Re: AMQP and Unicorn (mq gem)

[Jordan Ritter <jpr5@darkridge.com>]

We use unicorn + sinatra + tmm1/amqp, and I can say for a fact that you must initialize the AMQP connection in after_fork.  Eric is probably right about the fd sharing from before_fork.

In the rails-specific context, if preload_app is true, it's possible that using config.after_initialize {} will still result in the AMQP initialization occurring before the fork.

cheers,
--jordan


On Nov 9, 2010, at 4:55 PM, Eric Wong wrote:

> 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!

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