prometheus/vendor/github.com/sirupsen/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)&nbsp;[![GoDoc](https://godoc.org/github.com/sirupsen/logrus?status.svg)](https://godoc.org/github.com/sirupsen/logrus)

Logrus is a structured logger for Go (golang), completely API compatible with
the standard library logger.

**Seeing weird case-sensitive problems?** It's in the past been possible to
import Logrus as both upper- and lower-case. Due to the Go package environment,
this caused issues in the community and we needed a standard. Some environments
experienced problems with the upper-case variant, so the lower-case was decided.
Everything using `logrus` will need to use the lower-case:
`github.com/sirupsen/logrus`. Any package that isn't, should be changed.

To fix Glide, see [these
comments](https://github.com/sirupsen/logrus/issues/553#issuecomment-306591437).
For an in-depth explanation of the casing issue, see [this
comment](https://github.com/sirupsen/logrus/issues/570#issuecomment-313933276).

**Are you interested in assisting in maintaining Logrus?** Currently I have a
lot of obligations, and I am unable to provide Logrus with the maintainership it
needs. If you'd like to help, please reach out to me at `simon at author's
username dot com`.

Nicely color-coded in development (when a TTY is attached, otherwise just
plain text):

![Colored](http://i.imgur.com/PY7qMwd.png)

With `log.SetFormatter(&log.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.SetFormatter(&log.TextFormatter{})` when a TTY is not
attached, the output is compatible with the
[logfmt](http://godoc.org/github.com/kr/logfmt) format:

```text
time="2015-03-26T01:27:38-04:00" level=debug msg="Started observing beach" animal=walrus number=8
time="2015-03-26T01:27:38-04:00" level=info msg="A group of walrus emerges from the ocean" animal=walrus size=10
time="2015-03-26T01:27:38-04:00" level=warning msg="The group's number increased tremendously!" number=122 omg=true
time="2015-03-26T01:27:38-04:00" level=debug msg="Temperature changes" temperature=-4
time="2015-03-26T01:27:38-04:00" level=panic msg="It's over 9000!" animal=orca size=9009
time="2015-03-26T01:27:38-04:00" level=fatal msg="The ice breaks!" err=&{0x2082280c0 map[animal:orca size:9009] 2015-03-26 01:27:38.441574009 -0400 EDT panic It's over 9000!} number=100 omg=true
```
To ensure this behaviour even if a TTY is attached, set your formatter as follows:

```go
	log.SetFormatter(&log.TextFormatter{
		DisableColors: true,
		FullTimestamp: true,
	})
```

#### Logging Method Name

If you wish to add the calling method as a field, instruct the logger via:
```go
log.SetReportCaller(true)
```
This adds the caller as 'method' like so:

```json
{"animal":"penguin","level":"fatal","method":"github.com/sirupsen/arcticcreatures.migrate","msg":"a penguin swims by",
"time":"2014-03-10 19:57:38.562543129 -0400 EDT"}
```

```text
time="2015-03-26T01:27:38-04:00" level=fatal method=github.com/sirupsen/arcticcreatures.migrate msg="a penguin swims by" animal=penguin
```
Note that this does add measurable overhead - the cost will depend on the version of Go, but is
between 20 and 40% in recent tests with 1.6 and 1.7.  You can validate this in your
environment via benchmarks: 
```
go test -bench=.*CallerTracing
```


#### Case-sensitivity

The organization's name was changed to lower-case--and this will not be changed
back. If you are getting import conflicts due to case sensitivity, please use
the lower-case import: `github.com/sirupsen/logrus`.

#### Example

The simplest way to use Logrus is simply the package-level exported logger:

```go
package main

import (
  log "github.com/sirupsen/logrus"
)

func main() {
  log.WithFields(log.Fields{
    "animal": "walrus",
  }).Info("A walrus appears")
}
```

Note that it's completely api-compatible with the stdlib logger, so you can
replace your `log` imports everywhere with `log "github.com/sirupsen/logrus"`
and you'll now have the flexibility of Logrus. You can customize it all you
want:

```go
package main

import (
  "os"
  log "github.com/sirupsen/logrus"
)

func init() {
  // Log as JSON instead of the default ASCII formatter.
  log.SetFormatter(&log.JSONFormatter{})

  // Output to stdout instead of the default stderr
  // Can be any io.Writer, see below for File example
  log.SetOutput(os.Stdout)

  // Only log the warning severity or above.
  log.SetLevel(log.WarnLevel)
}

func main() {
  log.WithFields(log.Fields{
    "animal": "walrus",
    "size":   10,
  }).Info("A group of walrus emerges from the ocean")

  log.WithFields(log.Fields{
    "omg":    true,
    "number": 122,
  }).Warn("The group's number increased tremendously!")

  log.WithFields(log.Fields{
    "omg":    true,
    "number": 100,
  }).Fatal("The ice breaks!")

  // A common pattern is to re-use fields between logging statements by re-using
  // the logrus.Entry returned from WithFields()
  contextLogger := log.WithFields(log.Fields{
    "common": "this is a common field",
    "other": "I also should be logged always",
  })

  contextLogger.Info("I'll be logged with common and other field")
  contextLogger.Info("Me too")
}
```

