From: Daniel DeLorme <dan-ml@...42.com>
Date: 2010-05-19T10:56:13+09:00
Subject: Re: Asynchronous HTTP request

Daniel N wrote:
> On 18 May 2010 16:56, Daniel DeLorme <dan-ml@dan42.com> wrote:
> 
>> Daniel N wrote:
>>
>>> On 13 May 2010 17:37, Daniel DeLorme <dan-ml@dan42.com> wrote:
>>>
>>>  Does anyone know how to do the following, but without threads, purely
>>>> with
>>>> asynchronous IO?
>>>>
>>>> website = Thread.new{ Net::HTTP.get(URI.parse(url)) }
>>>> template = compute_lots_of_stuff()
>>>> puts template.sub("<content goes here>", website.value)
>>>>
>>> Event machine is perfect for this kind of stuff.  Weather it fits with the
>>> rest of your web framework is more likely the thing that makes it an
>>> unlikely selection (if you're using anything rack based for example)
>>>
>> If you could show me how to use EventMachine in this case I'd be grateful.
>> I couldn't figure out how to run compute_lots_of_stuff() while the http
>> requests are executing.
>>
> 
> What context are you trying to do this in?  Is it inside a rack request
> (rails / merb / sinatra /pancake / other)?  or is this in a stand alone
> script?
> 
> Could you perhaps provide a bit of context for what you're trying to achive?

Ok, here's the context. I didn't put this in my OP because I figured
it would just bore everybody to tears.

This is inside a rack request. The idea is that I'm assembling a web
page by doing a bunch of sub-requests for the various parts of the
page. So I'll have something like:

   action "index" do
     @news = subreq("http://news.server")
     @ad = subreq("http://ad.server")
     @blog = subreq("http://blog.server")
     @forum = subreq("http://forum.server")
   end

All these sub-requests are launched asynchronously and, while they are
executing, the app generates the layout within which the output of the
subrequests will be embedded. So I'll have something like:

   response = ['<html><body>',
     '<div>',@ad,'</div>',
     '<div>',@news,'</div>',
     '<div>',@blog,'</div>',
     '<div>',@forum,'</div>',
     '</body></html>']

And when rack finally outputs the response to the client it will block
on the various subrequests unless/until they have completed.

What I can't figure out with EventMachine is how to have the "main
thread" generate the layout while the subrequests are executing.