collective.taskqueue

   <taskqueue-server />

   <taskqueue>
   queue mailhost
   </taskqueue>
   <taskqueue-server>
   queue mailhost
   </taskqueue-server>

Preferred minimal configration with Redis:

.. code:: ini

eggs = collective.taskqueue [redis]

zope-conf-additional = %import collective.taskqueue

   <taskqueue-server>
     name ${:_buildout_section_name_}
   </taskqueue-server>

Redis-support gives you distributable queues, which can be shared between instances. All instances should have queue-specific <taskqueue />, but only the consuming instance requires <taskqueue-server />.

Example Redis configuration with multiple queues:

.. code:: ini

eggs = collective.taskqueue [redis]

zope-conf-additional = %import collective.taskqueue

   <taskqueue-server>
     name ${:_buildout_section_name_}
   </taskqueue-server>
   <taskqueue>
     type redis
     queue mailhost
     unix_socket_path ${buildout:directory}/var/redis.sock
   </taskqueue>
   <taskqueue-server>
     queue mailhost
     name ${:_buildout_section_name_}
   </taskqueue-server>

It's recommended to only use local Redis-installations, because remote connections can be killed by firewalls (there's no ping or heartbeat to keep the connection alive through enterprise firewalls).

Queue a task:

.. code:: python

from collective.taskqueue import taskqueue task_id = taskqueue.add('/Plone/path/to/my/view')

Tasks are queued (and consumed) after a successful transaction.

To make views visible only for asynchronous requests, views can be registered for a special layer collective.taskqueue.interfaces.ITaskQueueLayer, which is only found from requests dispatched by collective.taskqueue.

By default, taskqueue.add copies headers from the current requests to the asynchronous request. That should be enough to authenticate the requests in exactly the the same way as the current request was authenticated.

taskqueue.add returns uuid like id for the task, which can be used e.g. to track the task status later. Task id later provided as X-Task-Id header in the queued request. You can get it in your task view with self.request.getHeader('X-Task-Id').

More robust authentication can be implemented with a custom PAS-plugin. collective.taskqueue ships with an optionally installable PAS-plugin, which authenticates each request as the user who queued it. To achieve this, collective.taskqueue appends X-Task-User-Id-header into the queued request.

Taskqueue API has been inspired by Google AppEngine Task Queue API__.

__ https://developers.google.com/appengine/docs/python/taskqueue/

Introspecting queues

As a minimalistic asynchronous framework for Plone, collective.taskqueue does not provider any user interface for observing or introspecting queues. Yet, from trusted Python, it is possible to look up a current length of a named queue (name of the default queue is "default"):

.. code:: python

from zope.component import getUtility from collective.taskqueue.interfaces import ITaskQueue

len(getUtility(ITaskQueue, name='default'))

Advanced configuration

Supported <taskqueue />-settings are:

queue (default=default) Unique task queue name.

type (default=local) Task queue type ('local' or 'redis') or full class path to a custom type.

unix_socket_path Redis server unix socket path (use instead of host and port).

Other supported Redis-queue options are: host, port, db and password.

Supported <taskqueue-server />-settings are:

name (default=default) Consumer name, preferably instance name. Consumer is name used by Redis-queues for reserving messages from queue to achieve quaranteed delivery.

queue (default=default) Queue name for this consumer (consuming server). There must be a <taskqueue/> with matching queue-value registered.

concurrent_limit (default=1) Maximum concurrent task limit for this consumer. It's recommend to set this smaller than zserver-thread-count. Leaving this to the default (1) should give the best results in terms of minimal ConflictErrors.

retry_max_count (default=10) Maximum ZPublisher retry count for requests dispatched by this consumer.

.. note:: Once this limit has been exceeded by ZPublisher, the conflicting
   task is permanently trashed. (An alternative behavior is possible
   by implementing a custom queue class.)

Advanced usage

taskqueue.add accepts the following arguments (with default value):

url (required, no default) Target path representing the task to be called.

method (optional, default=GET) HTTP-method for the call. Must be either GET or POST.

params (optional, default=None) A dictionary of optional task arguments, which are appended as query string after the given url.

headers (optional, default=None) A dictionary of optional HTTP-headers to be appended to (or used to replace) the headers copied from the active request.

payload (optional, default=current) An optional payload for POST-request. Payload from the active request will be copied by default. Copying the active payload can be prevented with payload=None.

queue (optional, default=alphabetically-first-registered-queue) An optional queue name, when more than one queue is registered.

How Redis queueing works

1. taskqueue.add prepares a message, which will be pushed (lpush) into key collective.taskqueue.%(queue)s (where %(queue)s is the name of the queue) at the end of the transaction. If Redis connection is down during the transaction vote, the whole transaction is aborted.

2. <taskqueue-server /> reads each message (rpoplpush) from queue so that they will remain in key collective.taskqueue.%(queue)s.%(name)s (where %(name)s is the name of the <taskqueue-server/>) until each asynchronous processing request has returned a HTTP response.

3. On startup, and when all known messages have been processed, <taskqueue-server/> purges collective.taskqueue.%(queue)s.%(name)s into collective.taskqueue.%(queue)s (with rpoplpush) and those tasks are processed again. (E.g. if Plone was forced to restart in middle of task handling request.)

Redis integration uses PubSub to notify itself about new messages in queue (and get as instant handling as possible in terms of Plone's asyncore-loop).