Ruby: Issues running multiple threads?

Question

Is there something wrong with running multiple threads?

When I load the page using action1, it works.

HomeController

def action1
    threads = []
    threads << Thread.new {@lub = client.tag_recent_media('tag1')}
    Thread.new{@tags = client.tag_recent_media('encorebeach')}
    Thread.new{@location = client.location_recent_media('16565')}

    threads.each(&:join)  
end

Home View

<% (@lub+@tags+@location).each do |media| %>
<%= media %>
<% end %>

Here's another controller with a different view

AnotherController

def action1
    threads2 = []
    threads2 << Thread.new {@lub2 = client.tag_recent_media('tag1')}
    Thread.new{@tags2 = client.tag_recent_media('encorebeach')}
    Thread.new{@location2 = client.location_recent_media('16565')}

    threads2.each(&:join)  
end

Another View

<% (@lub2+@tags2+@location2).each do |media| %>
<%= media %>
<% end %>

For the second output I get an error

undefined method `+' for nil:NilClass

I think it is something wrong with the threads. Can someone assist me as to why this is happening? Is it because I already executed a thread in the home page, and then when I want to go to another page, it runs a thread again and it won't work?

Thanks!

clacke · Accepted Answer · 2013-04-19 05:59:20Z

2

threads only contains your first thread. It is by pure luck that your first method has its threads finish before you use the instance variables in the view.

threads = []
threads << Thread.new {@lub = client.tag_recent_media('tag1')}
threads << Thread.new{@tags = client.tag_recent_media('encorebeach')}
threads << Thread.new{@location = client.location_recent_media('16565')}

or even simpler:

threads = [
  Thread.new{@lub = client.tag_recent_media('tag1')},
  Thread.new{@tags = client.tag_recent_media('encorebeach')},
  Thread.new{@location = client.location_recent_media('16565')}
]

will accomplish what you want.

It is quite likely that you will not gain much performance from this compared to just running them in the controller thread. It is even possible that you will thrash the database server, slowing it down instead. Do measure whether the threads buy you any performance, and keep in mind that there are many pitfalls when using threads.

answered Apr 19, 2013 at 5:59

clacke

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3 Comments

hellomello Over a year ago

thanks for your response. What do you mean? I have this inside my controller? I'm new so I'm not sure what that statement means. Also, do you recommend another method if I want to run each variable simultaneously instead of running them line by line until each finish one by one to get the desired results?

clacke Over a year ago

The controller thread, the thread in which the controller runs, rather than in fresh background threads.

clacke Over a year ago

If you want to run all calls simultaneously, this is indeed how you do it. I am just suggesting that doing so may not buy you much if the database needs to serialize the writes anyway, as is probably the case with the two calls to the same method.

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