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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.

Inherits From

Constants

ANSI sequence colors

"\e[30m"
"\e[34m"
ActiveSupport::Deprecation::DeprecatedObjectProxy.new("\e[1m", "BOLD is deprecated! Use MODES[:bold] instead.", ActiveSupport.deprecator)

Embed in a String to clear all previous ANSI sequences.

ActiveSupport::Deprecation::DeprecatedObjectProxy.new("\e[0m", "CLEAR is deprecated! Use MODES[:clear] instead.", ActiveSupport.deprecator)
"\e[36m"
"\e[32m"
{
debug: -> (logger) { !logger.debug? },
info: -> (logger) { !logger.info? },
error: -> (logger) { !logger.error? },
}
"\e[35m"

ANSI sequence modes

{
clear:     0,
bold:      1,
italic:    3,
underline: 4,
}
"\e[31m"
"\e[37m"
"\e[33m"

Attributes

[W] logger

Public class methods

Flush all log_subscribers‘ logger.

Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 115
def flush_all!
  logger.flush if logger.respond_to?(:flush)
end
Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 110
def log_subscribers
  subscribers
end
Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 96
def logger
  @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
    Rails.logger
  end
end
Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 136
def initialize
  super
  @event_levels = {}
end

Public instance methods

Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 149
def call(event)
  super if logger
rescue => e
  log_exception(event.name, e)
end
Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 141
def logger
  LogSubscriber.logger
end
Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 155
def publish_event(event)
  super if logger
rescue => e
  log_exception(event.name, e)
end
Source code GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 145
def silenced?(event)
  logger.nil? || @event_levels[event]&.call(logger)
end

Private instance methods

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 GitHub
# File activesupport/lib/active_support/log_subscriber.rb, line 175
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

Namespace

Definition files