Question

I am getting this error While running this

LoadError: Expected /home/user/Desktop/Tripurari/myapp/app/models/host.rb to define Host##

But every thing on it’s place. Can some one tell me what the exact problem is below method.

def self.check_all(keyword)
  memo_mutex = Mutex.new
  memo       = {}
  threads    = []
  name       = keyword.keyword
  SITES.each do |site_and_options|
    threads << Thread.new do
      @host = Host.find_or_create_by_name(site)
      if keyword.unavailable_usernames.find_by_host_id(@host.id)
        memo[@host.name] = true
      else
        memo[@host.name] = false
      end
    end
  end
  threads.each { |t| t.join }
  memo
end

Answer 1

The issue is probably caused by the autoloader. If the Host class is not yet loaded when first entering the loop where you create a couple of new threads, it is autoloaded, i.e. Rails searches the loadpath for a file matching the naming conventions and requires it.

This process is not threadsave. In your case, as you are creating servral threads in quick succession, each trying to autoload the global class, you get race conditions and strange things happen. Basically, you have two options for tackling this:

1. You can explicitly load the model before starting your threads by using require 'host' before starting your loop.
2. Or you can set config.threadsave! in an initializer. This will (among other things) preload all your classes when starting your server. This is preferred as with this, you avoid a truckload of other difficult to debug concurrency issues. For more information about config.threadsafe!, please refer to the excellent article by Aaron Patterson arguing it should be removed altogether in Rails 4.