# Logrus

Logrus is a simple, opinionated logging package for Go which is completely API
compatible with the standard library logger. It has six logging levels: Debug,
Info, Warn, Error, Fatal and Panic. It supports custom logging formatters, and
ships with JSON and nicely formatted text by default. It encourages the use of
logging key value pairs for discoverability. Logrus allows you to add hooks to
logging events at different levels, for instance to notify an external error
tracker.

#### Fields

Logrus encourages careful, informative logging. It encourages the use of 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. The `WithFields` call is optional.

In general, with Logrus using any of the `printf`-family functions should be
seen as a hint you want to 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:

```go
log.AddHook("error", func(entry logrus.Entry) {
  err := airbrake.Notify(errors.New(entry.String()))
  if err != nil {
    log.WithFields(logrus.Fields{
      "source": "airbrake",
      "endpoint": airbrake.Endpoint,
    }).Info("Failed to send error to Airbrake")
  }
})
```

#### Errors

You can also use Logrus to return errors with fields. For instance:

```go
err := record.Destroy()
if err != nil {
  return log.WithFields(&logrus.Fields{
            "id": record.Id,
            "method": "destroy"
          }).AsError("Failed to destroy record")
}
```

Will return a `logrus.Error` object. Passing it to
`log.{Info,Warn,Error,Fatal,Panic}` will log it according to the formatter set
for the environment.

#### Level logging

Logrus has six 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.")
log.Fatal("Bye.")
log.Panic("I'm bailing.")
```

You can set the logging level:

```go
// Will log anything that is info or above, default.
logrus.Level = LevelInfo
```

#### 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`.
4. `file`. The file (and line) where the logging entry was created. E.g.,
   `main.go:82`.

#### 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

  if Environment == "production" {
    log.SetFormatter(logrus.JSONFormatter)
  } else {
    // The TextFormatter is default, you don't actually have to do this.
    log.SetFormatter(logrus.TextFormatter)
  }
}
```

#### Formats

The built in logging formatters are:

* `logrus.TextFormatter`. Logs the event in colors if stdout is a tty, otherwise
  without colors. Default for the development environment. <screenshot>
* `logrus.JSONFormatter`. Default for the production environment. <screnshot>

You can define your formatter taking an entry. `entry.Data` is a `Fields` type
which is a `map[string]interface{}` with all your fields as well as the default
ones (see Entries above):

```go
log.SetFormatter(func(entry *logrus.Entry) {
  serialized, err = json.Marshal(entry.Data)
  if err != nil {
    return nil, log.WithFields(&logrus.Fields{
      "source": "log formatter",
      "entry": entry.Data
    }).AsError("Failed to serialize log entry to JSON")
  }
})
```