Support filter backend based on operation type

This spec proposes to add attribute 'operation'
to RequestSpec when asking scheduler plugins
to perform filtering and weighing.

Partial-Implements: bp support-mark-pool-sold-out
Change-Id: I694ad2f8fab76b39ca1d9f4551102130faf5fc44

specs/rocky/support-filter-backend-on-operation-type.rst

@@ -0,0 +1,173 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==============================================
+Support filter backend based on operation type
+==============================================
+
+https://blueprints.launchpad.net/cinder/+spec/support-mark-pool-sold-out
+
+This blueprint proposes to support filter backend based on operation type.
+
+Problem description
+===================
+
+Now our filter and weigher plugins' algorithm are all designed based on
+two factors, backend information and user input. But there could be
+another factor that may also affect the scheduler result, operation
+type. For instance, if administrators want to disable some of the
+backend pool only for creating new volume operation, it's impossible
+based on our current logic.
+
+Use Cases
+=========
+
+We have discussed the concept of **sold out** during Rocky PTG `[1]`_ and
+that word is more related to administrative operation, when most of an
+specific pool's capacity has been consumed, administrators need the
+functionality to mark the backend pool sold out, that is to say, it
+will become unavailable for allocating new volumes, but end users still
+can perform any operations on their existing resources (extend volume,
+create snapshot).
+
+Adding this feature in cinder will help cloud vendors write their
+own plugins which can filter backend based on the operation type as well
+as backend information.
+
+Proposed change
+===============
+
+New attribute ``operation`` will be added into ``RequestSpec`` object
+and plugins can filter backend based on backend states as well as
+request type:
+
+.. code-block:: python
+
+        def backend_passes(self, backend_state, filter_properties):
+            #get request type from filter_properties
+            spec = filter_properties.get('request_spec', {})
+            request_type = spec.get('operation', None)
+            if request_type in ['create_volume']:
+                #check backend state when request is for creating new volume
+                return backend_state.free_capacity_gb > MIN_REQUIRED_GB
+            if request_type in ['extend_volume', 'retype_volume']
+                #perform other validation when request is ......
+
+**NOTE**: This spec doesn't propose to update any plugin's logic, the code
+above only intends to demonstrate how the ``operation`` can be used.
+
+Based on our current features, the value of ``operation`` can be one of
+these below:
+
+1. create_group
+2. manage_existing
+3. extend_volume
+4. create_volume
+5. create_snapshot
+6. migrate_volume
+7. retype_volume
+8. manage_existing_snapshot
+
+And we will inject scheduler manager's corresponding methods in order to
+append this info to ``RequestSpec`` automatically:
+
+.. code-block:: python
+
+        @append_operation
+        def extend_volume(self, context, volume, new_size, reservations,
+                          request_spec=None, filter_properties=None):
+            #other existing codes
+
+Alternatives
+------------
+
+The alternative is to implement a custom scheduler manager and configure it
+in ``scheduler_manager`` and then use either a custom filter or the JSON
+filter.
+
+Data model impact
+-----------------
+
+``RequestSpec`` will be updated to store this new attribute.
+
+REST API impact
+---------------
+
+None
+
+Cinder-client impact
+--------------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+Developers can develop new scheduler plugins based
+on backend state as well as request type.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  tommylikehu(tommylikehu@gmail.com)
+
+Work Items
+----------
+
+* Add ``operation`` to those actions which will be scheduled
+  to scheduler to perform filtering and weighing logic.
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+* Add unit tests to cover this change.
+
+Documentation Impact
+====================
+
+* Update the developer document to mention the new attribute
+  will be delivered to scheduler plugins as well.
+
+References
+==========
+
+
+_`[1]`: https://etherpad.openstack.org/p/cinder-ptg-rocky-wednesday
+_`[2]`: https://review.openstack.org/#/c/308869/