pq 1.2

PQ is a transactional queue system for PostgreSQL written in Python.

It allows you to push and pop items in and out of a queue in various
ways and also provides two scheduling options: delayed processing and
prioritization.

The system uses a single table that holds all tasks across queues; the
specifics are easy to customize.

The system currently supports only the `psycopg2
https://pypi.python.org/pypi/psycopg2`_ database driver – or
`psycopg2cffi <https://pypi.python.org/pypi/psycopg2cffi>`_ for PyPy.

The basic queue implementation is similar to Ryan Smith's
`queue_classic <https://github.com/ryandotsmith/queue_classic>`_
library written in Ruby, but uses `advisory locks
<http://www.postgresql.org/docs/current/static/explicit-locking.html#ADVISORY-LOCKS>`_
for concurrency control.

In terms of performance, the implementation clock in at about 1,000
operations per second. Using the `PyPy <http://pypy.org/>`_
interpreter, this scales linearly with the number of cores available.


Getting started
===============

All functionality is encapsulated in a single class ``PQ``.

``class PQ(conn=None, pool=None, table='queue', debug=False)``

Example usage:

::

from psycopg2 import connect
from pq import PQ

conn = connect('dbname=example user=postgres')
pq = PQ(conn)

For multi-threaded operation, use a connection pool such as
``psycopg2.pool.ThreadedConnectionPool``.

You probably want to make sure your database is created with the
``utf-8`` encoding.

To create and configure the queue table, call the ``create()`` method.

::

pq.create()

The table name defaults to ``'queue'``. To use a different name, pass
it as a string value as the ``table`` argument for the ```PQ`` class
(illustrated above).


Queues
======

The ``pq`` object exposes queues through Python's dictionary
interface:

::

queue = pq['apples']

The ``queue`` object provides ``get`` and ``put`` methods as explained
below, and in addition, it also works as a context manager where it
manages a transaction:

::

with queue as cursor:
...

The statements inside the context manager are either committed as a
transaction or rejected, atomically. This is useful when a queue is
used to manage tasks because it allows you to retrieve a task from the
queue, perform a task and write a result, with transactional
semantics.

Methods
=======

Use the ``put(data)`` method to insert an item into the queue. It
takes a JSON-compatible object such as a Python dictionary:

::

queue.put({'kind': 'Cox'})
queue.put({'kind': 'Arthur Turner'})
queue.put({'kind': 'Golden Delicious'})

Items are pulled out of the queue using ``get(block=True)``. The
default behavior is to block until an item is available with a default
timeout of one second after which a value of ``None`` is returned.

::

def eat(kind):
print 'umm, %s apples taste good.' % kind

task = queue.get()
eat(**task.data)

The ``task`` object provides additional metadata in addition to the
``data`` attribute as illustrated by the string representation:

>>> task
<pq.Task id=77709 size=1 enqueued_at="2014-02-21T16:22:06Z" schedule_at=None>

The ``get`` operation is also available through iteration:

::

for task in queue:
if task is None:
break

eat(**task.data)

The iterator blocks if no item is available. Again, there is a default
timeout of one second, after which the iterator yields a value of
``None``.

An application can then choose to break out of the loop, or wait again
for an item to be ready.

::

for task in queue:
if task is not None:
eat(**task.data)

# This is an infinite loop!


Scheduling
==========

Items can be scheduled such that they're not pulled until a later
time:

::

queue.put({'kind': 'Cox'}, '5m')

In this example, the item is ready for work five minutes later. The
method also accepts ``datetime`` and ``timedelta`` objects.


Priority
========

If some items are more important than others, a time expectation can
be expressed:

::

queue.put({'kind': 'Cox'}, expected_at='5m')

This tells the queue processor to give priority to this item over an
item expected at a later time, and conversely, to prefer an item with
an earlier expected time.

The scheduling and priority options can be combined:

::

queue.put({'kind': 'Cox'}, '1h', '2h')

This item won't be pulled out until after one hour, and even then,
it's only processed subject to it's priority of two hours.


Pickles
=======

If a queue name is provided as ``<name>/pickle``
(e.g. ``'jobs/pickle'``), items are automatically pickled and
unpickled using Python's built-in ``cPickle`` module:

::

queue = pq['apples/pickle']

class Apple(object):
def __init__(self, kind):
self.kind = kind

queue.put(Apple('Cox'))

The old pickle protocol ``0`` is used to ensure the pickled data is
encoded as ``ascii`` which should be compatible with any database
encoding.


Thread-safety
=============

All objects are thread-safe as long as a connection pool is provided
where each thread receives its own database connection.


Changes
=======


1.2 (2014-10-21)
----------------

Improvements:

- Fixed concurrency issue where a large number of locks would be held
as a queue grows in size.

- Fixed a database connection resource issue.


1.1 (2014-02-27)
----------------

Features:

- A queue is now also a context manager, providing transactional
semantics.

- A queues now returns task objects which provide metadata and allows
reading and writing task data.

Improvements:

- The same connection pool can now be used with different queues.

Bugs:

- The `Literal` string wrapper did not work correctly with `psycopg2`.

- The transaction manager now correctly returns connections to the
pool.


1.0 (2013-11-20)
----------------

- Initial public release.