Package: carrot
Version:0.10.7NOTEThis release contains backward-incompatible changes.
Please read theChangelogfor more information.Introductioncarrotis anAMQPmessaging queue framework. AMQP is the Advanced Message
Queuing Protocol, an open standard protocol for message orientation, queuing,
routing, reliability and security.The aim ofcarrotis to make messaging in Python as easy as possible by
providing a high-level interface for producing and consuming messages. At the
same time it is a goal to re-use what is already available as much as possible.carrothas pluggable messaging back-ends, so it is possible to support
several messaging systems. Currently, there is support forAMQP(py-amqplib,pika),STOMP(python-stomp). There’s also an
in-memory backend for testing purposes, using thePython queue module.Several AMQP message broker implementations exists, includingRabbitMQ,ZeroMQandApache ActiveMQ. You’ll need to have one of these installed,
personally we’ve been usingRabbitMQ.Before you start playing withcarrot, 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 theWikipedia article about AMQP.DocumentationCarrot is using Sphinx, and the latest documentation is available at GitHub:http://github.com/ask/carrot/InstallationYou can installcarroteither via the Python Package Index (PyPI)
or from source.To install usingpip,:$ pip install carrotTo install usingeasy_install,:$ easy_install carrotIf you have downloaded a source tarball you can install it
by doing the following,:$ python setup.py build
# python setup.py install # as rootTerminologyThere are some concepts you should be familiar with before starting:PublishersPublishers sends messages to an exchange.ExchangesMessages 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.ConsumersConsumers declares a queue, binds it to a exchange and receives
messages from it.QueuesQueues receive messages sent to exchanges. The queues are declared
by consumers.Routing keysEvery 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 exchangeMatches if the routing key property of the message and
therouting_keyattribute of the consumer are identical.Fan-out exchangeAlways matches, even if the binding does not have a routing
key.Topic exchangeMatches 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".ExamplesCreating a connectionYou can set up a connection by creating an instance ofcarrot.messaging.BrokerConnection, with the appropriate options for
your broker:>>> from carrot.connection import BrokerConnection
>>> conn = BrokerConnection(hostname="localhost", port=5672,
...                           userid="test", password="test",
...                           virtual_host="test")If you’re using Django you can use thecarrot.connection.DjangoBrokerConnectionclass instead, which loads
the connection settings from yoursettings.py:BROKER_HOST = "localhost"
BROKER_PORT = 5672
BROKER_USER = "test"
BROKER_PASSWORD = "secret"
BROKER_VHOST = "/test"Then create a connection by doing:>>> from carrot.connection import DjangoBrokerConnection
>>> conn = DjangoBrokerConnection()Receiving messages using a ConsumerFirst we open up a Python shell and start a message consumer.This consumer declares a queue named"feed", receiving messages with
the routing key"importer"from the"feed"exchange.The example then uses the consumerswait()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=conn, queue="feed",
...                     exchange="feed", routing_key="importer")
>>> def import_feed_callback(message_data, message):
...     feed_url = message_data["import_feed"]
...     print("Got feed import message for: %s" % feed_url)
...     # something importing this feed url
...     # import_feed(feed_url)
...     message.ack()
>>> consumer.register_callback(import_feed_callback)
>>> consumer.wait() # Go into the consumer loop.Sending messages using a PublisherThen we open up another Python shell to send some messages to the consumer
defined in the last section.>>> from carrot.messaging import Publisher
>>> publisher = Publisher(connection=conn,
...                       exchange="feed", routing_key="importer")
>>> publisher.send({"import_feed": "http://cnn.com/rss/edition.rss"})
>>> publisher.close()Look in the first Python shell again (whereconsumer.wait()is running),
where the following text has been printed to the screen:Got feed import message for: http://cnn.com/rss/edition.rssSerialization of DataBy default every message is encoded usingJSON, so sending
Python data structures like dictionaries and lists works.YAML,msgpackand Python’s built-inpicklemodule is also supported,
and if needed you can register any custom serialization scheme you
