Ougai

A structured logging system that is capable of handling a message, structured data, or an exception easily. It has JSON formatters compatible with Bunyan or pino for Node.js, and a human-readable formatter with Amazing Print for the console.

Installation

Add this line to your application's Gemfile:

gem 'ougai'

And then execute:

$ bundle

Or install it yourself as:

$ gem install ougai

Usage

Ougai::Logger is a sub-class of the standard Logger in Ruby. All arguments of the initialize pass through to ::Logger.

require 'ougai'

logger = Ougai::Logger.new($stdout)

TRACE level

The level of logger supports TRACE level lower than DEBUG.

logger.level = Ougai::Logger::TRACE # , :trace or 'trace'

log only a message

logger.info('Information!')

{"name":"main","hostname":"mint","pid":14607,"level":30,"time":"2016-10-16T22:26:48.835+09:00","v":0,"msg":"Information!"}

log only structured data

logger.info({
  msg: 'Request', method: 'GET', path: '/login',
  format: 'html', controller: 'LoginController',
  action: 'new', status: 200
})
logger.debug(user: { name: 'Taro', age: 19 })

{"name":"main","hostname":"mint","pid":9044,"level":30,"time":"2016-10-28T17:58:53.668+09:00","v":0,"msg":"Request","method":"GET","path":"/login","format":"html","controller":"LoginController","action":"new","status":200}
{"name":"main","hostname":"mint","pid":9044,"level":20,"time":"2016-10-28T17:58:53.668+09:00","v":0,"msg":"No message","user":{"name":"Taro","age":19}}

If a data does not contain msg field, msg is set default_message attribute value of a Logger. its default is 'No message'.

logger.default_message = 'User dump'
logger.debug(user: { name: 'Taro', age: 19 })

{"name":"main","hostname":"mint","pid":9303,"level":20,"time":"2016-10-28T18:03:50.118+09:00","v":0,"msg":"User dump","user":{"name":"Taro","age":19}}

log only an exception

begin
  raise StandardError, 'some error'
rescue => ex
  logger.error(ex)
end

{"name":"main","hostname":"mint","pid":4422,"level":50,"time":"2016-10-22T13:05:02.989+09:00","v":0,"msg":"some error","err":{"name":"StandardError","message":"some error","stack":"main.rb:24:in `<main>'"}}

log with a message and custom data

logger.debug('Debugging', data_id: 1, data_flag: true)
logger.debug('Debug!', custom_data: { id: 1, name: 'something' })

{"name":"main","hostname":"mint","pid":14607,"level":20,"time":"2016-10-16T22:26:48.836+09:00","v":0,"msg":"Debugging","data_id":1,"data_flag":true}
{"name":"main","hostname":"mint","pid":14607,"level":20,"time":"2016-10-16T22:26:48.836+09:00","v":0,"msg":"Debug!","custom_data":{"id":1,"name":"something"}}

log with a message and an exception

begin
  raise StandardError, 'fatal error'
rescue => ex
  logger.fatal('Unexpected!', ex)
end

{"name":"main","hostname":"mint","pid":14607,"level":60,"time":"2016-10-16T22:26:48.836+09:00","v":0,"msg":"Unexpected!","err":{"name":"StandardError","message":"fatal error","stack":"main.rb:12:in `<main>'"}}

log with an exception and custom data

begin
  raise StandardError, 'some error'
rescue => ex
  logger.error(ex, error_id: 999)
end

{"name":"main","hostname":"mint","pid":13962,"level":50,"time":"2016-10-28T23:44:52.144+09:00","v":0,"error_id":999,"err":{"name":"StandardError","message":"some error","stack":"main.rb:40:in `<main>'"}}

log with a message, an exception and custom data

begin
  1 / 0
rescue => ex
  logger.error('Caught error', ex, reason: 'zero spec')
end

{"name":"main","hostname":"mint","pid":14607,"level":50,"time":"2016-10-16T22:26:48.836+09:00","v":0,"msg":"Caught error","err":{"name":"ZeroDivisionError","message":"divided by 0","stack":"main.rb:18:in `/'\n ...'"},"reason":"zero spec"}

logs with blocks

logger.info { 'Hello!' }

logger.debug do
  ['User dump', { name: 'Taro', age: 15 }]
end

logger.error do
  ['Failed to fetch info', ex, { id: 10 }]
end

logger.fatal { ex }

logger.fatal do
  ['Unexpected', ex]
end

To specify more than one message, exception, and custom data, the block returns them as an array.

