Asynchronous consumer example¶
The following example implements a consumer that will respond to RPC commands sent from RabbitMQ. For example, it will reconnect if RabbitMQ closes the connection and will shutdown if RabbitMQ cancels the consumer or closes the channel. While it may look intimidating, each method is very short and represents a individual actions that a consumer can do.
# -*- coding: utf-8 -*- import logging import pika LOG_FORMAT = ('%(levelname) -10s %(asctime)s %(name) -30s %(funcName) ' '-35s %(lineno) -5d: %(message)s') LOGGER = logging.getLogger(__name__) class ExampleConsumer(object): """This is an example consumer that will handle unexpected interactions with RabbitMQ such as channel and connection closures. If RabbitMQ closes the connection, it will reopen it. You should look at the output, as there are limited reasons why the connection may be closed, which usually are tied to permission related issues or socket timeouts. If the channel is closed, it will indicate a problem with one of the commands that were issued and that should surface in the output as well. """ EXCHANGE = 'message' EXCHANGE_TYPE = 'topic' QUEUE = 'text' ROUTING_KEY = 'example.text' def __init__(self, amqp_url): """Create a new instance of the consumer class, passing in the AMQP URL used to connect to RabbitMQ. :param str amqp_url: The AMQP url to connect with """ self._connection = None self._channel = None self._closing = False self._consumer_tag = None self._url = amqp_url def connect(self): """This method connects to RabbitMQ, returning the connection handle. When the connection is established, the on_connection_open method will be invoked by pika. :rtype: pika.SelectConnection """ LOGGER.info('Connecting to %s', self._url) return pika.SelectConnection(pika.URLParameters(self._url), self.on_connection_open, stop_ioloop_on_close=False) def on_connection_open(self, unused_connection): """This method is called by pika once the connection to RabbitMQ has been established. It passes the handle to the connection object in case we need it, but in this case, we'll just mark it unused. :type unused_connection: pika.SelectConnection """ LOGGER.info('Connection opened') self.add_on_connection_close_callback() self.open_channel() def add_on_connection_close_callback(self): """This method adds an on close callback that will be invoked by pika when RabbitMQ closes the connection to the publisher unexpectedly. """ LOGGER.info('Adding connection close callback') self._connection.add_on_close_callback(self.on_connection_closed) def on_connection_closed(self, connection, reply_code, reply_text): """This method is invoked by pika when the connection to RabbitMQ is closed unexpectedly. Since it is unexpected, we will reconnect to RabbitMQ if it disconnects. :param pika.connection.Connection connection: The closed connection obj :param int reply_code: The server provided reply_code if given :param str reply_text: The server provided reply_text if given """ self._channel = None if self._closing: self._connection.ioloop.stop() else: LOGGER.warning('Connection closed, reopening in 5 seconds: (%s) %s', reply_code, reply_text) self._connection.add_timeout(5, self.reconnect) def reconnect(self): """Will be invoked by the IOLoop timer if the connection is closed. See the on_connection_closed method. """ # This is the old connection IOLoop instance, stop its ioloop self._connection.ioloop.stop() if not self._closing: # Create a new connection self._connection = self.connect() # There is now a new connection, needs a new ioloop to run self._connection.ioloop.start() def open_channel(self): """Open a new channel with RabbitMQ by issuing the Channel.Open RPC command. When RabbitMQ responds that the channel is open, the on_channel_open callback will be invoked by pika. """ LOGGER.info('Creating a new channel') self._connection.channel(on_open_callback=self.on_channel_open) def on_channel_open(self, channel): """This method is invoked by pika when the channel has been opened. The channel object is passed in so we can make use of it. Since the channel is now open, we'll declare the exchange to use. :param pika.channel.Channel channel: The channel object """ LOGGER.info('Channel opened') self._channel = channel self.add_on_channel_close_callback() self.setup_exchange(self.EXCHANGE) def add_on_channel_close_callback(self): """This method tells pika to call the on_channel_closed method if RabbitMQ unexpectedly closes the channel. """ LOGGER.info('Adding channel close callback') self._channel.add_on_close_callback(self.on_channel_closed) def on_channel_closed(self, channel, reply_code, reply_text): """Invoked by pika when RabbitMQ unexpectedly closes the channel. Channels are usually closed if you attempt to do something that violates the protocol, such as re-declare an exchange or queue with different parameters. In this case, we'll close the connection to shutdown the object. :param pika.channel.Channel: The closed channel :param int reply_code: The numeric reason the channel was closed :param str reply_text: The text reason the channel was closed """ LOGGER.warning('Channel %i was closed: (%s) %s', channel, reply_code, reply_text) self._connection.close() def setup_exchange(self, exchange_name): """Setup the exchange on RabbitMQ by invoking the Exchange.Declare RPC command. When it is complete, the on_exchange_declareok method will be invoked by pika. :param str|unicode exchange_name: The name of the exchange to declare """ LOGGER.info('Declaring exchange %s', exchange_name) self._channel.exchange_declare(self.on_exchange_declareok, exchange_name, self.EXCHANGE_TYPE) def on_exchange_declareok(self, unused_frame): """Invoked by pika when RabbitMQ has finished the Exchange.Declare RPC command. :param pika.Frame.Method unused_frame: Exchange.DeclareOk response frame """ LOGGER.info('Exchange