aggregation with tools like Splunk or Logstash. #### Formatters The built-in logging formatters are: * `logrus.TextFormatter`. Logs the event in colors if stdout is a tty, otherwise without colors. * *Note:* to force colored output when there is no TTY, set the `ForceColors` field to `true`. * `logrus.JSONFormatter`. Logs fields as JSON. Third party logging formatters: * [`zalgo`](https://github.com/aybabtme/logzalgo): invoking the P͉̫o̳̼̊w̖͈̰͎e̬͔̭͂r͚̼̹̲ ̫͓͉̳͈ō̠͕͖̚f̝͍̠ ͕̲̞͖͑Z̖̫̤̫ͪa͉̬͈̗l͖͎g̳̥o̰̥̅!̣͔̲̻͊̄ ̙̘̦̹̦. You can define your formatter by implementing the `Formatter` interface, requiring a `Format` method. `Format` takes an `*Entry`. `entry.Data` is a `Fields` type (`map[string]interface{}`) with all your fields as well as the default ones (see Entries section above): ```go type MyJSONFormatter struct { } log.Formatter = new(MyJSONFormatter) func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) { serialized, err := json.Marshal(entry.Data) if err != nil { return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err) } return append(serialized, '\n'), nil } ``` [godoc]: https://godoc.org/github.com/Sirupsen/logrus