.. _issue: https://github.com/mosquito/aio-pika/issues
.. _pull request: https://github.com/mosquito/aio-pika/compare
.. _aio-pika: https://github.com/mosquito/aio-pika
.. _official tutorial: https://www.rabbitmq.com/tutorials/tutorial-two-python.html
.. _work-queues:

Work Queues
===========

.. warning::

   This is a beta version of the port from `official tutorial`_. Please when you found an
   error create `issue`_ or `pull request`_ for me.

   This implementation is a part of official tutorial.
   Since version 1.7.0 `aio-pika`_ has :ref:`patterns submodule <patterns-worker>`.

   You might use :class:`aio_pika.patterns.Master` for real projects.

.. note::
   Using the `aio-pika`_ async Python client

.. note::

   **Prerequisites**

   This tutorial assumes RabbitMQ is installed_ and running on localhost on standard port (`5672`).
   In case you use a different host, port or credentials, connections settings would require adjusting.

   .. _installed: https://www.rabbitmq.com/download.html

   **Where to get help**

   If you're having trouble going through this tutorial you can `contact us`_ through the mailing list.

   .. _contact us: https://groups.google.com/forum/#!forum/rabbitmq-users

.. image:: /_static/tutorial/python-two.svg
   :align: center

In the :ref:`first tutorial <introduction>` we wrote programs to send and receive messages
from a named queue. In this one we'll create a Work Queue that will be used to distribute
time-consuming tasks among multiple workers.

The main idea behind Work Queues (aka: *Task Queues*) is to avoid doing a resource-intensive
task immediately and having to wait for it to complete. Instead we schedule the task to be
done later. We encapsulate a task as a message and send it to the queue. A worker process
running in the background will pop the tasks and eventually execute the job. When you
run many workers the tasks will be shared between them.

This concept is especially useful in web applications where it's impossible to handle a
complex task during a short HTTP request window.

Preparation
+++++++++++

In the previous part of this tutorial we sent a message containing `"Hello World!"`.
Now we'll be sending strings that stand for complex tasks. We don't have a real-world
task, like images to be resized or pdf files to be rendered, so let's fake it by just
pretending we're busy - by using the `asyncio.sleep()` function. We'll take the number of dots
in the string as its complexity; every dot will account for one second of "work".
For example, a fake task described by Hello... will take three seconds.

We will slightly modify the send.py code from our previous example, to allow arbitrary
messages to be sent from the command line. This program will schedule tasks to our work
queue, so let's name it *new_task.py*:

.. literalinclude:: examples/2-work-queues/new_task_initial.py
   :language: python
   :pyobject: main

Our old receive.py script also requires some changes: it needs to fake a second of work
for every dot in the message body. It will pop messages from the queue and perform the task,
so let's call it *tasks_worker.py*:

.. code-block:: python

    async def on_message(message: IncomingMessage):
        print(" [x] Received %r" % message.body)
        await asyncio.sleep(message.body.count(b'.'))
        print(" [x] Done")


Round-robin dispatching
+++++++++++++++++++++++

One of the advantages of using a Task Queue is the ability to easily parallelise work.
If we are building up a backlog of work, we can just add more workers and that way, scale easily.

First, let's try to run two *tasks_worker.py* scripts at the same time. They will
both get messages from the queue, but how exactly? Let's see.

You need three consoles open. Two will run the ``tasks_worker.py`` script.
These consoles will be our two consumers - C1 and C2.

::

    shell1$ python tasks_worker.py
    [*] Waiting for messages. To exit press CTRL+C

::

    shell2$ python tasks_worker.py
    [*] Waiting for messages. To exit press CTRL+C

In the third one we'll publish new tasks. Once you've started the consumers you can publish a few messages::

    shell3$ python new_task.py First message.
    shell3$ python new_task.py Second message..
    shell3$ python new_task.py Third message...
    shell3$ python new_task.py Fourth message....
    shell3$ python new_task.py Fifth message.....

Let's see what is delivered to our workers::

    shell1$ python tasks_worker.py
     [*] Waiting for messages. To exit press CTRL+C
     [x] Received 'First message.'
     [x] Received 'Third message...'
     [x] Received 'Fifth message.....'

::

    shell2$ python tasks_worker.py
     [*] Waiting for messages. To exit press CTRL+C
     [x] Received 'Second message..'
     [x] Received 'Fourth message....'

By default, RabbitMQ will send each message to the next consumer, in sequence.
On average every consumer will get the same number of messages. This way
of distributing messages is called round-robin. Try this out with three or more workers.

Message acknowledgment
++++++++++++++++++++++

Doing a task can take a few seconds. You may wonder what happens if one of the consumers starts a
long task and dies with it only partly done. With our current code once RabbitMQ delivers message
to the customer it immediately removes it from memory. In this case, if you kill a worker we will
lose the message it was just processing. We'll also lose all the messages that were dispatched to
this particular worker but were not yet handled.

But we don't want to lose any tasks. If a worker dies, we'd like the task to be delivered to another worker.

In order to make sure a message is never lost, RabbitMQ supports message acknowledgments.
An ack(nowledgement) is sent back from the consumer to tell RabbitMQ that a particular message
had been received, processed and that RabbitMQ is free to delete it.

If a consumer dies (its channel is closed, connection is closed, or TCP connection is lost)
without sending an ack, RabbitMQ will understand that a message wasn't processed fully and
will re-queue it. If there are other consumers online at the same time, it will then quickly
redeliver it to another consumer. That way you can be sure that no message is lost, even if
the workers occasionally die.

