如何让这个AMQP单消息订阅者稳定？

Question

作为大型应用程序的一部分，我必须设置一些跨多个工作人员的传出请求的基本速率限制。这背后的想法相当简单：通过发布带有“立即”标志的“令牌”消息，如果没有人在等待，则该消息将被自动丢弃。通过让工作人员仅在发送传出请求之前订阅令牌队列，令牌不会“保存”，并且每个令牌只能使用一次。我认为这相当优雅。

不幸的是，添加和删除订阅者并不完全稳定。我已经在 https://gist.github.com/1263921/ebdafa067ca09514183d3fc5d6e43c7094fc2733。代码如下：

require 'bundler'
Bundler.setup

require 'amqp'

puts "single-message consumer listening to rapid producer"

QUEUE_NAME   = 'test.rapid-queue-unsubscription'
PRODUCE_RATE = 1.0/10
CONSUME_RATE = 1.0/9

def start_producer
  exchange = AMQP::Exchange.new(AMQP::Channel.new, :direct, "") 

  n = 0
  EM::PeriodicTimer.new(PRODUCE_RATE) do
    message = "msg #{n}"
    exchange.publish(message,
                     :immediate   => true, # IMPORTANT, messages are dropped if nobody listening now
                     :routing_key => QUEUE_NAME)
    puts "> PUT #{message}"
    n += 1
  end
end

def start_consumer

  EM::PeriodicTimer.new(CONSUME_RATE) do

    started = Time.now
    AMQP::Channel.new do |channel_consumer|
      channel_consumer.prefetch(1)
      tick_queue = channel_consumer.queue(QUEUE_NAME)

      consumer = AMQP::Consumer.new(channel_consumer, tick_queue, nil, exclusive = false, no_ack = true)
      consumer.on_delivery do |_, message|

        took = Time.now - started
        puts "< GET #{message} [waited #{took.round(2)}s][#{(1.0/took).round(2)} reqs/sec]"

        consumer.cancel
        channel_consumer.close
      end
      consumer.consume
    end
  end
end

EM.run do
  EM.set_quantum(50)

  start_producer
  start_consumer
end

运行该示例几分钟最终会因以下两个错误之一而终止：

1. amq-client-0.8.3/lib/amq/client/async/consumer.rb:246:in `阻止': 未定义方法“handle_delivery” nil:NilClass (NoMethodError)

2. amq-client-0.8.3/lib/amq/client/async/adapter.rb:244:in `send_frame'：尝试通过关闭的连接发送帧。 帧是#(AMQ::Client::ConnectionClosedError)

第一个错误是由于订阅者已被删除，但消息仍然传递给它，而 amq-client 库从来没有预料到这种情况发生。第二个错误来自发布者，它突然关闭了连接。

要使其始终按预期工作，我缺少什么？

使用的版本：

• OS X 10.7.1
• ruby​​ 1.9.2p312（2011-08-11 修订版 32926）[x86_64-darwin11.1.0]
• RabbitMQ 2.6。 1

Gemfile：

source 'http://rubygems.org'

gem 'amqp'

Gemfile.lock：

GEM
  remote: http://rubygems.org/
  specs:
    amq-client (0.8.3)
      amq-protocol (>= 0.8.0)
      eventmachine
    amq-protocol (0.8.1)
    amqp (0.8.0)
      amq-client (~> 0.8.3)
      amq-protocol (~> 0.8.0)
      eventmachine
    eventmachine (0.12.10)

PLATFORMS
  ruby

DEPENDENCIES
  amqp
  eventmachine

Answer 1

来自#rabbitmq 通道（amqp 作者antares_）：只需使用单个通道，就可以正常工作。略有变化，但稳定版本：

require 'bundler'
Bundler.setup

require 'amqp'

puts "single-message consumer listening to rapid producer"

QUEUE_NAME   = 'test.rapid-queue-unsubscription'
PRODUCE_RATE = 1.0/10
CONSUME_RATE = 1.0/9

def start_producer channel
  exchange = AMQP::Exchange.new(channel, :direct, "") 

  n = 0
  EM::PeriodicTimer.new(PRODUCE_RATE) do
    message = "msg #{n}"
    exchange.publish(message,
                     :immediate   => true, # IMPORTANT, messages are dropped if nobody listening now
                     :routing_key => QUEUE_NAME)
    puts "> PUT #{message}"
    n += 1
  end
end

def start_consumer channel
  EM::PeriodicTimer.new(CONSUME_RATE) do

    started = Time.now
    tick_queue = channel.queue(QUEUE_NAME)

    consumer = AMQP::Consumer.new(channel, tick_queue, nil, exclusive = false, no_ack = true)
    consumer.on_delivery do |_, message|

      took = Time.now - started
      puts "< GET #{message} [waited #{took.round(2)}s][#{(1.0/took).round(2)} reqs/sec]"

      consumer.cancel do
        puts "< GET #{message} (CANCEL DONE)"
      end
    end
    consumer.consume
  end
end

EM.run do
  EM.set_quantum(50)

  AMQP::Channel.new do |channel|
    start_producer channel
  end

  AMQP::Channel.new do |channel|
    channel.prefetch(1)
    start_consumer channel
  end

end