want to use.Each option has its advantages and disadvantages.json– JSON is supported in many programming languages, is nowa standard part of Python (since 2.6), and is fairly fast to
decode using the modern Python libraries such ascjson or ``simplejson.The primary disadvantage toJSONis that it limits you to
the following data types: strings, unicode, floats, boolean,
dictionaries, and lists.  Decimals and dates are notably missing.Also, binary data will be transferred using base64 encoding, which
will cause the transferred data to be around 34% larger than an
encoding which supports native binary types.However, if your data fits inside the above constraints and
you need cross-language support, the default setting ofJSONis probably your best choice.pickle– If you have no desire to support any language other thanPython, then using thepickleencoding will gain you
the support of all built-in Python data types (except class instances),
smaller messages when sending binary files, and a slight speedup
overJSONprocessing.yaml– YAML has many of the same characteristics asjson,except that it natively supports more data types (including dates,
recursive references, etc.)However, the Python libraries for YAML are a good bit slower
than the libraries for JSON.If you need a more expressive set of data types and need to maintain
cross-language compatibility, thenYAMLmay be a better fit
than the above.To instruct carrot to use an alternate serialization method,
use one of the following options.Set the serialization option on a per-Publisher basis:>>> from carrot.messaging import Publisher
>>> publisher = Publisher(connection=conn,
...                       exchange="feed", routing_key="importer",
...                       serializer="yaml")Set the serialization option on a per-call basis>>> from carrot.messaging import Publisher
>>> publisher = Publisher(connection=conn,
...                       exchange="feed", routing_key="importer")
>>> publisher.send({"import_feed": "http://cnn.com/rss/edition.rss"},
...                serializer="pickle")
>>> publisher.close()Note thatConsumer``sdo not need the serialization method specified in their code.  They canauto-detectthe serialization method since we supply the``Content-typeheader as part of the AMQP message.Sending raw data without SerializationIn some cases, you don’t need your message data to be serialized. If you
pass in a plain string or unicode object as your message, then carrot will
not waste cycles serializing/deserializing the data.You can optionally specify acontent_typeandcontent_encodingfor the raw data:>>> from carrot.messaging import Publisher
>>> publisher = Publisher(connection=conn,
...                       exchange="feed",
                          routing_key="import_pictures")
>>> publisher.send(open('~/my_picture.jpg','rb').read(),
                   content_type="image/jpeg",
                   content_encoding="binary")
>>> publisher.close()Themessageobject returned by theConsumerclass will have acontent_typeandcontent_encodingattribute.Receiving messages without a callbackYou can also poll the queue manually, by using thefetchmethod.
This method returns aMessageobject, 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=conn, queue="feed",
...                     exchange="feed", routing_key="importer")
>>> message = consumer.fetch()
>>> if message:
...    message_data = message.payload
...    message.ack()
... else:
...     # No messages waiting on the queue.
>>> consumer.close()Sub-classing the messaging classesTheConsumer, andPublisherclasses can also be sub classed. 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 import_feed(self, 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["action"]
...         if action == "import_feed":
...             # something importing this feed
...             # import_feed(message_data["feed_url"])
                message.ack()
...         else:
...             raise Exception("Unknown action: %s" % action)>>> publisher = FeedPublisher(connection=conn)
>>> publisher.import_feed("http://cnn.com/rss/edition.rss")
>>> publisher.close()>>> consumer = FeedConsumer(connection=conn)
>>> consumer.wait() # Go into the consumer loop.Getting HelpMailing listJoin thecarrot-usersmailing list.Bug trackerIf you have any suggestions, bug reports or annoyances please report them
to our issue tracker athttp://github.com/ask/carrot/issues/ContributingDevelopment ofcarrothappens at Github:http://github.com/ask/carrotYou are highly encouraged to participate in the development. If you don’t
like Github (for some reason) you’re welcome to send regular patches.LicenseThis software is licensed under theNew BSD License. See theLICENSEfile in the top distribution directory for the full license text.