declared') self.setup_queue(self.QUEUE) def setup_queue(self, queue_name): """Setup the queue on RabbitMQ by invoking the Queue.Declare RPC command. When it is complete, the on_queue_declareok method will be invoked by pika. :param str|unicode queue_name: The name of the queue to declare. """ LOGGER.info('Declaring queue %s', queue_name) self._channel.queue_declare(self.on_queue_declareok, queue_name) def on_queue_declareok(self, method_frame): """Method invoked by pika when the Queue.Declare RPC call made in setup_queue has completed. In this method we will bind the queue and exchange together with the routing key by issuing the Queue.Bind RPC command. When this command is complete, the on_bindok method will be invoked by pika. :param pika.frame.Method method_frame: The Queue.DeclareOk frame """ LOGGER.info('Binding %s to %s with %s', self.EXCHANGE, self.QUEUE, self.ROUTING_KEY) self._channel.queue_bind(self.on_bindok, self.QUEUE, self.EXCHANGE, self.ROUTING_KEY) def on_bindok(self, unused_frame): """Invoked by pika when the Queue.Bind method has completed. At this point we will start consuming messages by calling start_consuming which will invoke the needed RPC commands to start the process. :param pika.frame.Method unused_frame: The Queue.BindOk response frame """ LOGGER.info('Queue bound') self.start_consuming() def start_consuming(self): """This method sets up the consumer by first calling add_on_cancel_callback so that the object is notified if RabbitMQ cancels the consumer. It then issues the Basic.Consume RPC command which returns the consumer tag that is used to uniquely identify the consumer with RabbitMQ. We keep the value to use it when we want to cancel consuming. The on_message method is passed in as a callback pika will invoke when a message is fully received. """ LOGGER.info('Issuing consumer related RPC commands') self.add_on_cancel_callback() self._consumer_tag = self._channel.basic_consume(self.on_message, self.QUEUE) def add_on_cancel_callback(self): """Add a callback that will be invoked if RabbitMQ cancels the consumer for some reason. If RabbitMQ does cancel the consumer, on_consumer_cancelled will be invoked by pika. """ LOGGER.info('Adding consumer cancellation callback') self._channel.add_on_cancel_callback(self.on_consumer_cancelled) def on_consumer_cancelled(self, method_frame): """Invoked by pika when RabbitMQ sends a Basic.Cancel for a consumer receiving messages. :param pika.frame.Method method_frame: The Basic.Cancel frame """ LOGGER.info('Consumer was cancelled remotely, shutting down: %r', method_frame) if self._channel: self._channel.close() def on_message(self, unused_channel, basic_deliver, properties, body): """Invoked by pika when a message is delivered from RabbitMQ. The channel is passed for your convenience. The basic_deliver object that is passed in carries the exchange, routing key, delivery tag and a redelivered flag for the message. The properties passed in is an instance of BasicProperties with the message properties and the body is the message that was sent. :param pika.channel.Channel unused_channel: The channel object :param pika.Spec.Basic.Deliver: basic_deliver method :param pika.Spec.BasicProperties: properties :param str|unicode body: The message body """ LOGGER.info('Received message # %s from %s: %s', basic_deliver.delivery_tag, properties.app_id, body) self.acknowledge_message(basic_deliver.delivery_tag) def acknowledge_message(self, delivery_tag): """Acknowledge the message delivery from RabbitMQ by sending a Basic.Ack RPC method for the delivery tag. :param int delivery_tag: The delivery tag from the Basic.Deliver frame """ LOGGER.info('Acknowledging message %s', delivery_tag) self._channel.basic_ack(delivery_tag) def stop_consuming(self): """Tell RabbitMQ that you would like to stop consuming by sending the Basic.Cancel RPC command. """ if self._channel: LOGGER.info('Sending a Basic.Cancel RPC command to RabbitMQ') self._channel.basic_cancel(self.on_cancelok, self._consumer_tag) def on_cancelok(self, unused_frame): """This method is invoked by pika when RabbitMQ acknowledges the cancellation of a consumer. At this point we will close the channel. This will invoke the on_channel_closed method once the channel has been closed, which will in-turn close the connection. :param pika.frame.Method unused_frame: The Basic.CancelOk frame """ LOGGER.info('RabbitMQ acknowledged the cancellation of the consumer') self.close_channel() def close_channel(self): """Call to close the channel with RabbitMQ cleanly by issuing the Channel.Close RPC command. """ LOGGER.info('Closing the channel') self._channel.close() def run(self): """Run the example consumer by connecting to RabbitMQ and then starting the IOLoop to block and allow the SelectConnection to operate. """ self._connection = self.connect() self._connection.ioloop.start() def stop(self): """Cleanly shutdown the connection to RabbitMQ by stopping the consumer with RabbitMQ. When RabbitMQ confirms the cancellation, on_cancelok will be invoked by pika, which will then closing the channel and connection. The IOLoop is started again because this method is invoked when CTRL-C is pressed raising a KeyboardInterrupt exception. This exception stops the IOLoop which needs to be running for pika to communicate with RabbitMQ. All of the commands issued prior to starting the IOLoop will be buffered but not processed. """ LOGGER.info('Stopping') self._closing = True self.stop_consuming() self._connection.ioloop.start() LOGGER.info('Stopped') def close_connection(self): """This method closes the connection to RabbitMQ.""" LOGGER.info('Closing connection') self._connection.close() def main(): logging.basicConfig(level=logging.INFO, format=LOG_FORMAT) example = ExampleConsumer('amqp://guest:guest@localhost:5672/%2F') try: example.run() except KeyboardInterrupt: example.stop() if __name__ == '__main__': main()