Using Tornado with Pika for Asynchronous Queue Monitoring

Question

I have an AMQP server (RabbitMQ) that I would like to both publish and read from in a Tornado web server. To do this, I figured I would use an asynchronous amqp python library; in particular Pika (a variation of it that supposedly supports Tornado).

I have written code that appears to successfully read from the queue, except that at the end of the request, I get an exception (the browser returns fine):

[E 101219 01:07:35 web:868] Uncaught exception GET / (127.0.0.1)
    HTTPRequest(protocol='http', host='localhost:5000', method='GET', uri='/', version='HTTP/1.1', remote_ip='127.0.0.1', remote_ip='127.0.0.1', body='', headers={'Host': 'localhost:5000', 'Accept-Language': 'en-us,en;q=0.5', 'Accept-Encoding': 'gzip,deflate', 'Keep-Alive': '115', 'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8', 'User-Agent': 'Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.13) Gecko/20101206 Ubuntu/10.10 (maverick) Firefox/3.6.13', 'Accept-Charset': 'ISO-8859-1,utf-8;q=0.7,*;q=0.7', 'Connection': 'keep-alive', 'Cache-Control': 'max-age=0', 'If-None-Match': '"58f554b64ed24495235171596351069588d0260e"'})
    Traceback (most recent call last):
      File "/home/dave/devel/lib/python2.6/site-packages/tornado/web.py", line 810, in _stack_context
        yield
      File "/home/dave/devel/lib/python2.6/site-packages/tornado/stack_context.py", line 77, in StackContext
        yield
      File "/usr/lib/python2.6/contextlib.py", line 113, in nested
        yield vars
      File "/home/dave/lib/python2.6/site-packages/tornado/stack_context.py", line 126, in wrapped
        callback(*args, **kwargs)
      File "/home/dave/devel/src/pika/pika/tornado_adapter.py", line 42, in _handle_events
        self._handle_read()
      File "/home/dave/devel/src/pika/pika/tornado_adapter.py", line 66, in _handle_read
        self.on_data_available(chunk)
      File "/home/dave/devel/src/pika/pika/connection.py", line 521, in on_data_available
        self.channels[frame.channel_number].frame_handler(frame)
    KeyError: 1

I'm not entirely sure I am using this library correctly, so I might be doing something blatantly wrong. The basic flow of my code is:

1. Request comes in
2. Create connection to RabbitMQ using TornadoConnection; specify a callback
3. In connection callback, create a channel, declare/bind my queue, and call basic_consume; specify a callback
4. In consume callback, close the channel and call Tornado's finish function.
5. See exception.

My questions are a few:

1. Is this flow even correct? I'm not sure what the purpose of the connection callback is except that it doesn't work if I don't use it.
2. Should I be creating one AMQP connection per web request? RabbitMQ's documentation suggests that no, I should not but rather I should stick to creating just channels. But where would I create the connection, and how do I attempt reconnects should it go down briefly?
3. If I am creating one AMQP connection per Web request, where should I be closing it? Calling amqp.close() in my callback seems to screw things up even more.

I will try to have some sample code up a little later, but the steps I described above lay out the consuming side of things fairly completely. I am having issues with the publishing side as well, but the consuming of queues is more pressing.

Answer 1

It would help to see some source code, but I use this same tornado-supporting pika module without issue in more than one production project.

You don't want to create a connection per request. Create a class that wraps all of your AMQP operations, and instantiate it as a singleton at the tornado Application level that can be used across requests (and across request handlers). I do this in a 'runapp()' function that does some stuff like this and then starts the main tornado ioloop.

Here's a class called 'Events'. It's a partial implementation (specifically, I don't define 'self.handle_event' here. That's up to you.

class Event(object):
  def __init__(self, config):
    self.host = 'localhost'
    self.port = '5672'
    self.vhost = '/'
    self.user = 'foo'
    self.exchange = 'myx'
    self.queue = 'myq'
    self.recv_routing_key = 'msgs4me'
    self.passwd = 'bar'

    self.connected = False 
    self.connect()


  def connect(self):

    credentials = pika.PlainCredentials(self.user, self.passwd)

    parameters = pika.ConnectionParameters(host = self.host,
                                         port = self.port,
                                         virtual_host = self.vhost,
                                         credentials = credentials)

    srs = pika.connection.SimpleReconnectionStrategy()

    logging.debug('Events: Connecting to AMQP Broker: %s:%i' % (self.host,
                                                              self.port))
    self.connection = tornado_adapter.TornadoConnection(parameters,
                                                      wait_for_open = False,
                                                      reconnection_strategy = srs,
                                                      callback = self.on_connected)

  def on_connected(self):

    # Open the channel
    logging.debug("Events: Opening a channel")
    self.channel = self.connection.channel()

    # Declare our exchange
    logging.debug("Events: Declaring the %s exchange" %  self.exchange)
    self.channel.exchange_declare(exchange = self.exchange,
                                type = "fanout",
                                auto_delete = False,
                                durable = True)

    # Declare our queue for this process
    logging.debug("Events: Declaring the %s queue" %  self.queue)
    self.channel.queue_declare(queue = self.queue,
                             auto_delete = False,
                             exclusive = False,
                             durable = True)


    # Bind to the exchange
    self.channel.queue_bind(exchange = self.exchange,
                          queue = self.queue,
                          routing_key = self.recv_routing_key)

    self.channel.basic_consume(consumer = self.handle_event, queue = self.queue, no_ack = True)

    # We should be connected if we made it this far
    self.connected = True

And then I put that in a file called 'events.py'. My RequestHandlers and any back end code all utilize a 'common.py' module that wraps code that's useful to both (my RequestHandlers don't call any amqp module methods directly -- same for db, cache, etc as well), so I define 'events=None' at the module level in common.py, and I instantiate the Event object kinda like this:

import events

def runapp(config):
    if myapp.common.events is None: 
       myapp.common.events = myapp.events.Event(config)
    logging.debug("MYAPP.COMMON.EVENTS: %s", myapp.common.events)
    http_server = tornado.httpserver.HTTPServer(app,
                                            xheaders=config['HTTPServer']['xheaders'],
                                            no_keep_alive=config['HTTPServer']['no_keep_alive'])
    http_server.listen(port) 
    main_loop = tornado.ioloop.IOLoop.instance()
    logging.debug("MAIN IOLOOP: %s", main_loop)
    main_loop.start()

Happy new year :-D

Answer 2

Someone has reported success in merging Tornado and Pika here. From what I can tell, it isn't as simple as just calling Pika from Tornado, since both libraries want to have their own event loops in charge.