logrus/README.md

# Logrus <img src="http://i.imgur.com/hTeVwmJ.png" width="40" height="40" alt=":walrus:" class="emoji" title=":walrus:"/>&nbsp;[![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]. **Please note the Logrus API is not
yet stable (pre 1.0), the core API is unlikely change much but please version
control your Logrus to make sure you aren't fetching latest `master` on every
build.**

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!")
}
```

#### Package logging

Alike the stdlib logger, logrus exposes functions that you can use to log
to a default global logger. This is convenient to avoid passing a
`logrus.Logger` thorough your app's packages; you can simply setup `logrus
from your main package and use the package function directly accross your app.

#### 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.ErrorLevel,
    logrus.FatalLevel,
    logrus.PanicLevel,
  }
}
```

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
}
```

#### Rotation

Log rotation is not provided with Logrus. Log rotation should be done by an
external program (like `logrotated(8)`) that can compress and delete old log
entries. It should not be a feature of the application-level logger.


[godoc]: https://godoc.org/github.com/Sirupsen/logrus