Spec: Optimize driver loading in v2 API

See specs/ussuri/optimize_driver_loading_in_v2_api.rst for details.

Story: 2006654
Task 36904

Change-Id: I6d88b6c4bc674fb5dab145e05b6b512c443f5e56

specs/ussuri/optimize_driver_loading_in_v2_api.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=================================
+Optimize driver loading in v2 API
+=================================
+
+https://storyboard.openstack.org/#!/story/2006654
+
+Problem Description
+===================
+
+Currently, required drivers are loaded in the ``__init__`` method of v2 API
+resources. However, ``flask_restful`` instanciates resources on
+`each request`_, meaning that drivers are also loaded on each request. This
+leads to sub-optimal performance.
+
+.. _`each request`: https://github.com/flask-restful/flask-restful/blob/20b220e9c4690a7a25474b6521ead342aaed0235/flask_restful/__init__.py#L465
+
+Proposed Change
+===============
+
+Implement two new classmethods in ``cloudkitty.api.v2.base.BaseResource``:
+
+* ``reload``: Reloads all required drivers.
+
+* ``load``: Loads all required drivers by calling ``reload``. In case the
+  drivers are already loaded, does nothing
+
+In order to be able to persist drivers between class instantiations, they will
+be set as class attributes.
+
+Alternatives
+------------
+
+None.
+
+Data model impact
+-----------------
+
+None.
+
+REST API impact
+---------------
+
+None.
+
+Security impact
+---------------
+
+None.
+
+Notifications Impact
+--------------------
+
+None.
+
+Other end user impact
+---------------------
+
+The average response time for v2 API endpoints will be reduced.
+
+Performance Impact
+------------------
+
+Loading the drivers only once will result in a performance gain.
+
+Other deployer impact
+---------------------
+
+Log messages that appear on driver instantiation will appear only once rather
+than on each request.
+
+Developer impact
+----------------
+
+Driver loading will have to be done in the ``reload`` method rather than in
+``__init__``.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  <lukapeschke/peschk_l>
+
+Work Items
+----------
+
+* Implement the ``reload`` and ``load`` methods.
+
+Dependencies
+============
+
+None.
+
+Testing
+=======
+
+Existing unit tests for the v2 API along with planned tempest tests will test
+this.
+
+Documentation Impact
+====================
+
+The developer documentation for the v2 API will be updated to include
+information about the ``load`` and ``reload`` methods
+
+References
+==========
+
+None.