Re: AMQP and Unicorn (mq gem)

[Eric Wong <normalperson@yhbt.net>]

Marcelo de Moraes Serpa <fullofcaffeine@gmail.com> wrote:
> Hi all,
> 
> I'm having issues with Unicorn and connecting to 
> RabbitMQ using the tmm1-amqp gem.
> 
>  I've tried lots of  approaches.
> 
> The classic initializer with Thread.new { EM.run } 
> for the Rails app and even tried using 
> the Qusion library. 
> 
>  (https://github.com/danielsdeleo/qusion)
> 
> I've made a simple mod to Qusion for it to monkey patch 
> unicorn too. 
> 
> The code looks like:
> 
> /vendor/plugins/qusion/lib/amqp.rb
> 
> module AMQP
>   def self.start_web_dispatcher(amqp_settings={})
>     @settings = settings.merge(amqp_settings)
>     case Qusion::ServerSpy.server_type
>     when :passenger
>       PhusionPassenger.on_event(:starting_worker_process) do |forked| 

<side note>maybe adding a similar hook to Unicorn for library/app authors
to use would be a good idea.</side note>

>         if forked
>           EM.kill_reactor
>           Thread.current[:mq], @conn = nil, nil
>         end 
>         Thread.new { start }
>         die_gracefully_on_signal
>       end
>     when :standard
>       Thread.new { start }
>       die_gracefully_on_signal
>     when :evented
>       die_gracefully_on_signal
>     when :none # << HERE
>       Thread.new { start }
>       die_gracefully_on_signal
>     else
>       raise ArgumentError, "AMQP#start_web_dispatcher
>  requires an argument of 
> [:standard|:evented|:passenger|:none]"
>     end
>   end
>   
>   def self.die_gracefully_on_signal
>     Signal.trap("INT")  { AMQP.stop { EM.stop } }
>     Signal.trap("TERM") { AMQP.stop { EM.stop } }
>   end

Registering a standard at_exit {} block in the Unicorn
after_fork hook would be a good idea than trapping INT
and TERM, which Unicorn uses.

> end
> 
> See the :none case? It used to be empty and return nil. 
> 
> Now it has the same behavior as the non-
> evented :standard server. 
> 
> The problem is that the "server_spy" wasn't 
> recognizing Unicorn. So far so 
> good.
> 
> Originally, the AMQP/Qusion conf goes into config/environment.rb
> 
>    config.after_initialize do
>       
>       Qusion.start(:user => 'guest',:pass => 'mypass') 
>  end
> 
> 
> However, when I publish messages to the queue,
>  it just isn't posted. It silently fails. 
> 
> I've then tried to put this on config/unicorn.rb:
> 
> before_fork do |server,worker|
>  Qusion.start(:user => 'guest',:pass => 'mypass')
>  MQ.new.queue('jobs').publish('hey!')
> end
> 
> And the hey! message is posted. 
> 
> However, it only works from here, the MQ.new.queue on the app code 
> fails silently.
> 
> Any ideas on what might be happenig?

Is preload_app true?  Then you're probably trying to share sockets
across processes and confusing EM or AMQP.

Instead of using before_fork above, you should probably
be using after_fork:

  before_fork do |server,worker|
    Qusion.start(:user => 'guest',:pass => 'mypass')
    MQ.new.queue('jobs').publish('hey!')
  end

I'm not very familiar with AMQP, but the Unicorn process is strictly an
AMQP sender/client and doesn't listen as a server on a port, right?
If so, the above before_fork config *should* work for you.

Let us know how it goes!

-- 
Eric Wong
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