[Spec] Spec class-based-scenarios.rst is implemented

Change-Id: If50cc5a341029b72ff92eeac41b1fc4559c6974b

doc/specs/implemented/class-based-scenarios.rst

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+
+===================================
+Class-based Scenario Implementation
+===================================
+
+Introduce scenarios implementation as classes, not methods.
+
+Problem description
+===================
+
+Current scenario implementation transforms method to class at runtime,
+so this overcomplicates the code.
+
+Method-based extensions mechanism is not a common practice in frameworks,
+so this is a bit confusing.
+
+Most Rally plugins like Context, SLA, Runner, OutputChart (except Scenario)
+are implemented as classes, not methods.
+
+Proposed change
+===============
+
+Add an ability to implement scenarios as classes, keeping full backward
+compatibility with existing code.
+
+This means that class represents single scenario which is actually implemented
+in method *Scenario.run()*.
+
+So input task can contain scenario names that does not have method part
+splitted by dot from class part.
+
+For example, here we have two scenarios, first one is in old manner
+and another is class-based:
+
+.. code-block:: json
+
+ {
+   "Dummy.dummy": [
+     {
+       "runner": {
+         "type": "serial",
+         "times": 20
+       }
+     }
+   ],
+   "another_dummy_scenario": [
+     {
+       "runner": {
+         "type": "serial",
+         "times": 20
+       }
+     }
+   ]
+ }
+
+Class AnotherDummyScenario should have method run():
+
+.. code-block:: python
+
+  from rally.task import scenario
+
+  @scenario.configure(name="another_dummy_scenario")
+  class AnotherDummyScenario(scenario.Scenario):
+
+      def run(self):
+          """Scenario implementation."""
+
+Modules *rally.task.engine* and *rally.task.processing* should be modified to
+make them working with class-based scenarios.
+
+Alternatives
+------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+
+  Alexander Maretskiy <amaretskiy@mirantis.com>
+
+
+Work Items
+----------
+
+ - Update task.engine and task.processing for class-based scenarios
+ - Transform all Dummy scenarios into class-based implementations as first
+   stage of usage class-based scenarios.
+
+Dependencies
+============
+
+None