class ActiveSupport::LogSubscriber

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.

# File activesupport/lib/active_support/log_subscriber.rb, line 116
def flush_all!
  logger.flush if logger.respond_to?(:flush)
end

# File activesupport/lib/active_support/log_subscriber.rb, line 111
def log_subscribers
  subscribers
end

# File activesupport/lib/active_support/log_subscriber.rb, line 97
def logger
  @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
    Rails.logger
  end
end

# File activesupport/lib/active_support/log_subscriber.rb, line 137
def initialize
  super
  @event_levels = {}
end

# File activesupport/lib/active_support/log_subscriber.rb, line 150
def call(event)
  super if logger
rescue => e
  log_exception(event.name, e)
end

# File activesupport/lib/active_support/log_subscriber.rb, line 142
def logger
  LogSubscriber.logger
end

# File activesupport/lib/active_support/log_subscriber.rb, line 156
def publish_event(event)
  super if logger
rescue => e
  log_exception(event.name, e)
end

# File activesupport/lib/active_support/log_subscriber.rb, line 146
def silenced?(event)
  logger.nil? || @event_levels[event]&.call(logger)
end

# File activesupport/lib/active_support/log_subscriber.rb, line 176
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