Re: Forking off the unicorn master process to create a background worker

[Russell Branca <chewbranca@gmail.com>]

Hello Eric,


Sorry for the delayed response, with the combination of being sick and
heading out of town for a while, this project got put on the
backburner. I really appreciate your response and think its a clean
solution for what I'm trying to do. I've started back in getting the
job queue working this week, and will hopefully have a working
solution in the next day or two. A little more information about what
I'm doing, I'm trying to create a centralized resque job queue server
that each of the different applications can queue work into, so I'll
be using redis behind resque for storing jobs and what not, which
brings me an area I'm not sure of the best approach on. So when we hit
the job queue endpoint in the rack app, it spawns the new worker, and
then immediately returns the 200 ok started background job message,
which cuts off communication back to the job queue. My plan is to save
a status message of the result of the background task into redis, and
have resque check that to verify the task was successful. Is there a
better approach for returning the resulting status code with unicorn,
or is this a reasonable approach? Thanks again for your help.


-Russell

On Wed, May 26, 2010 at 2:05 PM, Eric Wong <normalperson@yhbt.net> wrote:
> Russell Branca <chewbranca@gmail.com> wrote:
>> Hello,
>>
>> I'm trying to find an efficient way to create a new instance of a
>> rails application to perform some background tasks without having to
>> load up the entire rails stack every time, so I figured forking off
>> the master process would be a good way to go. Now I can easily just
>> increment the worker count and then send a web request in, but the new
>> worker would be part of the main worker pool, so in the time between
>> spawning a new worker and sending the request, another request could
>> have come in and snagged that worker. Is it possible to create a new
>> worker and not have it enter the main worker pool so I could access it
>> directly?
>
> Hi Russell,
>
> You could try having an endpoint in your webapp (with authentication, or
> have it reject env['REMOTE_ADDR'] != '127.0.0.1') that runs the
> background task for you.  Since it's a background app, you should
> probably fork + Process.setsid + fork (or Process.daemon in 1.9), and
> return an HTTP response immediately so your app can serve other
> requests.
>
> The following example should be enough to get you started (totally
> untested)
>
> ------------ config.ru -------------
> require 'rack/lobster'
>
> map "/.seekrit_endpoint" do
>  use Rack::ContentLength
>  use Rack::ContentType, 'text/plain'
>  run(lambda { |env|
>    return [ 403, {}, [] ] if env['REMOTE_ADDR'] != '127.0.0.1'
>    pid = fork
>    if pid
>      Process.waitpid(pid)
>
>      # cheap way to avoid unintentional fd sharing with our children,
>      # this causes the current Unicorn worker to exit after sending
>      # the response:
>      # Otherwise you'd have to be careful to disconnect+reconnect
>      # databases/memcached/redis/whatever (in both the parent and
>      # child) to avoid unintentional sharing that'll lead to headaches
>      Process.kill(:QUIT, $$)
>
>      [ 200, {}, [ "started background process\n" ] ]
>    else
>      # child, daemonize it so the unicorn master won't need to
>      # reap it (that's the job of init)
>      Process.setsid
>      exit if fork
>
>      begin
>        # run your background code here instead of sleeping
>        sleep 5
>        env["rack.logger"].info "done sleeping"
>      rescue => e
>        env["rack.logger"].error(e.inspect)
>      end
>      # make sure we don't enter the normal response cycle back in the
>      # worker...
>      exit!(0)
>    end
>  })
> end
>
> map "/" do
>  run Rack::Lobster.new
> end
>
>> I know this is not your typical use case for unicorn, and you're
>> probably thinking there is a lot better ways to do this, however, I
>> currently have a rails framework that powers a handful of standalone
>> applications on a server with limited resources, and I'm trying to
>> make a centralized queue that all the applications use, so the queue
>> needs to be able to spawn a new worker for each of the applications
>> efficiently, and incrementing/decrementing worker counts in unicorn is
>> the most efficient way I've found to spawn a new rails instance.
>
> Yeah, it's definitely an odd case and there are ways to shoot yourself
> in the foot with it (especially with unintentional fd sharing), but Ruby
> exposes all the Unix process management goodies better than most
> languages (probably better than anything else I've used).
>
>> Any help, suggestions or insight into this would be greatly appreciated.
>
> Let us know how it goes :)
>
> --
> Eric Wong
>
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