celery/docs/getting-started/next-steps.rst

============
 Next Steps
============

The :ref:`first-steps` guide is intentionally minimal.  In this guide
we will demonstrate what Celery offers in more detail, including
how to add Celery support for your application and library.

.. contents::
    :local:


Our Project
===========

Project layout::

    proj/__init__.py
        /celery.py
        /tasks.py

:file:`proj/celery.py`
----------------------

.. literalinclude:: ../../examples/next-steps/proj/celery.py
    :language: python

In this module we created our :class:`@Celery` instance (sometimes
referred to as the *app*).  To use Celery within your project
you simply import this instance.

- The ``broker`` argument specifies the URL of the broker to use.

    See :ref:`celerytut-broker` for more information.

- The ``backend`` argument specifies the result backend to use,

    It's used to keep track of task state and results.
    While results are disabled by default we use the amqp backend here
    to demonstrate how retrieving the results work, you may want to use
    a different backend for your application, as they all have different
    strenghts and weaknesses.  If you don't need results it's best
    to disable them.  Results can also be disabled for individual tasks
    by setting the ``@task(ignore_result=True)`` option.

    See :ref:`celerytut-keeping-results` for more information.

- The ``include`` argument is a list of modules to import when
  the worker starts.  We need to add our tasks module here so
  that the worker is able to find our tasks.

:file:`proj/tasks.py`
---------------------

.. literalinclude:: ../../examples/next-steps/proj/tasks.py
    :language: python


Starting the worker
===================

The :program:`celery` program can be used to start the worker::

   $ celery worker --app=proj -l info

The :option:`--app` argument specifies the Celery app instance to use,
it must be in the form of ``module.path:celery``, where the part before the colon
is the name of the module, and the attribute name comes last.
If a package name is specified instead it will automatically
try to find a ``celery`` module in that package, and if the name
is a module it will try to find a ``celery`` attribute in that module.
This means that the following all results in the same::

    $ celery --app=proj
    $ celery --app=proj.celery:
    $ celery --app=proj.celery:celery


Subtasks
========

A :func:`~celery.subtask` wraps the signature of a single task invocation:
arguments, keyword arguments and execution options.

A subtask for the ``add`` task can be created like this::

    >>> from celery import subtask
    >>> subtask(add.name, args=(4, 4))

or you can create one from the task itself::

    >>> from proj.tasks import add
    >>> add.subtask(args=(4, 4))

It takes the same arguments as the :meth:`~@Task.apply_async` method::

    >>> add.apply_async(args, kwargs, **options)
    >>> add.subtask(args, kwargs, **options)

    >>> add.apply_async((2, 2), countdown=1)
    >>> add.subtask((2, 2), countdown=1)

And like there is a :meth:`~@Task.delay` shortcut for `apply_async`
there is an :meth:`~@Task.s` shortcut for subtask::

    >>> add.s(*args, **kwargs)

    >>> add.s(2, 2)
    proj.tasks.add(2, 2)

    >>> add.s(2, 2) == add.subtask((2, 2))
    True

You can't define options with :meth:`~@Task.s`, but a chaining
``set`` call takes care of that::

    >>> add.s(2, 2).set(countdown=1)
    proj.tasks.add(2, 2)

Partials
--------

A subtask can be applied too::

    >>> add.s(2, 2).delay()
    >>> add.s(2, 2).apply_async(countdown=1)

Specifying additional args, kwargs or options to ``apply_async``/``delay``
creates partials:

- Any arguments added will be prepended to the args in the signature::

    >>> partial = add.s(2)          # incomplete signature
    >>> partial.delay(4)            # 2 + 4
    >>> partial.apply_async((4, ))  # same

- Any keyword arguments added will be merged with the kwargs in the signature,
  with the new keyword arguments taking precedence::

    >>> s = add.s(2, 2)
    >>> s.delay(debug=True)                    # -> add(2, 2, debug=True)
    >>> s.apply_async(kwargs={"debug": True})  # same

- Any options added will be merged with the options in the signature,
  with the new options taking precedence::

    >>> s = add.subtask((2, 2), countdown=10)
    >>> s.apply_async(countdown=1)  # countdown is now 1

You can also clone subtasks to augment these::

    >>> s = add.s(2)
    proj.tasks.add(2)

    >>> s.clone(args=(4, ), kwargs={"debug": True})
    proj.tasks.add(2, 4, debug=True)

Partials are meant to be used with callbacks, any tasks linked or chord
callbacks will be applied with the result of the parent task.
Sometimes you want to specify a callback that does not take
additional arguments, and in that case you can set the subtask
to be immutable::

    >>> add.apply_async((2, 2), link=reset_buffers.subtask(immutable=True))

The ``.si()`` shortcut can also be used to create immutable subtasks::

    >>> add.apply_async((2, 2), link=reset_buffers.si())

Only the execution options can be set when a subtask is immutable,
and it's not possible to apply the subtask with partial args/kwargs.

