Class: GovukMessageQueueConsumer::Consumer

Inherits:
Object
  • Object
show all
Defined in:
lib/govuk_message_queue_consumer/consumer.rb

Direct Known Subclasses

TestConsumer

Constant Summary collapse

NUMBER_OF_MESSAGES_TO_PREFETCH =

Only fetch one message at a time on the channel.

By default, queues will grab messages eagerly, which reduces latency. However, that also means that if multiple workers are running one worker can starve another of work. We’re not expecting a high throughput on this queue, and a small bit of latency isn’t a problem, so we fetch one at a time to share the work evenly.

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Instance Method Summary collapse

Constructor Details

#initialize(queue_name:, exchange_name:, processor:, routing_key: '#', statsd_client: NullStatsd.new) ⇒ Consumer

Create a new consumer

Parameters:

  • queue_name (String) —

    Your queue name. This is specific to your application.

  • exchange_name (String) —

    Name of the exchange to bind to, for example published_documents

  • processor (Object) —

    An object that responds to process

  • routing_key (String) (defaults to: '#') —

    The RabbitMQ routing key to bind the queue to

  • statsd_client (Statsd) (defaults to: NullStatsd.new) —

    An instance of the Statsd class



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# File 'lib/govuk_message_queue_consumer/consumer.rb', line 22

def initialize(queue_name:, exchange_name:, processor:, routing_key: '#', statsd_client: NullStatsd.new)
  @queue_name = queue_name
  @exchange_name = exchange_name
  @processor = processor
  @routing_key = routing_key
  @statsd_client = statsd_client
end

Instance Method Details

#run ⇒ Object 



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# File 'lib/govuk_message_queue_consumer/consumer.rb', line 30

def run
  queue.subscribe(block: true, manual_ack: true) do |delivery_info, headers, payload|
    begin
      message = Message.new(payload, headers, delivery_info)
      @statsd_client.increment("#{@queue_name}.started")
      processor_chain.process(message)
      @statsd_client.increment("#{@queue_name}.#{message.status}")
    rescue Exception => e
      @statsd_client.increment("#{@queue_name}.uncaught_exception")
      Airbrake.notify_or_ignore(e) if defined?(Airbrake)
      $stderr.puts "Uncaught exception in processor: \n\n #{e.class}: #{e.message}\n\n#{e.backtrace.join("\n")}"
      exit(1) # Ensure rabbitmq requeues outstanding messages
    end
  end
end