4.8 KiB
Logrus
Logrus is a simple, opinionated structured logging package for Go which is 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 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:
// Will log anything that is info or above, default.
log.Level = logrus.Info
Entries
Besides the fields added with WithField
or WithFields
some fields are
automatically added to all logging events:
time
. The timestamp when the entry was created.msg
. The logging message passed to{Info,Warn,Error,Fatal,Panic}
after theAddFields
call. E.g.Failed to send event.
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)
}
}
Formats
The built in 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 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):
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
}