Watcher Planner Selector

This component is responsible for selecting an appropriate Planner based
on config options passed by user for distinct Strategy.

Change-Id: I8dd84cef4b9eb25a6cdf448d55a2dd8da3f10f9c
Implements: bp watcher-planner-selector

specs/rocky/approved/watcher-planner-selector.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
+
+========================
+Watcher Planner Selector
+========================
+
+https://blueprints.launchpad.net/watcher/+spec/watcher-planner-selector
+
+`Watcher Planner`_ takes the set of `Actions`_ generated by a `Strategy`_ and
+builds the design of a workflow which defines how-to schedule in time those
+different Actions and for each Action what are the prerequisite conditions.
+It is only possible to choose only one (default) `Planner`_ for all strategies
+now.
+
+Custom Planners could be added to Watcher, thereby users can adapt Watcher
+workflow to their needs. As one of the ways for customization, the option
+could be added to set concrete Planner for specific Strategy that takes
+features of users' workflow into account.
+
+
+Problem description
+===================
+
+Currently there is no opportunity to set unique Planner to specific Strategy.
+User can set default Planner in config file that would be used alongside every
+Audit.
+
+If using different Planner is the case, user should change default
+value to desired one. By doing this we restrict ourselves to use this new
+Planner with all of the Strategies, what is not desired because some of
+`Solutions`_ generated by Strategies could be not compatible with the schedule
+made by this Planner.
+
+Use Cases
+----------
+
+* As an Strategy developer, I want to choose what Planner would be applied to the
+  Solution generated by my Strategy.
+
+* As an Strategy developer, I expect that if I provide no Planner to my Strategy, it
+  will use default Planner that is set in base Strategy..
+
+
+Proposed change
+===============
+
+We propose to add property to base Strategy class, say 'planner', and set
+default value to this option in concrete Strategies where appropriate (for
+example, 'workload_stabilization' Planner as default for Strategy with the
+same name).
+
+Planner should be load in AuditHandler. Currently it's done with the help of
+PlannerManager, that loads sole planner for every Strategy which set by
+CONF.watcher_planner.planner value. Definitely, PlannerManager do only
+loading job, so it's better to load Planner without this intermediate layer.
+Thus we propose to exchange AuditHandler._planner_manager with
+DefaultPlannerLoader. As a result, PlannerManager could be removed from
+Watcher.
+
+Therefore, AuditHandler now should pass desired Planner name to Loader.
+We propose to get this name from property of the Strategy used in current
+Audit. In this case we ought to get Strategy object associated with current
+Audit.
+
+For this propose, we need to be able to get Strategy prior to
+AuditHandler._planner_loader.load() method call. It could be be done with the
+help of StrategyContext class that loads Strategy based on names of Goal and
+Audit. The problem is that this loading is done during do_execute_strategy()
+what's a bad design. We propose to move *selecting* of strategy to distinct
+method, say 'select_strategy':
+
+::
+
+    def select_strategy(audit, request_context):
+        osc = clients.OpenStackClients()
+        goal = objects.Goal.get_by_id(request_context, audit.goal_id)
+
+        strategy_name = None
+        if audit.strategy_id:
+            strategy = objects.Strategy.get_by_id(
+                request_context, audit.strategy_id)
+            strategy_name = strategy.name
+
+        strategy_selector = default.DefaultStrategySelector(
+            goal_name=goal.name,
+            strategy_name=strategy_name,
+            osc=osc)
+        return strategy_selector.select()
+
+Now we could call this method not only in do_execute_strategy() to pick
+strategy only for execution, but also call it in AuditHandler.
+
+Another benefit to do it in this manner is that if Audit was created without
+specified Strategy, StrategyContext will take care to retrieve default
+Strategy for this Goal.
+
+By design, whole AuditHandler class passes current Audit as an argument of
+many of its methods. It would be convenient to have current Audit as a field of
+AuditHandler class. Also, it is good design decision, because it's true OOP
+paradigm. To sum up, we propose to remove all these arguments and replace
+them with class field.
+
+As a result, we can get Planner for current Strategy without adding new
+argument to AuditHandler.planner() or storing name of Strategy in additional
+field.
+
+Alternatives
+------------
+
+Alternatively, we could pass Planner name as argument during creation of
+AuditTemplate.
+
+As an alternative to changing signatures of AuditHandler methods we could add a
+new field only for Strategy name.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+Signature of PlannerManager.load() method changes.
+AuditHandler methods change their signatures, 'audit' argument would be
+replaced with same class field.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  Egor Panfilov <erakli>
+
+Work Items
+----------
+
+* Refactor DefaultStrategyContext.do_execute_strategy() and add
+  select_strategy() method
+* Add 'planner' property to BaseStrategy
+* Replace PlannerManager with PlannerLoader in AuditHandler
+* Rewrite AuditHandler methods so that 'audit' argument would be replaced
+  with class' field
+
+
+Dependencies
+============
+
+https://blueprints.launchpad.net/watcher/+spec/planner-storage-action-plan
+
+
+Testing
+=======
+
+Tests with different Planners assigned to different Strategies should be
+implemented.
+
+
+Documentation Impact
+====================
+
+Documentation should be updated to explain how to set 'planner' option.
+
+
+References
+==========
+
+None
+
+
+History
+=======
+
+None
+
+
+.. _Actions: https://docs.openstack.org/watcher/latest/glossary.html#action
+.. _Strategy: https://docs.openstack.org/watcher/latest/glossary.html#strategy
+.. _Solutions: https://docs.openstack.org/watcher/latest/glossary.html#solution
+.. _Planner: https://docs.openstack.org/watcher/latest/glossary.html#watcher-planner
+.. _Watcher Planner: https://docs.openstack.org/watcher/latest/glossary.html#watcher-planner