From: Rick DeNatale <rick.denatale@...>
Date: 2007-09-06T12:17:16+09:00
Subject: Re: Ruby Threads question

On 9/5/07, Cd Cd <cdalten@gmail.com> wrote:
> The following code is a slight modification of the code found at the top
> of page 136 in the book "Programming Ruby The Pragmatic Programmers'
> Guide" by Dave Thomas with Chad Fowler and Andy Hunt
>
> #!/usr/bin/ruby -w
>
> require 'net/http'
>
> pages = %w(www.google.com www.slashdot.org www.mit.edu)
> threads = []
>
> for page_to_fetch in pages
>         threads << Thread.new(page_to_fetch) do |url|
>                 h=Net::HTTP.new(url, 80)
>                 puts "Fetching: #{url}"
>                 resp = h.get('/', nil)
>                 puts "Got #{url}: #{resp.message}"
>         end
> end
>
> threads.each {|thr| thr.join }
>
> How come when I run this code, the following output goes line by line?
> Fetching: www.google.com
> Fetching: www.slashdot.org
> Fetching: www.mit.edu
> Got www.slashdot.org: Moved Permanently
> Got www.google.com: OK
> Got www.mit.edu: OK
>
>
> Shouldn't the output be all done at once since the code is running in
> parallel?

I'm not sure what you mean "all done at once".

If the code wasn't running in parallel threads then I'd expect to see:

Fetching: www.google.com
Got www.google.com: OK
Fetching: www.slashdot.org
Got www.slashdot.org: Moved Permanently
Fetching: www.mit.edu
Got www.mit.edu: OK

What's really happening is something like:

Thread 1                                        Thread 2
                     Thread 3
opens HTTP
prints Fetching:www.google.com
issues get
                                                       opens http
                                                       prints
Fetching: www.slashdot.org
                                                       issues get

                                 prints Fetching: www.mit.edu

                                 issues get
                                                      get finishes
                                                      prints Got
www.slashdot.org: Moved Permanently
get finishes
prints Got www.google.com: OK

                                get finishes

                                prints Got www.mit.edu: OK

So the output order is evidence that there is indeed parallel activity.
-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/