Adding custom fields to all logs

The fields of with_fields add to all logs as is.

logger.with_fields = { version: '1.1.0' }
logger.debug(user: { name: 'Taro', age: 19 })
logger.info('Hello!', user: { name: 'Jiro' }, version: '2.3')

{"name":"test","hostname":"mint","pid":30182,"level":20,"time":"2017-07-22T20:52:12.332+09:00","v":0,"version":"1.1.0","msg":"No message","user":{"name":"Taro","age":19}}
{"name":"test","hostname":"mint","pid":30308,"level":30,"time":"2017-07-22T20:53:54.314+09:00","v":0,"version":"2.3","user":{"name":"Jiro"},"msg":"Hello!"}

If any field of with_fields is specified in each log, the field is overridden. If the field's type is Array, both with_field value and logging value are merged with concat and uniq.

If the field's type is Hash, then values are merged recursively.

logger.with_fields = { version: '1.1.0', user: { name: 'Taro' } }
logger.debug(user: { age: 19 })

{"name":"test","hostname":"mint","pid":30182,"level":20,"time":"2017-07-22T20:52:12.332+09:00","v":0,"version":"1.1.0","msg":"No message","user":{"name":"Taro","age":19}}

Create a child logger

logger.child(with_fields) creates a child logger of self. Its argument with_fields add to all logs the child logger outputs. A child logger can also create its child logger. If you pass a block to this method, the child logger will be yielded to it.

logger = Ougai::Logger.new(STDOUT)
logger.with_fields = { app: 'yourapp', tags: ['service'], kind: 'main' }

child_logger = logger.child({ tags: ['user'], kind: 'logic' })
logger.info('Created child logger')

child_logger.info('Created a user', name: 'Mike')

gc_logger = child_logger.child({ kind: 'detail' })
child_logger.info('Created grand child logger')

gc_logger.debug('something detail', age: 34, weight: 72)

gc_logger.child({ mode: 'processed' }) do |gcc_logger|
  gcc_logger.info('Great-grandchild logger that will be cleaned up on block exit.')

  :some_return_value
end

child_logger.sev_threshold = :error # alias of level
child_logger.info('This is not outputted')
gc_logger.info('This is not outputted')
child_logger.error('This is outputted')
gc_logger.error('This is outputted')

child_logger.level = :debug # does not work because the level is below parent one
child_logger.debug('This is not outputted')

{"name":"main","hostname":"mint","pid":8342,"level":30,"time":"2017-08-01T22:07:20.400+09:00","v":0,"app":"yourapp","tags":["service"],"kind":"main","msg":"Created child logger"}
{"name":"Mike","hostname":"mint","pid":8342,"level":30,"time":"2017-08-01T22:07:20.400+09:00","v":0,"app":"yourapp","tags":["service","user"],"kind":"logic","msg":"Created a user"}
{"name":"main","hostname":"mint","pid":8342,"level":30,"time":"2017-08-01T22:07:20.400+09:00","v":0,"app":"yourapp","tags":["service","user"],"kind":"logic","msg":"Created grand child logger"}
{"name":"main","hostname":"mint","pid":8342,"level":20,"time":"2017-08-01T22:07:20.400+09:00","v":0,"app":"yourapp","tags":["service","user"],"kind":"detail","age":34,"weight":72,"msg":"something detail"}
{"name":"main","hostname":"mint","pid":8342,"level":20,"time":"2017-08-01T22:07:20.400+09:00","v":0,"app":"yourapp","tags":["service","user"],"kind":"detail","mode":"processed","msg":"Great-grandchild logger that will be cleaned up on block exit."}
{"name":"main","hostname":"mint","pid":4894,"level":50,"time":"2017-08-01T22:07:20.401+09:00","v":0,"app":"yourapp","tags":["service","user"],"kind":"logic","msg":"This is outputed"}
{"name":"main","hostname":"mint","pid":4894,"level":50,"time":"2017-08-01T22:07:20.401+09:00","v":0,"app":"yourapp","tags":["service","user"],"kind":"detail","msg":"This is outputed"}

If any field exists in both parent log and child log, the parent value is overridden or merged by child value.

If the field's type is Hash, then values are merged recursively.

Hook before logging

Setting before_log of logger or child an lambda with data field, a process can be run before log each output.

• Adding variable data (like Thread ID) to logging data can be defined in common.
• Returning false in lambda, the log is cancelled and does not output.
• The before_log of child logger is run ahead of the parent logger's.

logger.before_log = lambda do |data|
  data[:thread_id] = Thread.current.object_id.to_s(36)
end

logger.debug('on main thread')
Thread.new { logger.debug('on another thread') }

{"name":"main","hostname":"mint","pid":13975,"level":20,"time":"2017-08-06T15:35:53.435+09:00","v":0,"msg":"on main thread","thread_id":"gqe0ava6c"}
{"name":"main","hostname":"mint","pid":13975,"level":20,"time":"2017-08-06T15:35:53.435+09:00","v":0,"msg":"on another thread","thread_id":"gqe0cb14g"}

Using broadcast, log output plural targets

Ougai::Logger.broadcast c