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README.rst

carrot - AMQP Messaging Framework for Python

Version: 0.4.0-pre1

Introduction

carrot is an AMQP messaging queue framework. AMQP is the Advanced Message Queuing Protocol, an open standard protocol for message orientation, queuing, routing, reliability and security.

The aim of carrot is to make messaging in Python as easy as possible by providing a high-level interface form producing and consuming messages. At the same time it is a goal to re-use what is already available as much as possible.

carrot supprots pluggable messaging back-ends, so it is possible to support several messaging systems. At the time of release, the py-amqplib based backend is considered suitable for production use.

Several AMQP message broker implementations exists, including RabbitMQ, ZeroMQ and Apache ActiveMQ. You'll need to have one of these installed, personally we've been using RabbitMQ.

Before you start playing with carrot, you should probably read up on AMQP, and you could start with the excellent article about using RabbitMQ under Python, Rabbits and warrens. For more detailed information, you can refer to the Wikipedia article about AMQP.

Installation

You can install carrot either via the Python Package Index (PyPI) or from source.

To install using pip,:

$ pip install carrot

To install using easy_install,:

$ easy_install carrot

If you have downloaded a source tarball you can install it by doing the following,:

$ python setup.py build
# python setup.py install # as root

Terminology

There are some concepts you should be familiar with before starting:

  • Publishers

    Publishers sends messages to an exchange.

  • Exchanges

    Messages are sent to exchanges. Exchanges are named and can be configured to use one of several routing algorithms. The exchange routes the messages to consumers by matching the routing key in the message with the routing key the consumer provides when binding to the exchange.

  • Consumers

    Consumers declares a queue, binds it to a exchange and receives messages from it.

  • Queues

    Queues receive messages sent to exchanges. The queues are declared by consumers.

  • Routing keys

    Every message has a routing key. The interpretation of the routing key depends on the exchange type. There are four default exchange types defined by the AMQP standard, and vendors can define custom types (so see your vendors manual for details).

    These are the default exchange types defined by AMQP/0.8:

    • Direct exchange

      Matches if the routing key property of the message and the routing_key attribute of the consumer are identical.

    • Fan-out exchange

      Always matches, even if the binding does not have a routing key.

    • Topic exchange

      Matches the routing key property of the message by a primitive pattern matching scheme. The message routing key then consists of words separated by dots (".", like domain names), and two special characters are available; star ("*") and hash ("#"). The star matches any word, and the hash matches zero or more words. For example "*.stock.#" matches the routing keys "usd.stock" and "eur.stock.db" but not "stock.nasdaq".

Examples

Creating a connection

You can set up a connection by creating an instance of carrot.messaging.AMQPConnection, with the appropriate options for your AMQP server:

>>> from carrot.connection import AMQPConnection
>>> amqpconn = AMQPConnection(hostname="localhost", port=5672,
...                           userid="test", password="test",
...                           vhost="test")

If you're using Django you can use the carrot.connection.DjangoAMQPConnection class instead, which loads the connection settings from your settings.py:

AMQP_SERVER = "localhost"
AMQP_PORT = 5672
AMQP_USER = "test"
AMQP_PASSWORD = "secret"
AMQP_VHOST = "/test"

Then create a connection by doing:

>>> from carrot.connection import DjangoAMQPConnection
>>> amqpconn = DjangoAMQPConnection()

Sending messages using a Publisher

Here we create a publisher sending messages with a routing key of importer to the "feed" exchange,

>>> from carrot.messaging import Publisher
>>> publisher = Publisher(connection=amqpconn,
...                       exchange="feed", routing_key="importer")
>>> publisher.send({"import_feed": "http://cnn.com/rss/edition.rss"})
>>> publisher.close()

By default every message is encoded using JSON, so sending Python data structures like dictionaries and lists works. If you want to support more complicated data, you might want to configure the publisher and consumer to use something like pickle, by providing them with an encoder and decoder respectively.

Receiving messages using a Consumer

This consumer declares a queue named "feed", receiving messages with the routing key "importer" from the "feed" exchange.

The example then uses the consumers wait() method to go into consume mode, where it continuously polls the queue for new messages, and when a message is received it passes the message to all registered callbacks.

>>> from carrot.messaging import Consumer
>>> consumer = Consumer(connection=amqpconn, queue="feed",
...                     exchange="feed", routing_key="importer")
>>> def import_feed_callback(message_data, message)
...     feed_url = message_data.get("import_feed")
...     if not feed_url:
...         message.reject()
...     # something importing this feed url
...     # import_feed(feed_url)
...     message.ack()
>>> consumer.register_callback(import_feed_callback)
>>> consumer.wait() # Go into the consumer loop.

Receiving messages without a callback

You can also poll the queue manually, by using the fetch method. This method returns a Message object, from where you can get the message body, de-serialize the body to get the data, acknowledge, reject or re-queue the message.

>>> consumer = Consumer(connection=amqpconn, queue="feed",
...                     exchange="feed", routing_key="importer")
>>> message = consumer.fetch()
>>> if message:
...    message_data = message.decode()
...    message.ack()
... else:
...     # No messages waiting on the queue.
>>> consumer.close()

Sub-classing the messaging classes

The Consumer, and Publisher classes can also be subclassed. Thus you can define the above publisher and consumer like so:

>>> from carrot.messaging import Publisher, Consumer
>>> class FeedPublisher(Publisher):
...     exchange = "feed"
...     routing_key = "importer"
...
...     def feed_import(feed_url):
...         return self.send({"action": "import_feed",
...                           "feed_url": feed_url})
>>> class FeedConsumer(Consumer):
...     queue = "feed"
...     exchange = "feed"
...     routing_key = "importer"
...
...     def receive(self, message_data, message):
...         action = message_data.get("action")
...         if not action:
...             message.reject()
...         if action == "import_feed":
...             # something importing this feed
...             # import_feed(message_data["feed_url"])
...         else:
...             raise Exception("Unknown action: %s" % action)
>>> publisher = FeedPublisher(connection=amqpconn)
>>> publisher.import_feed("http://cnn.com/rss/edition.rss")
>>> publisher.close()
>>> consumer = FeedConsumer(connection=amqpconn)
>>> consumer.wait() # Go into the consumer loop.

License

This software is licensed under the New BSD License. See the LICENSE file in the top distribution directory for the full license text.

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