How to start bunny thread in Rails and Thin - ruby-on-rails-3

Question

I'm integrating Bunny gem for RabbitMQ with Rails, should I start Bunny thread in an initializer that Rails starts with application start or do it in a separate rake task so I can start it in a separate process ?
I think if I'm producing messages only then I need to do it in Rails initializer so it can be used allover the app, but if I'm consuming I should do it in a separate rake task, is this correct ?

Answer 1

You are correct: you should not be consuming from the Rails application itself. The Rails application should be a producer, in which case, an initializer is the correct place to start the Bunny instance.
I essentially have this code in my Rails applications which publish messages to RabbitMQ:
# config/initializers/bunny.rb
MESSAGING_SERVICE = MessagingService.new(ENV.fetch("AMQP_URL"))
MESSAGING_SERVICE.start
# app/controllers/application_controller.rb
class ApplicationController
def messaging_service
MESSAGING_SERVICE
end
end
# app/controllers/uploads_controller.rb
class UploadsController < ApplicationController
def create
# save the model
messaging_service.publish_resize_image_request(model.id)
redirect_to uploads_path
end
end
# lib/messaging_service.rb
class MessagingService
def initialize(amqp_url)
#bunny = Bunny.new(amqp_url)
#bunny.start
at_exit { #bunny.stop }
end
attr_reader :bunny
def publish_resize_image_request(image_id)
resize_image_exchange.publish(image_id.to_s)
end
def resize_image_exchange
#resize_image_exchange ||=
channel.exchange("resize-image", passive: true)
end
def channel
#channel ||= bunny.channel
end
end
For consuming messages, I prefer to start executables without Rake involved. Rake will fork a new process, which will use more memory.
# bin/image-resizer-worker
require "bunny"
bunny = Bunny.new(ENV.fetch("AMQP_URL"))
bunny.start
at_exit { bunny.stop }
channel = bunny.channel
# Tell RabbitMQ to send this worker at most 2 messages at a time
# Else, RabbitMQ will send us as many messages as we can absorb,
# which would be 100% of the queue. If we have multiple worker
# instances, we want to load-balance between each of them.
channel.prefetch(2)
exchange = channel.exchange("resize-image", type: :direct, durable: true)
queue = channel.queue("resize-image", durable: true)
queue.bind(exchange)
queue.subscribe(manual_ack: true, block: true) do |delivery_info, properties, payload|
begin
upload = Upload.find(Integer(payload))
# somehow, resize the image and/or post-process the image
# Tell RabbitMQ we processed the message, in order to not see it again
channel.acknowledge(delivery_info.delivery_tag, false)
rescue ActiveRecord::RecordNotFound => _
STDERR.puts "Model does not exist: #{payload.inspect}"
# If the model is not in the database, we don't want to see this message again
channel.acknowledge(delivery_info.delivery_tag, false)
rescue Errno:ENOSPC => e
STDERR.puts "Ran out of disk space resizing #{payload.inspect}"
# Do NOT ack the message, in order to see it again at a later date
# This worker, or another one on another host, may have free space to
# process the image.
rescue RuntimeError => e
STDERR.puts "Failed to resize #{payload}: #{e.class} - #{e.message}"
# The fallback should probably be to ack the message.
channel.acknowledge(delivery_info.delivery_tag, false)
end
end
Given all that though, you may be better off with pre-built gems and using Rails' abstraction, ActiveJob.