.. note::

    In this tutorial we use the prefix operator `~` to subtasks.
    You probably shouldn't use it in your production code, but it's a handy shortcut
    when testing with the Python shell::

        >>> ~subtask

        >>> # is the same as
        >>> subtask.delay().get()

Groups
------

A group can be used to execute several tasks in parallel.

The :class:`~celery.group` function takes a list of subtasks::

    >>> from celery import group
    >>> from proj.tasks import add

    >>> group(add.s(2, 2), add.s(4, 4))
    (proj.tasks.add(2, 2), proj.tasks.add(4, 4))

If you **call** the group, the tasks will be applied
one after one in the current process, and a :class:`~@TaskSetResult`
instance is returned which can be used to keep track of the results,
or tell how many tasks are ready and so on::

    >>> g = group(add.s(2, 2), add.s(4, 4))
    >>> res = g()
    >>> res.get()
    [4, 8]

However, if you call ``apply_async`` on the group it will
send a special grouping task, so that the action of applying
the tasks happens in a worker instead of the current process::

    >>> res = g.apply_async()
    >>> res.get()
    [4, 8]

Group also supports iterators::

    >>> group(add.s(i, i) for i in xrange(100))()


A group is a subclass instance, so it can be used in combination
with other subtasks.

Map & Starmap
-------------

:class:`~celery.map` and :class:`~celery.starmap` are built-in tasks
that calls the task for every element in a sequence.

They differ from group in that

- only one task message is sent

- the operation is sequential.

For example using ``map``:

.. code-block:: python

    >>> from proj.tasks import add

    >>> ~xsum.map([range(10), range(100)])
    [45, 4950]

is the same as having a task doing:

.. code-block:: python

    @celery.task
    def temp():
        return [xsum(range(10)), xsum(range(100))]

and using ``starmap``::

    >>> ~add.starmap(zip(range(10), range(10)))
    [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

is the same as having a task doing:

.. code-block:: python

    @celery.task
    def temp():
        return [add(i, i) for i in range(10)]

Both ``map`` and ``starmap`` are subtasks, so they can be used as
other subtasks and combined in groups etc., for example
to apply the starmap after 10 seconds::

    >>> add.starmap(zip(range(10), range(10))).apply_async(countdown=10)

Chunking
--------

-- Chunking lets you divide a iterable of work into pieces,
   so that if you have one million objects, you can create
   10 tasks with hundred thousand objects each.

Some may worry that chunking your tasks results in a degradation
of parallelism, but this is rarely true for a busy cluster
and in practice since you are avoiding the overhead  of messaging
it may considerably increase performance.

To create a chunks subtask you can use :meth:`@Task.chunks`:

.. code-block:: python

    >>> add.chunks(zip(range(100), range(100)), 10)

As with :class:`~celery.group` the act of **calling**
the chunks will apply the tasks in the current process:

.. code-block:: python

    >>> from proj.tasks import add

    >>> res = add.chunks(zip(range(100), range(100)), 10)()
    >>> res.get()
    [[0, 2, 4, 6, 8, 10, 12, 14, 16, 18],
     [20, 22, 24, 26, 28, 30, 32, 34, 36, 38],
     [40, 42, 44, 46, 48, 50, 52, 54, 56, 58],
     [60, 62, 64, 66, 68, 70, 72, 74, 76, 78],
     [80, 82, 84, 86, 88, 90, 92, 94, 96, 98],
     [100, 102, 104, 106, 108, 110, 112, 114, 116, 118],
     [120, 122, 124, 126, 128, 130, 132, 134, 136, 138],
     [140, 142, 144, 146, 148, 150, 152, 154, 156, 158],
     [160, 162, 164, 166, 168, 170, 172, 174, 176, 178],
     [180, 182, 184, 186, 188, 190, 192, 194, 196, 198]]

while calling ``.apply_async`` will create a dedicated
task so that the individual tasks are applied in a worker
instead::

    >>> add.chunks(zip(range(100), range(100), 10)).apply_async()

You can also convert chunks to a group::

    >>> group = add.chunks(zip(range(100), range(100), 10)).group()

and with the group skew the countdown of each task by increments
of one::

    >>> group.skew(start=1, stop=10)()

which means that the first task will have a countdown of 1, the second
a countdown of 2 and so on.