For more advanced usage such as logging to multiple locations from the same
application, you can also create an instance of the `logrus` Logger:

```go
package main

import (
  "os"
  "github.com/sirupsen/logrus"
)

// Create a new instance of the logger. You can have any number of instances.
var log = logrus.New()

func main() {
  // The API for setting attributes is a little different than the package level
  // exported logger. See Godoc.
  log.Out = os.Stdout

  // You could set this to any `io.Writer` such as a file
  // file, err := os.OpenFile("logrus.log", os.O_CREATE|os.O_WRONLY, 0666)
  // if err == nil {
  //  log.Out = file
  // } else {
  //  log.Info("Failed to log to file, using default stderr")
  // }

  log.WithFields(logrus.Fields{
    "animal": "walrus",
    "size":   10,
  }).Info("A group of walrus emerges from the ocean")
}
```

#### Fields

Logrus encourages careful, structured logging through 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.WithFields(log.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.

#### Default Fields

Often it's helpful to have fields _always_ attached to log statements in an
application or parts of one. For example, you may want to always log the
`request_id` and `user_ip` in the context of a request. Instead of writing
`log.WithFields(log.Fields{"request_id": request_id, "user_ip": user_ip})` on
every line, you can create a `logrus.Entry` to pass around instead:

```go
requestLogger := log.WithFields(log.Fields{"request_id": request_id, "user_ip": user_ip})
requestLogger.Info("something happened on that request") # will log request_id and user_ip
requestLogger.Warn("something not great happened")
```

#### 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.

Logrus comes with [built-in hooks](hooks/). Add those, or your custom hook, in
`init`:

```go
import (
  log "github.com/sirupsen/logrus"
  "gopkg.in/gemnasium/logrus-airbrake-hook.v2" // the package is named "airbrake"
  logrus_syslog "github.com/sirupsen/logrus/hooks/syslog"
  "log/syslog"
)

func init() {

  // Use the Airbrake hook to report errors that have Error severity or above to
  // an exception tracker. You can create custom hooks, see the Hooks section.
  log.AddHook(airbrake.NewHook(123, "xyz", "production"))

  hook, err := logrus_syslog.NewSyslogHook("udp", "localhost:514", syslog.LOG_INFO, "")
  if err != nil {
    log.Error("Unable to connect to local syslog daemon")
  } else {
    log.AddHook(hook)
  }
}
```
Note: Syslog hook also support connecting to local syslog (Ex. "/dev/log" or "/var/run/syslog" or "/var/run/log"). For the detail, please check the [syslog hook README](hooks/syslog/README.md).

A list of currently known of service hook can be found in this wiki [page](https://github.com/sirupsen/logrus/wiki/Hooks)


#### Level logging

Logrus has seven logging levels: Trace, Debug, Info, Warning, Error, Fatal and Panic.

```go
log.Trace("Something very low level.")
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.SetLevel(log.InfoLevel)
```

It may be useful to set `log.Level = logrus.DebugLevel` 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
import (
  log "github.com/sirupsen/logrus"
)

init() {
  // do something here to set environment depending on an environment variable
  // or command-line flag
  if Environment == "production" {
    log.SetFormatter(&log.JSONFormatter{})
  } else {
    // The TextFormatter is default, you don't actually have to do this.
    log.SetFormatter(&log.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`.  To force no colored output even if there is a TTY  set the
    `DisableColors` field to `true`. For Windows, see
    [github.com/mattn/go-colorable](https://github.com/mattn/go-colorable).
  * When colors are enabled, levels are truncated to 4 characters by default. To disable
    truncation set the `DisableLevelTruncation` field to `true`.
  * All options are listed in the [generated docs](https://godoc.org/github.com/sirupsen/logrus#TextFormatter).
* `logrus.JSONFormatter`. Logs fields as JSON.
  * All options are listed in the [generated docs](https://godoc.org/github.com/sirupsen/logrus#JSONFormatter).

