# Ruby & Threads

**URL:** <https://www.ruby-forum.com/t/ruby-threads/143042>\
**Category:** Ruby\
**Created:** [July 14, 2008, 7:08pm UTC](https://www.ruby-forum.com/t/ruby-threads/143042 "2008-07-14T19:08:27Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![michaelb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/b19c9b/32.png) [@michaelb](https://www.ruby-forum.com/u/michaelb)\
**Post date:** [July 14, 2008, 7:08pm UTC](https://www.ruby-forum.com/t/ruby-threads/143042/1 "2008-07-14T19:08:27Z")

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Hello all,

I’m building an application that has to branch out and call about 10  
other Ruby scripts. Since each script will run for a few seconds,  
waiting for each one to finish will take a while, which is too much. So,  
I’ve been looking into threads and I have a system that’s working (in  
tests), but I have a few questions. First of all, the system:

require ‘enumerator’

holder = []

array = (1…10).to\_a  
puts array.inspect

array.each\_slice(3) do |group|  
group.each do |number|  
@thread = Thread.new do  
puts “Starting #{number}…\n”  
sleep(5)  
holder \<\< number  
end  
end  
end

@thread.join  
puts holder.inspect

In theory, the output should look something like this:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]  
Starting 1…  
Starting 2…  
Starting 3…  
Starting 4…  
Starting 5…  
Starting 6…  
Starting 7…  
Starting 8…  
Starting 9…  
Starting 10…  
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

However, sometimes the second thread may finish before the first, etc.,  
but the order doesn’t matter. What matters is that the script’s  
execution time just went from over 20 seconds to under two! However, as  
you can see, I have to call ‘@thread.join’. I do this because if I  
don’t, the script will exit before all of the threads are done  
executing, so holder is always an emtpy array. Am I right? Or is there  
some other way to keep the main script from exiting until all the  
threads are done? Is there anything else I’m doing wrong?

Thanks,  
Michael B.

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<div class="post-metadata">

**Author:** ![michaelb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/b19c9b/32.png) [@michaelb](https://www.ruby-forum.com/u/michaelb)\
**Post date:** [July 14, 2008, 7:31pm UTC](https://www.ruby-forum.com/t/ruby-threads/143042/2 "2008-07-14T19:31:07Z")

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Michael B. wrote:

> ```
> puts "Starting #{number}...\n"
> sleep(5)
> holder << number
> end
> 
> ```
> 
> end  
> end
> 
> @thread.join

#join is definitely a good idea, because otherwise (as you observed) the  
main thread will exit before the others have finished, but you are  
overwriting the @thread variable on each iteration through the loop.

The usual idiom for this is something like:

threads = array.map { Thread.new {…} }  
threads.each {|th| th.join}

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<div class="post-metadata">

**Author:** ![michaelb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/b19c9b/32.png) [@michaelb](https://www.ruby-forum.com/u/michaelb)\
**Post date:** [July 14, 2008, 8:07pm UTC](https://www.ruby-forum.com/t/ruby-threads/143042/3 "2008-07-14T20:07:52Z")

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Joel VanderWerf wrote:

> Michael B. wrote:
> 
> > ```
> > puts "Starting #{number}...\n"
> > sleep(5)
> > holder << number
> > end
> > 
> > ```
> > 
> > end  
> > end
> > 
> > @thread.join
> 
> #join is definitely a good idea, because otherwise (as you observed) the  
> main thread will exit before the others have finished, but you are  
> overwriting the @thread variable on each iteration through the loop.
> 
> The usual idiom for this is something like:
> 
> threads = array.map { Thread.new {…} }  
> threads.each {|th| th.join}

Joel,

Initially I meant to do that because I thought that I would only need to  
“join” one thread to get them all to continue, until I realized that  
some might finish before others, so I altered the code to the method  
that you described.

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<div class="post-metadata">

**Author:** ![michaelb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/b19c9b/32.png) [@michaelb](https://www.ruby-forum.com/u/michaelb)\
**Post date:** [July 14, 2008, 10:36pm UTC](https://www.ruby-forum.com/t/ruby-threads/143042/4 "2008-07-14T22:36:11Z")

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In plain Ruby, you might want to rewrite this to a more  
functional style:

holder =  
array.collect do |number|  
Thread.new do  
puts “Starting #{number}…\n”  
sleep(5)  
number  
end  
end.collect do |thread|  
thread.value  
end

And, using ThreadLimiter [1,2], you can reduce it to:

require “threadlimiter”

holder =  
array.threaded\_collect do |number|  
puts “Starting #{number}…\n”  
sleep(5)  
number  
end

gegroet,  
Erik V. - [http://www.erikveen.dds.nl/](http://www.erikveen.dds.nl/)

[1] [threadlimiter (0.2.1)](http://www.erikveen.dds.nl/threadlimiter/doc/index.html)  
[2] [http://rubyforge.org/projects/threadlimiter/](http://rubyforge.org/projects/threadlimiter/)