Question

Is it 0K to use class variables to communicate with threads in Rails?

Are there any pros or cons compared to using queues?

I’m not sure if this works in any environment, it worked in mine: Ruby Enterprise + Passenger + nginx + Rails 3

Here’s an example:

# rails runner "ThreadJob.new.run"
class ThreadJob

  @@counter = 0

  def run
    producer = Thread.new do
      1.upto 10 do
        sleep 1
        @@counter+= 1
        puts "Producer: #{@@counter}"
      end
    end

    consumer = Thread.new do
      sleep 0.5
      1.upto 10 do
        sleep 1
        @@counter-= 1
        puts "Consumer: #{@@counter}"
      end
    end

    producer.join
    consumer.join

    puts @@counter # result should be 0
  end
end

I’m wondering if this can work if I modify @@counter from within another process.

Answer 1

No, it’s not ok to update class variables from multiple threads like your example.

Take a look at the explanation of why, and how to solve this using the Mutex class:

http://www.ruby-doc.org/docs/ProgrammingRuby/html/tut_threads.html