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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

LEVEL_CHECKS = { debug: -> (logger) { !logger.debug? }, info: -> (logger) { !logger.info? }, error: -> (logger) { !logger.error? }, }
 

Attributes

[W] logger

Class Public methods

flush_all!()

Flush all log_subscribers‘ logger.

# File activesupport/lib/active_support/log_subscriber.rb, line 95
def flush_all!
  logger.flush if logger.respond_to?(:flush)
end

log_subscribers()

# File activesupport/lib/active_support/log_subscriber.rb, line 90
def log_subscribers
  subscribers
end

logger()

# File activesupport/lib/active_support/log_subscriber.rb, line 76
def logger
  @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
    Rails.logger
  end
end

new()

# File activesupport/lib/active_support/log_subscriber.rb, line 120
def initialize
  super
  @event_levels = {}
end

Instance Public methods

call(event)

# File activesupport/lib/active_support/log_subscriber.rb, line 129
def call(event)
  super if logger
rescue => e
  log_exception(event.name, e)
end

logger()

# File activesupport/lib/active_support/log_subscriber.rb, line 116
def logger
  LogSubscriber.logger
end

silenced?(event)

# File activesupport/lib/active_support/log_subscriber.rb, line 125
def silenced?(event)
  logger.nil? || @event_levels[event]&.call(logger)
end