This gem provides a logging device for the lumberjack gem that will log JSON formatted output to a stream with one JSON document per line. This can be used as part of a log processing pipeline to ship the log to a structured data store or logging service.

You can send the JSON output either to a stream or to another Lumberjack device.

# Send to STDOUT
device = Lumberjack::JsonDevice.new(STDOUT)

# Send to another logging device
log_file = Lumberjack::Device::LogFile.new("/var/log/app.log")
device = Lumberjack::JsonDevice.new(log_file)

By default, the JSON document will map to the Lumberjack::LogEntry data structure.

{"timestamp": "2020-01-02T19:47:45.123455", "severity": "INFO", "progname": "web", "pid": 101, "message": "test", "tags": {"foo": "bar"}}

You can specify a mapping to the device to customize the JSON document data structure. You can map the standard field names (time, severity, progname, pid, message, and tags) to custom field names.

If you map a field to an array, it will be mapped into a nested hash in the JSON document.

Any keys beyond the standard field names will be populated by a tag with the same name. These tags will not be included with the rest of the tags.

device = Lumberjack::JsonDevice.new(STDOUT, mapping: {
  time: "timestamp",
  severity: "level",
  progname: ["app", "name"],
  pid: ["app", "pid"],
  message: "message",
  duration: "duration",
  tags: "tags"
})

{"timestamp": "2020-01-02T19:47:45.123455", "level": "INFO", "app": {"name": "web", "pid": 101}, "message": "test", "duration": 5, "tags": {"foo": "bar"}}

If you omit any fields in the mapping, they will not appear in the JSON document.

device = Lumberjack::JsonDevice.new(STDOUT, mapping: {
  time: "timestamp",
  severity: "level",
  message: "message",
})

{"timestamp": "2020-01-02T19:47:45.123455", "level": "INFO", "message": "test"}

You can also provide a block or any object that responds to call in a mapping. The block will be called with the value and should emit a hash that will be merged into the JSON document.

device = Lumberjack::JsonDevice.new(STDOUT, mapping: {
  time: lambda { |val| (timestamp: val.to_f * 1000).round} },
  severity: "level",
  message: "test",
})

{"timestamp": 1578125375588, "level": "INFO", "message": "test"}

Finally, you can specify true in the mapping as a short cut to map the field to the same name. If the field name contains periods, it will be mapped to a nested structure.

device = Lumberjack::JsonDevice.new(STDOUT, mapping: {
  "message" => true,
  "http.status" => true,
  "http.method" => true,
  "http.path" => true
})

{"message": "test", "http": {"status": 200, "method": "GET", "path": "/resource"}}

The device will have a Lumberjack::Formatter that will be used to format objects before serializing them as JSON. You can add additional formatters for classes to the default formatter, or supply a custom one when creating the device.

device.formatter.add(Exception, LumberjacK::Formatter::InspectFormatter.new)
device.formatter.add(ActiveRecord::Base, LumberjacK::Formatter::IdFormatter.new)

You can also specify the datetime_format that will be used to serialize Time and DateTime objects.

device.datetime_format = "%Y-%m-%dT%H:%M:%S.%3N"

Log entries with no message will not be written to the log.

Add this line to your application's Gemfile:

gem 'lumberjack_json_device'

And then execute:

$ bundle

Or install it yourself as:

$ gem install lumberjack_json_device

Open a pull request on GitHub.

Please use the standardrb syntax and lint your code with standardrb --fix before submitting.

The gem is available as open source under the terms of the MIT License.