watcher/doc/source/dev/plugin/strategy-plugin.rst

..
      Except where otherwise noted, this document is licensed under Creative
      Commons Attribution 3.0 License.  You can view the license at:

          https://creativecommons.org/licenses/by/3.0/

.. _implement_strategy_plugin:

=================================
Build a new optimization strategy
=================================

Watcher Decision Engine has an external :ref:`strategy <strategy_definition>`
plugin interface which gives anyone the ability to integrate an external
strategy in order to make use of placement algorithms.

This section gives some guidelines on how to implement and integrate custom
strategies with Watcher.


Pre-requisites
==============

Before using any strategy, you should make sure you have your Telemetry service
configured so that it would provide you all the metrics you need to be able to
use your strategy.


Creating a new plugin
=====================

First of all you have to:

- Extend :py:class:`~.BaseStrategy`
- Implement its :py:meth:`~.BaseStrategy.execute` method

Here is an example showing how you can write a plugin called ``DummyStrategy``:

.. code-block:: python

    import uuid

    class DummyStrategy(BaseStrategy):

        DEFAULT_NAME = "dummy"
        DEFAULT_DESCRIPTION = "Dummy Strategy"

        def __init__(self, name=DEFAULT_NAME, description=DEFAULT_DESCRIPTION):
            super(DummyStrategy, self).__init__(name, description)

        def execute(self, model):
            migration_type = 'live'
            src_hypervisor = 'compute-host-1'
            dst_hypervisor = 'compute-host-2'
            instance_id = uuid.uuid4()
            parameters = {'migration_type': migration_type,
                          'src_hypervisor': src_hypervisor,
                          'dst_hypervisor': dst_hypervisor}
            self.solution.add_action(action_type="migration",
                                     resource_id=instance_id,
                                     input_parameters=parameters)
            # Do some more stuff here ...
            return self.solution

As you can see in the above example, the :py:meth:`~.BaseStrategy.execute`
method returns a :py:class:`~.BaseSolution` instance as required. This solution
is what wraps the abstract set of actions the strategy recommends to you. This
solution is then processed by a :ref:`planner <planner_definition>` to produce
an action plan which shall contain the sequenced flow of actions to be
executed by the :ref:`Watcher Applier <watcher_applier_definition>`.

Please note that your strategy class will be instantiated without any
parameter. Therefore, you should make sure not to make any of them required in
your ``__init__`` method.


Abstract Plugin Class
=====================

Here below is the abstract :py:class:`~.BaseStrategy` class that every single
strategy should implement:

.. autoclass:: watcher.decision_engine.strategy.strategies.base.BaseStrategy
    :members:
    :noindex:


Add a new entry point
=====================

In order for the Watcher Decision Engine to load your new strategy, the
strategy must be registered as a named entry point under the
``watcher_strategies`` entry point of your ``setup.py`` file. If you are using
pbr_, this entry point should be placed in your ``setup.cfg`` file.

The name you give to your entry point has to be unique.

Here below is how you would proceed to register ``DummyStrategy`` using pbr_:

.. code-block:: ini

    [entry_points]
    watcher_strategies =
        dummy = thirdparty.dummy:DummyStrategy


To get a better understanding on how to implement a more advanced strategy,
have a look at the :py:class:`~.BasicConsolidation` class.

.. _pbr: http://docs.openstack.org/developer/pbr/

Using strategy plugins
======================

The Watcher Decision Engine service will automatically discover any installed
plugins when it is restarted. If a Python package containing a custom plugin is
installed within the same environment as Watcher, Watcher will automatically
make that plugin available for use.

At this point, Watcher will use your new strategy if you reference it in the
``goals`` under the ``[watcher_goals]`` section of your ``watcher.conf``
configuration file. For example, if you want to use a ``dummy`` strategy you
just installed, you would have to associate it to a goal like this:

.. code-block:: ini

    [watcher_goals]
    goals = BALANCE_LOAD:basic,MINIMIZE_ENERGY_CONSUMPTION:dummy


You should take care when installing strategy plugins. By their very nature,
there are no guarantees that utilizing them as is will be supported, as
they may require a set of metrics which is not yet available within the
Telemetry service. In such a case, please do make sure that you first
check/configure the latter so your new strategy can be fully functional.

Querying metrics
----------------

A large set of metrics, generated by OpenStack modules, can be used in your
strategy implementation. To collect these metrics, Watcher provides a
`Helper`_ to the Ceilometer API, which makes this API reusable and easier
to used.

If you want to use your own metrics database backend, please refer to the
`Ceilometer developer guide`_. Indeed, Ceilometer's pluggable model allows
for various types of backends.  A list of the available backends is located
here_. The Ceilosca project is a good example of how to create your own
pluggable backend.


Finally, if your strategy requires new metrics not covered by Ceilometer, you
can add them through a Ceilometer `plugin`_.

.. _`Helper`: https://github.com/openstack/watcher/blob/master/watcher/metrics_engine/cluster_history/ceilometer.py#L31
.. _`Ceilometer developer guide`: http://docs.openstack.org/developer/ceilometer/architecture.html#storing-the-data
.. _`here`: http://docs.openstack.org/developer/ceilometer/install/dbreco.html#choosing-a-database-backend
.. _`plugin`: http://docs.openstack.org/developer/ceilometer/plugins.html
.. _`Ceilosca`: https://github.com/openstack/monasca-ceilometer/blob/master/ceilosca/ceilometer/storage/impl_monasca.py


Read usage metrics using the Python binding
-------------------------------------------

You can find the information about the Ceilometer Python binding on the
OpenStack `ceilometer client python API documentation
<http://docs.openstack.org/developer/python-ceilometerclient/api.html>`_

To facilitate the process, Watcher provides the ``osc`` attribute to every
strategy which includes clients to major OpenStack services, including
Ceilometer. So to access it within your strategy, you can do the following:

.. code-block:: py

    # Within your strategy "execute()"
    cclient = self.osc.ceilometer
    # TODO: Do something here

Using that you can now query the values for that specific metric:

.. code-block:: py

    query = None  # e.g. [{'field': 'foo', 'op': 'le', 'value': 34},]
    value_cpu = cclient.samples.list(
        meter_name='cpu_util',
        limit=10, q=query)


Read usage metrics using the Watcher Cluster History Helper
-----------------------------------------------------------

Here below is the abstract ``BaseClusterHistory`` class of the Helper.

.. autoclass:: watcher.metrics_engine.cluster_history.api.BaseClusterHistory
    :members:
    :noindex:


The following code snippet shows how to create a Cluster History class:

.. code-block:: py

    from watcher.metrics_engine.cluster_history import ceilometer as ceil

    query_history  = ceil.CeilometerClusterHistory()

Using that you can now query the values for that specific metric:

.. code-block:: py

    query_history.statistic_aggregation(resource_id=hypervisor.uuid,
                                  meter_name='compute.node.cpu.percent',
                                  period="7200",
                                  aggregate='avg'
                                  )