Lager is a logging library for go.

Instantiate a logger with the name of your component.

import (
  "github.com/pivotal-golang/lager"
)

logger := lager.NewLogger("my-app")

Lager can write logs to a variety of destinations. You can specify the destinations using Lager sinks:

To write to an arbitrary Writer object:

logger.RegisterSink(lager.NewWriterSink(myWriter, lager.INFO))

Lager supports the usual level-based logging, with an optional argument for arbitrary key-value data.

logger.Info("doing-stuff", logger.Data{
  "informative": true,
})

output:

{ "source": "my-app", "message": "doing-stuff", "data": { "informative": true }, "timestamp": 1232345, "log_level": 1 }

Error messages also take an Error object:

logger.Error("failed-to-do-stuff", errors.New("Something went wrong"))

output:

{ "source": "my-app", "message": "failed-to-do-stuff", "data": { "error": "Something went wrong" }, "timestamp": 1232345, "log_level": 1 }

You can avoid repetition of contextual data using 'Sessions':

contextualLogger := logger.Session("my-task", logger.Data{
  "request-id": 5,
})

contextualLogger.Info("my-action")

output:

{ "source": "my-app", "message": "my-task.my-action", "data": { "request-id": 5 }, "timestamp": 1232345, "log_level": 1 }

Lager is Apache 2.0 licensed.