Third party logging formatters:

* [`FluentdFormatter`](https://github.com/joonix/log). Formats entries that can be parsed by Kubernetes and Google Container Engine.
* [`GELF`](https://github.com/fabienm/go-logrus-formatters). Formats entries so they comply to Graylog's [GELF 1.1 specification](http://docs.graylog.org/en/2.4/pages/gelf.html).
* [`logstash`](https://github.com/bshuster-repo/logrus-logstash-hook). Logs fields as [Logstash](http://logstash.net) Events.
* [`prefixed`](https://github.com/x-cray/logrus-prefixed-formatter). Displays log entry source along with alternative layout.
* [`zalgo`](https://github.com/aybabtme/logzalgo). Invoking the P͉̫o̳̼̊w̖͈̰͎e̬͔̭͂r͚̼̹̲ ̫͓͉̳͈ō̠͕͖̚f̝͍̠ ͕̲̞͖͑Z̖̫̤̫ͪa͉̬͈̗l͖͎g̳̥o̰̥̅!̣͔̲̻͊̄ ̙̘̦̹̦.
* [`nested-logrus-formatter`](https://github.com/antonfisher/nested-logrus-formatter). Converts logrus fields to a nested structure.

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.SetFormatter(new(MyJSONFormatter))

func (f *MyJSONFormatter) Format(entry *Entry) ([]byte, error) {
  // Note this doesn't include Time, Level and Message which are available on
  // the Entry. Consult `godoc` on information about those fields or read the
  // source of the official loggers.
  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
}
```

#### Logger as an `io.Writer`

Logrus can be transformed into an `io.Writer`. That writer is the end of an `io.Pipe` and it is your responsibility to close it.

```go
w := logger.Writer()
defer w.Close()

srv := http.Server{
    // create a stdlib log.Logger that writes to
    // logrus.Logger.
    ErrorLog: log.New(w, "", 0),
}
```

Each line written to that writer will be printed the usual way, using formatters
and hooks. The level for those entries is `info`.

This means that we can override the standard library logger easily:

```go
logger := logrus.New()
logger.Formatter = &logrus.JSONFormatter{}

// Use logrus for standard log output
// Note that `log` here references stdlib's log
// Not logrus imported under the name `log`.
log.SetOutput(logger.Writer())
```

#### Rotation

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

#### Tools

| Tool | Description |
| ---- | ----------- |
|[Logrus Mate](https://github.com/gogap/logrus_mate)|Logrus mate is a tool for Logrus to manage loggers, you can initial logger's level, hook and formatter by config file, the logger will generated with different config at different environment.|
|[Logrus Viper Helper](https://github.com/heirko/go-contrib/tree/master/logrusHelper)|An Helper around Logrus to wrap with spf13/Viper to load configuration with fangs! And to simplify Logrus configuration use some behavior of [Logrus Mate](https://github.com/gogap/logrus_mate). [sample](https://github.com/heirko/iris-contrib/blob/master/middleware/logrus-logger/example) |

#### Testing

Logrus has a built in facility for asserting the presence of log messages. This is implemented through the `test` hook and provides:

* decorators for existing logger (`test.NewLocal` and `test.NewGlobal`) which basically just add the `test` hook
* a test logger (`test.NewNullLogger`) that just records log messages (and does not output any):

```go
import(
  "github.com/sirupsen/logrus"
  "github.com/sirupsen/logrus/hooks/test"
  "github.com/stretchr/testify/assert"
  "testing"
)

func TestSomething(t*testing.T){
  logger, hook := test.NewNullLogger()
  logger.Error("Helloerror")

  assert.Equal(t, 1, len(hook.Entries))
  assert.Equal(t, logrus.ErrorLevel, hook.LastEntry().Level)
  assert.Equal(t, "Helloerror", hook.LastEntry().Message)

  hook.Reset()
  assert.Nil(t, hook.LastEntry())
}
```

#### Fatal handlers

Logrus can register one or more functions that will be called when any `fatal`
level message is logged. The registered handlers will be executed before
logrus performs a `os.Exit(1)`. This behavior may be helpful if callers need
to gracefully shutdown. Unlike a `panic("Something went wrong...")` call which can be intercepted with a deferred `recover` a call to `os.Exit(1)` can not be intercepted.

```
...
handler := func() {
  // gracefully shutdown something...
}
logrus.RegisterExitHandler(handler)
...
```

#### Thread safety

By default, Logger is protected by a mutex for concurrent writes. The mutex is held when calling hooks and writing logs.
If you are sure such locking is not needed, you can call logger.SetNoLock() to disable the locking.

Situation when locking is not needed includes:

* You have no hooks registered, or hooks calling is already thread-safe.

* Writing to logger.Out is already thread-safe, for example:

  1) logger.Out is protected by locks.

  2) logger.Out is a os.File handler opened with `O_APPEND` flag, and every write is smaller than 4k. (This allow multi-thread/multi-process writing)

     (Refer to http://www.notthewizard.com/2014/06/17/are-files-appends-really-atomic/)