class ActiveSupport::LogSubscriber

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

And it's finally registered as:

ActiveRecord::LogSubscriber.attach_to :active_record

Since we need to know all instance methods before attaching the log subscriber, the line above should be called after your ActiveRecord::LogSubscriber definition.

A logger also needs to be set with ActiveRecord::LogSubscriber.logger=. This 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

Log subscriber also has some helpers to deal with logging and automatically flushes all logs when the request finishes (via action_dispatch.callback notification) in a Rails environment.

Constants

BLACK

Colors

BLUE
BOLD
CLEAR

Embed in a String to clear all previous ANSI sequences.

CYAN
GREEN
MAGENTA
RED
WHITE
YELLOW

Attributes

logger[W]

Public Class Methods

flush_all!() click to toggle source

Flush all log_subscribers' logger.

# File lib/active_support/log_subscriber.rb, line 96
def flush_all!
  logger.flush if logger.respond_to?(:flush)
end
log_subscribers() click to toggle source
# File lib/active_support/log_subscriber.rb, line 91
def log_subscribers
  subscribers
end
logger() click to toggle source
# File lib/active_support/log_subscriber.rb, line 83
def logger
  @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
    Rails.logger
  end
end

Private Class Methods

fetch_public_methods(subscriber, inherit_all) click to toggle source
# File lib/active_support/log_subscriber.rb, line 101
def fetch_public_methods(subscriber, inherit_all)
  subscriber.public_methods(inherit_all) - LogSubscriber.public_instance_methods(true)
end

Public Instance Methods

finish(name, id, payload) click to toggle source
Calls superclass method
# File lib/active_support/log_subscriber.rb, line 114
def finish(name, id, payload)
  super if logger
rescue => e
  if logger
    logger.error "Could not log #{name.inspect} event. #{e.class}: #{e.message} #{e.backtrace}"
  end
end
logger() click to toggle source
# File lib/active_support/log_subscriber.rb, line 106
def logger
  LogSubscriber.logger
end
start(name, id, payload) click to toggle source
Calls superclass method
# File lib/active_support/log_subscriber.rb, line 110
def start(name, id, payload)
  super if logger
end

Private Instance Methods

color(text, color, bold = false) click to toggle source

Set color by using a symbol or one of the defined constants. If a third option is set to true, it also adds bold to the string. This is based on the Highline implementation and will automatically append CLEAR to the end of the returned String.

# File lib/active_support/log_subscriber.rb, line 135
def color(text, color, bold = false) # :doc:
  return text unless colorize_logging
  color = self.class.const_get(color.upcase) if color.is_a?(Symbol)
  bold  = bold ? BOLD : ""
  "#{bold}#{color}#{text}#{CLEAR}"
end