config.ru

# sometimes you really _do_ want a global... 
# alternately, stick it as a method on a Module somewhere, App.logger or whatever, but it's still just as much of a smell.

# replace STDERR with your filename of choice.
$logger = Logger.new(STDERR)
$logger.formatter = proc { |severity, datetime, progname, msg|
  "#{severity} - #{progname} - [#{datetime.strftime("%d/%m/%Y %H:%M:%S")}] #{msg}\n"
}

run(Rack::Builder.new {
  use Rack::CommonLogger, $logger
  # ...
})

# and then just $logger.debug('hi there!') from wherever you need to debug shit.