Logrus

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

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

With log.Formatter = new(logrus.JSONFormatter), for easy parsing by logstash or Splunk:

{"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"}

Fields

Logrus encourages careful, structured 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:

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 on Error, Fatal and Panic.

log = logrus.New()

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 *Entry) (error) {
  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")
  }

  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
  }
}

Level logging

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

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 on a Logger, then it will only log entries with that severity or anything above it:

// 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:

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 logging formatters are:

• logrus.TextFormatter. Logs the event in colors if stdout is a tty, otherwise without colors.
• logrus.JSONFormatter. Logs fields as JSON.

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):

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
}