There aren't any message timeouts; RabbitMQ will redeliver the message when the consumer dies.
It's fine even if processing a message takes a very, very long time.

Message acknowledgments are turned on by default. In previous examples we explicitly turned
them off via the `no_ack=True` flag. It's time to remove this flag and send a proper acknowledgment
from the worker, once we're done with a task.

.. code-block:: python

    async def on_message(message: IncomingMessage):
        print(" [x] Received %r" % message.body)
        await asyncio.sleep(message.body.count(b'.'))
        print(" [x] Done")
        await message.ack()

.. code-block:: python

    # Declaring queue
        queue = await channel.declare_queue("hello")
    # Start listening the queue with name 'hello'
        await queue.consume(on_message)

or using special context processor:

.. literalinclude:: examples/2-work-queues/tasks_worker.py
   :language: python
   :lines: 7-11


If context processor will catch an exception, the message will be returned to the queue.

Using this code we can be sure that even if you kill a worker using CTRL+C while
it was processing a message, nothing will be lost. Soon after the worker dies all
unacknowledged messages will be redelivered.

.. note::
    **Forgotten acknowledgment**

    It's a common mistake to miss the ack. It's an easy error, but the
    consequences are serious. Messages will be redelivered when your client quits
    (which may look like random redelivery), but RabbitMQ will eat more and more
    memory as it won't be able to release any unacked messages.

    In order to debug this kind of mistake you can use rabbitmqctl to print the
    messages_unacknowledged field::

        $ sudo rabbitmqctl list_queues name messages_ready messages_unacknowledged
        Listing queues ...
        hello    0       0
        ...done.


Message durability
++++++++++++++++++

We have learned how to make sure that even if the consumer dies, the task isn't lost.
But our tasks will still be lost if RabbitMQ server stops.

When RabbitMQ quits or crashes it will forget the queues and messages unless you tell it not to.
Two things are required to make sure that messages aren't lost: we need to mark both the queue and messages as durable.

First, we need to make sure that RabbitMQ will never lose our queue. In order to do so,
we need to declare it as *durable*:

.. code-block:: python

   queue = await channel.declare_queue("hello", durable=True)


Although this command is correct by itself, it won't work in our setup.
That's because we've already defined a queue called hello which is not durable.
RabbitMQ doesn't allow you to redefine an existing queue with different parameters
and will return an error to any program that tries to do that.
But there is a quick workaround - let's declare a queue with different name, for example task_queue:

.. literalinclude:: examples/2-work-queues/tasks_worker.py
   :language: python
   :lines: 23-27

This queue_declare change needs to be applied to both the producer and consumer code.

At that point we're sure that the task_queue queue won't be lost even if RabbitMQ restarts.
Now we need to mark our messages as persistent - by supplying a delivery_mode
property with a value `PERSISTENT` (see enum :class:`aio_pika.DeliveryMode`).

.. literalinclude:: examples/2-work-queues/new_task.py
   :language: python
   :pyobject: main

.. note::

   **Note on message persistence**

   Marking messages as persistent doesn't fully guarantee that a message won't be lost.
   Although it tells RabbitMQ to save the message to disk, there is still a short time
   window when RabbitMQ has accepted a message and hasn't saved it yet. Also,
   RabbitMQ doesn't do fsync(2) for every message -- it may be just saved to cache and
   not really written to the disk. The persistence guarantees aren't strong, but
   it's more than enough for our simple task queue. If you need a stronger guarantee
   then you can use `publisher confirms`_.

   `aio-pika`_ supports `publisher confirms`_ out of the box.

   .. _publisher confirms: https://www.rabbitmq.com/confirms.html


Fair dispatch
+++++++++++++

You might have noticed that the dispatching still doesn't work exactly as we want.
For example in a situation with two workers, when all odd messages are heavy and
even messages are light, one worker will be constantly busy and the other one will
do hardly any work. Well, RabbitMQ doesn't know anything about that and will still
dispatch messages evenly.

This happens because RabbitMQ just dispatches a message when the message enters
the queue. It doesn't look at the number of unacknowledged messages for a consumer.
It just blindly dispatches every n-th message to the n-th consumer.


.. image:: /_static/tutorial/prefetch-count.svg
   :align: center


In order to defeat that we can use the basic.qos method with the `prefetch_count=1` setting.
This tells RabbitMQ not to give more than one message to a worker at a time. Or,
in other words, don't dispatch a new message to a worker until it has processed and
acknowledged the previous one. Instead, it will dispatch it to the next worker that is not still busy.

.. literalinclude:: examples/2-work-queues/tasks_worker.py
   :language: python
   :lines: 18-21


.. note::
    **Note about queue size**

    If all the workers are busy, your queue can fill up. You will want to keep an eye
    on that, and maybe add more workers, or have some other strategy.


Putting it all together
+++++++++++++++++++++++

Final code of our :download:`new_task.py <examples/2-work-queues/new_task.py>` script:

.. literalinclude:: examples/2-work-queues/new_task.py
   :language: python

And our :download:`tasks_worker.py <examples/2-work-queues/tasks_worker.py>`:

.. literalinclude:: examples/2-work-queues/tasks_worker.py
   :language: python

Using message acknowledgments and prefetch_count you can set up a work queue. The durability
options let the tasks survive even if RabbitMQ is restarted.

Now we can move on to :ref:`tutorial 3 <publish-subscribe>` and learn how to deliver the
same message to many consumers.


.. note::

    This material was adopted from `official tutorial`_ on **rabbitmq.org**.
