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Active Support Log Subscriber ↑

ActiveSupport::LogSubscriber is an object set to consume ActiveSupport::Notifications with the sole purpose of logging them. The log subscriber dispatches notifications to a registered object based on its given namespace.

An example would be Active Record log subscriber responsible for logging queries:

module ActiveRecord
  class LogSubscriber < ActiveSupport::LogSubscriber
    attach_to :active_record

    def sql(event)
      info "#{event.payload[:name]} (#{event.duration}) #{event.payload[:sql]}"
    end
  end
end

ActiveRecord::LogSubscriber.logger must be set as well, but it is assigned automatically in a Rails environment.

After configured, whenever a "sql.active_record" notification is published, it will properly dispatch the event (ActiveSupport::Notifications::Event) to the sql method.

Being an ActiveSupport::Notifications consumer, ActiveSupport::LogSubscriber exposes a simple interface to check if instrumented code raises an exception. It is common to log a different message in case of an error, and this can be achieved by extending the previous example:

module ActiveRecord
  class LogSubscriber < ActiveSupport::LogSubscriber
    def sql(event)
      exception = event.payload[:exception]

      if exception
        exception_object = event.payload[:exception_object]

        error "[ERROR] #{event.payload[:name]}: #{exception.join(', ')} " \
              "(#{exception_object.backtrace.first})"
      else
        # standard logger code
      end
    end
  end
end

ActiveSupport::LogSubscriber also has some helpers to deal with logging. For example, ActiveSupport::LogSubscriber.flush_all! will ensure that all logs are flushed, and it is called in Rails::Rack::Logger after a request finishes.

Namespace
Methods
C
F
L
N
S

Constants

BLACK = "\e[30m"
 

ANSI sequence colors
BLUE = "\e[34m"
 
CYAN = "\e[36m"
 
GREEN = "\e[32m"
 
LEVEL_CHECKS = { debug: -> (logger) { !logger.debug? }, info: -> (logger) { !logger.info? }, error: -> (logger) { !logger.error? }, }
 
MAGENTA = "\e[35m"
 
MODES = { clear: 0, bold: 1, italic: 3, underline: 4, }
 

ANSI sequence modes
RED = "\e[31m"
 
WHITE = "\e[37m"
 
YELLOW = "\e[33m"
 

Attributes

[W] logger

Class Public methods

flush_all!()

Flush all log_subscribers‘ logger.

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 112
def flush_all!
  logger.flush if logger.respond_to?(:flush)
end

log_subscribers()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 107
def log_subscribers
  subscribers
end

logger()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 93
def logger
  @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
    Rails.logger
  end
end

new()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 133
def initialize
  super
  @event_levels = {}
end

Instance Public methods

call(event)

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 146
def call(event)
  super if logger
rescue => e
  log_exception(event.name, e)
end

logger()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 138
def logger
  LogSubscriber.logger
end

silenced?(event)

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 142
def silenced?(event)
  logger.nil? || @event_levels[event]&.call(logger)
end

Instance Private methods

color(text, color, mode_options = {})

Set color by using a symbol or one of the defined constants. Set modes by specifying bold, italic, or underline options. Inspired by Highline, this method will automatically clear formatting at the end of the returned String.

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 166
def color(text, color, mode_options = {}) # :doc:
  return text unless colorize_logging
  color = self.class.const_get(color.upcase) if color.is_a?(Symbol)
  mode = mode_from(mode_options)
  clear = "\e[#{MODES[:clear]}m"
  "#{mode}#{color}#{text}#{clear}"
end