kombu/docs/includes/introduction.txt

:Version: 5.5.0rc2
:Web: https://kombu.readthedocs.io/
:Download: https://pypi.org/project/kombu/
:Source: https://github.com/celery/kombu/
:Keywords: messaging, amqp, rabbitmq, redis, mongodb, python, queue

About
=====

`Kombu` is a messaging library for Python.

The aim of `Kombu` is to make messaging in Python as easy as possible by
providing an idiomatic high-level interface for the AMQ protocol, and also
provide proven and tested solutions to common messaging problems.

`AMQP`_ is the Advanced Message Queuing Protocol, an open standard protocol
for message orientation, queuing, routing, reliability and security,
for which the `RabbitMQ`_ messaging server is the most popular implementation.

Features
========

* Allows application authors to support several message server
  solutions by using pluggable transports.

    * AMQP transport using the `py-amqp`_, `redis`_, or `SQS`_ libraries.

    * Virtual transports makes it really easy to add support for non-AMQP
      transports. There is already built-in support for `Redis`_,
      `Amazon SQS`_, `Azure Storage Queues`_, `Azure Service Bus`_,
      `ZooKeeper`_, `SoftLayer MQ`_, `MongoDB`_ and `Pyro`_.

    * In-memory transport for unit testing.

* Supports automatic encoding, serialization and compression of message
  payloads.

* Consistent exception handling across transports.

* The ability to ensure that an operation is performed by gracefully
  handling connection and channel errors.

* Several annoyances with `amqplib`_ has been fixed, like supporting
  timeouts and the ability to wait for events on more than one channel.

* Projects already using `carrot`_ can easily be ported by using
  a compatibility layer.

For an introduction to AMQP you should read the article `Rabbits and warrens`_,
and the `Wikipedia article about AMQP`_.

.. _`RabbitMQ`: https://www.rabbitmq.com/
.. _`AMQP`: https://amqp.org
.. _`py-amqp`: https://pypi.org/project/amqp/
.. _`qpid-python`: https://pypi.org/project/qpid-python/
.. _`Redis`: https://redis.io/
.. _`Amazon SQS`: https://aws.amazon.com/sqs/
.. _`Azure Storage Queues`: https://azure.microsoft.com/en-us/services/storage/queues/
.. _`Azure Service Bus`: https://azure.microsoft.com/en-us/services/service-bus/
.. _`Zookeeper`: https://zookeeper.apache.org/
.. _`Rabbits and warrens`: http://web.archive.org/web/20160323134044/http://blogs.digitar.com/jjww/2009/01/rabbits-and-warrens/
.. _`amqplib`: http://barryp.org/software/py-amqplib/
.. _`Wikipedia article about AMQP`: http://en.wikipedia.org/wiki/AMQP
.. _`carrot`: https://pypi.org/project/carrot/
.. _`librabbitmq`: https://pypi.org/project/librabbitmq/
.. _`Pyro`: https://pyro4.readthedocs.io/
.. _`SoftLayer MQ`: http://www.softlayer.com/services/additional/message-queue
.. _`MongoDB`: https://www.mongodb.com/

.. _transport-comparison:

Transport Comparison
====================

+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| **Client**    | **Type** | **Direct** | **Topic**  | **Fanout**    | **Priority** | **TTL**               |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *amqp*        | Native   | Yes        | Yes        | Yes           | Yes [#f3]_   | Yes [#f4]_            |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *qpid*        | Native   | Yes        | Yes        | Yes           | No           | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *redis*       | Virtual  | Yes        | Yes        | Yes (PUB/SUB) | Yes          | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *mongodb*     | Virtual  | Yes        | Yes        | Yes           | Yes          | Yes                   |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *SQS*         | Virtual  | Yes        | Yes [#f1]_ | Yes [#f2]_    | No           | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *zookeeper*   | Virtual  | Yes        | Yes [#f1]_ | No            | Yes          | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *in-memory*   | Virtual  | Yes        | Yes [#f1]_ | No            | No           | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *SLMQ*        | Virtual  | Yes        | Yes [#f1]_ | No            | No           | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+
| *Pyro*        | Virtual  | Yes        | Yes [#f1]_ | No            | No           | No                    |
+---------------+----------+------------+------------+---------------+--------------+-----------------------+


.. [#f1] Declarations only kept in memory, so exchanges/queues
         must be declared by all clients that needs them.

.. [#f2] Fanout supported via storing routing tables in SimpleDB.
         Disabled by default, but can be enabled by using the
         ``supports_fanout`` transport option.

.. [#f3] AMQP Message priority support depends on broker implementation.

.. [#f4] AMQP Message/Queue TTL support depends on broker implementation.

Documentation
-------------

Kombu is using Sphinx, and the latest documentation can be found here:

    https://kombu.readthedocs.io/

Quick overview
--------------

.. code-block:: python

    from kombu import Connection, Exchange, Queue

    media_exchange = Exchange('media', 'direct', durable=True)
    video_queue = Queue('video', exchange=media_exchange, routing_key='video')

    def process_media(body, message):
        print(body)
        message.ack()

    # connections
    with Connection('amqp://guest:guest@localhost//') as conn:

        # produce
        producer = conn.Producer(serializer='json')
        producer.publish({'name': '/tmp/lolcat1.avi', 'size': 1301013},
                          exchange=media_exchange, routing_key='video',
                          declare=[video_queue])

        # the declare above, makes sure the video queue is declared
        # so that the messages can be delivered.
        # It's a best practice in Kombu to have both publishers and
        # consumers declare the queue. You can also declare the
        # queue manually using:
        #     video_queue(conn).declare()

        # consume
        with conn.Consumer(video_queue, callbacks=[process_media]) as consumer:
            # Process messages and handle events on all channels
            while True:
                conn.drain_events()

    # Consume from several queues on the same channel:
    video_queue = Queue('video', exchange=media_exchange, key='video')
    image_queue = Queue('image', exchange=media_exchange, key='image')

    with connection.Consumer([video_queue, image_queue],
                             callbacks=[process_media]) as consumer:
        while True:
            connection.drain_events()


Or handle channels manually:

.. code-block:: python

    with connection.channel() as channel:
        producer = Producer(channel, ...)
        consumer = Consumer(channel)


All objects can be used outside of with statements too,
just remember to close the objects after use:

.. code-block:: python

    from kombu import Connection, Consumer

    connection = Connection()
        # ...
    connection.release()

    consumer = Consumer(channel_or_connection, ...)
    consumer.register_callback(my_callback)
    consumer.consume()
        # ....
    consumer.cancel()


`Exchange` and `Queue` are simply declarations that can be pickled
and used in configuration files etc.

They also support operations, but to do so they need to be bound
to a channel.

Binding exchanges and queues to a connection will make it use
that connections default channel.

.. code-block:: pycon

    >>> exchange = Exchange('tasks', 'direct')

    >>> connection = Connection()
    >>> bound_exchange = exchange(connection)
    >>> bound_exchange.delete()

    # the original exchange is not affected, and stays unbound.
    >>> exchange.delete()
    raise NotBoundError: Can't call delete on Exchange not bound to
        a channel.

Terminology
===========

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

    * Producers

        Producers 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

        A consumer declares a queue, binds it to an 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"`.