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

Module

Methods

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

log_subscribers()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 107
      def log_subscribers
        subscribers
      end
🔎 See on GitHub

logger()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 93
      def logger
        @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
          Rails.logger
        end
      end
🔎 See on GitHub

new()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 133
    def initialize
      super
      @event_levels = {}
    end
🔎 See on GitHub

Instance Public methods

call(event)

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 146
    def call(event)
      super if logger
    rescue => e
      log_exception(event.name, e)
    end
🔎 See on GitHub

logger()

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 138
    def logger
      LogSubscriber.logger
    end
🔎 See on GitHub

publish_event(event)

📝 Source code 
# File activesupport/lib/active_support/log_subscriber.rb, line 152
    def publish_event(event)
      super if logger
    rescue => e
      log_exception(event.name, e)
    end
🔎 See on GitHub

silenced?(event)

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

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 code 
# File activesupport/lib/active_support/log_subscriber.rb, line 172
    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
🔎 See on GitHub