Chaining tasks
--------------

Tasks can be linked together, which in practice means adding
a callback task::

    >>> res = add.apply_async((2, 2), link=mul.s(16))
    >>> res.get()
    4

The linked task will be applied with the result of its parent
task as the first argument, which in the above case will result
in ``mul(4, 16)`` since the result is 4.

The results will keep track of what subtasks a task applies,
and this can be accessed from the result instance::

    >>> res.children
    [<AsyncResult: 8c350acf-519d-4553-8a53-4ad3a5c5aeb4>]

    >>> res.children[0].get()
    64

The result instance also has a :meth:`~@AsyncResult.collect` method
that treats the result as a graph, enabling you to iterate over
the results::

    >>> list(res.collect())
    [(<AsyncResult: 7b720856-dc5f-4415-9134-5c89def5664e>, 4),
     (<AsyncResult: 8c350acf-519d-4553-8a53-4ad3a5c5aeb4>, 64)]

By default :meth:`~@AsyncResult.collect` will raise an
:exc:`~@IncompleteStream` exception if the graph is not fully
formed (one of the tasks has not completed yet),
but you can get an intermediate representation of the graph
too::

    >>> for result, value in res.collect(intermediate=True)):
    ....

You can link together as many tasks as you like,
and subtasks can be linked too::

    >>> s = add.s(2, 2)
    >>> s.link(mul.s(4))
    >>> s.link(log_result.s())

You can also add *error callbacks* using the ``link_error`` argument::

    >>> add.apply_async((2, 2), link_error=log_error.s())

    >>> add.subtask((2, 2), link_error=log_error.s())

Since exceptions can only be serialized when pickle is used
the error callbacks take the id of the parent task as argument instead:

.. code-block:: python

    from proj.celery import celery

    @celery.task
    def log_error(task_id):
        result = celery.AsyncResult(task_id)
        result.get(propagate=False)  # make sure result written.
        with open("/var/errors/%s" % (task_id, )) as fh:
            fh.write("--\n\n%s %s %s" % (
                task_id, result.result, result.traceback))

To make it even easier to link tasks together there is
a special subtask called :class:`~celery.chain` that lets
you chain tasks together:

.. code-block:: python

    >>> from celery import chain
    >>> from proj.tasks import add, mul

    # (4 + 4) * 8 * 10
    >>> res = chain(add.s(4, 4), mul.s(8), mul.s(10))
    proj.tasks.add(4, 4) | proj.tasks.mul(8)


Calling the chain will apply the tasks in the current process
and return the result of the last task in the chain::

    >>> res = chain(add.s(4, 4), mul.s(8), mul.s(10))
    >>> res.get()
    640

And calling ``apply_async`` will create a dedicated
task so that the act of applying the chain happens
in a worker::

    >>> res = chain(add.s(4, 4), mul.s(8), mul.s(10))
    >>> res.get()
    640

It also sets ``parent`` attributes so that you can
work your way up the chain to get intermediate results::

    >>> res.parent.get()
    64

    >>> res.parent.parent.get()
    8

    >>> res.parent.parent
    <AsyncResult: eeaad925-6778-4ad1-88c8-b2a63d017933>


Chains can also be made using the ``|`` (pipe) operator::

    >>> (add.s(2, 2) | mul.s(8) | mul.s(10)).apply_async()

Graphs
~~~~~~

In addition you can work with the result graph as a
:class:`~celery.datastructures.DependencyGraph`:

.. code-block:: python

    >>> res = chain(add.s(4, 4), mul.s(8), mul.s(10))()

    >>> res.parent.parent.graph
    285fa253-fcf8-42ef-8b95-0078897e83e6(1)
        463afec2-5ed4-4036-b22d-ba067ec64f52(0)
    872c3995-6fa0-46ca-98c2-5a19155afcf0(2)
        285fa253-fcf8-42ef-8b95-0078897e83e6(1)
            463afec2-5ed4-4036-b22d-ba067ec64f52(0)

You can even convert these graphs to *dot* format::

    >>> with open("graph.dot", "w") as fh:
    ...     res.parent.parent.graph.to_dot(fh)


and create images::

    $ dot -Tpng graph.dot -o graph.png

.. image:: ../images/graph.png


Chords
------