amqp-dispatcher 0.3.3

===============
AMQP Dispatcher
===============

A daemon to run AMQP consumers

Requirements
============

* Python 2.6+

Installation
============

Using PIP:

From Github::

pip install git+git://github.com/philipcristiano/amqp-dispatcher.git@0.3.3#egg=beaver

From PyPI::

pip install amqp-dispatcher==0.3.0


Running
=======

.. code:: bash

amqp-dispatcher --config amqp-dispatcher-config.yml

The environment variable ``RABBITMQ_URL`` can also be used which will cause
attempt to connect to the defined data source name. Hosts are separated
via commas, and they are connected to in random order.

Consumers
---------

Consumers are a class with 2 required methods: ``consume`` and ``shutdown``.
amqp-dispatcher will not monkey patch the environment, you will have to do
that yourself.

- ``consume``: ``consume`` is called once for each message being handled. It should take 2 parameters, a proxy for AMQP operations (``amqp``) and the message (``msg``).
- ``shutdown`` - ``shutdown`` is called before the instance of the consumer is removed. It takes a single argument ``exception`` which may be ``None``. If your consumer raises an exception while consuming the ``shutdown`` method will be called. Once ``shutdown`` is finished a new instance of your consumer will be created to replace the one that raised the exception. If you would like to rate limit instance replacement you can call ``gevent.sleep(X)`` to sleep for ``X`` seconds after a failure.


Example consumer:

.. code:: bash

class Consumer(object):

def __init__(self):
self.init_msg = "I've been initiliazed"

def consume(self, amqp, msg):
print 'Consuming message', msg.body
gevent.sleep(1)
val = random.random()
if val > .8:
print 'publishing'
amqp.publish('test_exchange', 'test_routing_key', {}, 'New body!')
if val < .5:
raise ValueError()
print 'Done sleeping'
amqp.ack()

def shutdown(self, exception=None):
print 'Shut down'


Configuration
-------------

amqp-dispatcher will read environment variable for connection information and a
YAML file for worker configuration.

Environment Variables
---------------------

- ``RABBITMQ_URL``: Connection string of the form ``amqp://USER:PASS@HOST:PORT/VHOST``

Startup Configuration
---------------------

If you need to perform custom actions (configure your logging, create initial objects) you can add a startup handler.

This is configured in the config yml with the ``startup_handler`` option.

.. code:: yaml

startup_handler: amqpdispatcher.example_startup:startup

Queue configuration
------------------

Queues can be created on the fly by amqp dispatcher, and may bind existing exchanges on the fly as well.

There are a few obvious constraints:

* To create a non-passive queue (typical behavior) the current user must have ``configure=queue`` permission
* To bind to an exchange, the current user must have ``read`` permission on the binding exchange

Queue configuration is as follows:

- ``queue``: (required) name of the queue
- ``durable``: (optional) queue created in "durable" mode (default = True)
- ``auto_delete``: (optional) queue created in "auto_delete" mode (default = False), meaning it will be deleted automatically once all consumers disconnect from it (e.g. on restart)
- ``exclusive``: (optional) queue created in "exclusive" mode (default = False) meaning it will only be accessible by this process
- ``x_dead_letter_exchange``: (optional) name of dead letter exchange
- ``x_dead_letter_routing_key``: (optional) dead letter routing key
- ``x_max_length``: (optional) maximum length of ready messages. (default = INFINITE)
- ``x_expires``: (optional) How long a queue can be unused for before it is automatically deleted (milliseconds) (default=INFINITE)
- ``x_message_ttl``: (optional) How long a message published to a queue can live before it is discarded (milliseconds) (default=INFINITE)

Bindings
--------

``bindings`` should contain a list of ``exchange``/``routing_key`` pairs and defines the binding for the queue (there can be multiple)

A complete configuration example would look like:

.. code:: yaml

queues:
- queue: notify_mat_job
durable: true
auto_delete: false
passive: true
exclusive: false
x_dead_letter_exchange: null
x_dead_letter_routing_key: null
x_max_length: null
x_expires: null
x_message_ttl: null
bindings:
- exchange: notify
routing_key: transaction.*
- exchange: notify
routing_key: click.*

- queue: notify_apsalar_job
bindings:
- exchange: notify
routing_key: transaction.*
- exchange: notify
routing_key: click.*

Worker configuration
--------------------

Workers are autoloaded when AMQP Dispatcher starts. This means your worker must
be importable from the environment.

A complete configuration example would look like:

.. code:: yaml

consumers:
- consumer: workers.module:Consumer
consumer_count: 1
queue: test_queue
prefetch_count: 2
- consumer: workers.module_2:Consumer
consumer_count: 2
queue: test_queue_2
prefetch_count: 10

``prefetch_count`` is the AMQP ``prefetch_count`` when consuming. The
``consumer_count`` is the number of instances of your consumer to handle messages
from that queue. Connection pools are highly recommended. MySQL will require the
`MySQL Connector <http://pypi.python.org/pypi/mysql-connector-python>`_ instead of
``mysqldb`` in order for gevent to switch properly.

Pools can be created and attached to the consumer class during the ``__init__``. Example with SQLAlchemy

.. code:: python

class Consumer(object):

session_maker = None

def __init__(self):
self.session = None

if Consumer._engine is None:
print 'Creating session maker'
Consumer._engine = create_engine(...)
Consumer.sessionmaker = sessionmaker(bind=Consumer._engine)

And then a session created during the consume method.

.. code:: python

def consume(self, proxy, msg):
session = self.sessionmaker()
# Do something with the session
session.close()

Logging
-------

Logging is performed on the logger ``amqp-dispatcher``. The RabbitMQ connection
provided by Haigha will log on ``amqp-dispatcher.haigha``.