deb-python-taskflow/doc/source/atoms.rst

------------------------
Atoms, tasks and retries
------------------------

Atom
====

An :py:class:`atom <taskflow.atom.Atom>` is the smallest unit in TaskFlow which
acts as the base for other classes (its naming was inspired from the
similarities between this type and `atoms`_ in the physical world). Atoms
have a name and may have a version. An atom is expected to name desired input
values (requirements) and name outputs (provided values).

.. note::

    For more details about atom inputs and outputs please visit
    :doc:`arguments and results <arguments_and_results>`.

.. automodule:: taskflow.atom

.. _atoms: http://en.wikipedia.org/wiki/Atom

Task
=====

A :py:class:`task <taskflow.task.BaseTask>` (derived from an atom) is the
smallest possible unit of work that can have an execute & rollback sequence
associated with it. These task objects all derive
from :py:class:`~taskflow.task.BaseTask` which defines what a task must
provide in terms of properties and methods.

Currently the following *provided* types of task subclasses are:

* :py:class:`~taskflow.task.Task`: useful for inheriting from and creating your
  own subclasses.
* :py:class:`~taskflow.task.FunctorTask`: useful for wrapping existing
  functions into task objects.

.. note::

    :py:class:`~taskflow.task.FunctorTask` task types can not currently be used
    with the :doc:`worker based engine <workers>` due to the fact that
    arbitrary functions can not be guaranteed to be correctly
    located (especially if they are lambda or anonymous functions) on the
    worker nodes.

Retry
=====

A :py:class:`retry <taskflow.retry.Retry>` (derived from an atom) is a special
unit that handles errors, controls flow execution and can (for example) retry
other atoms with other parameters if needed. When an associated atom
fails, these retry units are *consulted* to determine what the resolution
method should be. The goal is that with this *consultation* the retry atom
will suggest a method for getting around the failure (perhaps by retrying,
reverting a single item, or reverting everything contained in the retries
associated scope).

Currently derivatives of the :py:class:`retry <taskflow.retry.Retry>` base
class must provide a ``on_failure`` method to determine how a failure should
be handled.

The current enumeration set that can be returned from this method is:

* ``RETRY`` - retries the surrounding subflow (a retry object is associated
  with a flow, which is typically converted into a graph hierarchy at
  compilation time) again.

* ``REVERT`` - reverts only the surrounding subflow but *consult* the
  parent atom before doing this to determine if the parent retry object
  provides a different reconciliation strategy (retry atoms can be nested, this
  is possible since flows themselves can be nested).

* ``REVERT_ALL`` - completely reverts a whole flow.

To aid in the reconciliation process the
:py:class:`retry <taskflow.retry.Retry>` base class also mandates ``execute``
and ``revert`` methods (although subclasses are allowed to define these methods
as no-ops) that can be used by a retry atom to interact with the runtime
execution model (for example, to track the number of times it has been
called which is useful for the :py:class:`~taskflow.retry.ForEach` retry
subclass).

To avoid recreating common retry patterns the following provided retry
subclasses are provided:

* :py:class:`~taskflow.retry.AlwaysRevert`: Always reverts subflow.
* :py:class:`~taskflow.retry.AlwaysRevertAll`: Always reverts the whole flow.
* :py:class:`~taskflow.retry.Times`: Retries subflow given number of times.
* :py:class:`~taskflow.retry.ForEach`: Allows for providing different values
  to subflow atoms each time a failure occurs (making it possibly to resolve
  the failure by altering subflow atoms inputs).
* :py:class:`~taskflow.retry.ParameterizedForEach`: Same as
  :py:class:`~taskflow.retry.ForEach` but extracts values from storage
  instead of the :py:class:`~taskflow.retry.ForEach` constructor.

Examples
--------

.. testsetup::

    import taskflow
    from taskflow import task
    from taskflow import retry
    from taskflow.patterns import linear_flow
    from taskflow import engines

.. doctest::

    >>> class EchoTask(task.Task):
    ...     def execute(self, *args, **kwargs):
    ...         print(self.name)
    ...         print(args)
    ...         print(kwargs)
    ...
    >>> flow = linear_flow.Flow('f1').add(
    ...     EchoTask('t1'),
    ...     linear_flow.Flow('f2', retry=retry.ForEach(values=['a', 'b', 'c'], name='r1', provides='value')).add(
    ...         EchoTask('t2'),
    ...         EchoTask('t3', requires='value')),
    ...     EchoTask('t4'))

In this example the flow ``f2`` has a retry controller ``r1``, that is an
instance of the default retry controller :py:class:`~taskflow.retry.ForEach`,
it accepts a collection of values and iterates over this collection when
each failure occurs. On each run :py:class:`~taskflow.retry.ForEach` retry
returns the next value from the collection and stops retrying a subflow if
there are no more values left in the collection. For example if tasks ``t2`` or
``t3`` fail, then the flow ``f2`` will be reverted and retry ``r1`` will retry
it with the next value from the given collection ``['a', 'b', 'c']``. But if
the task ``t1`` or the task ``t4`` fails, ``r1`` won't retry a flow, because
tasks ``t1`` and ``t4`` are in the flow ``f1`` and don't depend on
retry ``r1`` (so they will not *consult* ``r1`` on failure).

.. doctest::

    >>> class SendMessage(task.Task):
    ...     def execute(self, message):
    ...         print("Sending message: %s" % message)
    ...
    >>> flow = linear_flow.Flow('send_message', retry=retry.Times(5)).add(
    ...      SendMessage('sender'))

In this example the ``send_message`` flow will try to execute the
``SendMessage`` five times when it fails. When it fails for the sixth time (if
it does) the task will be asked to ``REVERT`` (in this example task reverting
does not cause anything to happen but in other use cases it could).

.. doctest::

    >>> class ConnectToServer(task.Task):
    ...     def execute(self, ip):
    ...         print("Connecting to %s" % ip)
    ...
    >>> server_ips = ['192.168.1.1', '192.168.1.2', '192.168.1.3' ]
    >>> flow = linear_flow.Flow('send_message',
    ...                         retry=retry.ParameterizedForEach(rebind={'values': 'server_ips'},
    ...                                                          provides='ip')).add(
    ...     ConnectToServer(requires=['ip']))

In this example the flow tries to connect a server using a list (a tuple
can also be used) of possible IP addresses. Each time the retry will return
one IP from the list. In case of a failure it will return the next one until
it reaches the last one, then the flow will be reverted.

Interfaces
==========

.. automodule:: taskflow.task
.. automodule:: taskflow.retry

Hierarchy
=========

.. inheritance-diagram::
    taskflow.atom
    taskflow.task
    taskflow.retry
    :parts: 1