Merge "Finish work to remove cast to schedule_run_instance"

specs/juno/remove-cast-to-schedule-run-instance.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
+
+============================================
+Stop using the scheduler run_instance method
+============================================
+
+https://blueprints.launchpad.net/nova/+spec/remove-cast-to-schedule-run-instance
+
+Currently the scheduler is used to both pick a host for an instance to be built
+on and to handle some setup and failure conditions for booting an instance.
+The scheduler should be responsible for placement logic and everything else
+should be moved elsewhere.  This will make efforts to introduce new scheduler
+drivers or split the scheduler out of Nova easier to tackle by keeping a clean
+interface with a clear responsibility.
+
+
+Problem description
+===================
+
+The flow of execution for spawning an instance is complicated and highly
+distributed.  Some amount of distribution is necessary but there is work
+happening and decisions being made in unexpected parts of the code.  This makes
+it very difficult to look at separating the scheduler out, and means that it
+will need intimate integration with Nova that should be unnecessary.  It is
+also unecessarily difficult to reason about what is happening at which point in
+the code which makes it challenging to improve those parts of the code.
+
+
+Proposed change
+===============
+
+In Havana it became possible to query the scheduler for a list of hosts to
+provision an instance to.  The conductor service also emerged as a place to
+help orchestrate tasks that don't logically belong in either the api or compute
+nodes.  There has already been work to move some of the spawn instance workflow
+into the conductor and the final part of that effort is to have the conductor
+communicate with compute nodes rather than the scheduler.
+
+There is a new, currently unused, build_and_run_instance method in the compute
+manager which mimics the currently used run_instance method, but handles a
+failed build by sending an RPC cast to the conductor service rather than the
+scheduler.  The proposed change is to have the conductor query the scheduler
+and send a message to a compute which invokes the new build_and_run_instance
+method.  Because the new method is unused and therefore untested by Tempest
+there will likely be some work required to achieve full compatibility with the
+current run_instance method.
+
+Alternatives
+------------
+
+An alternative would be to rework the run_instance method to cast back to
+conductor rather than use the new build_and_run_instance method.  This was
+decided against because the amount of refactoring that would need to happen
+there meant it was easier to rebuild that method from scratch.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None.  The notifications being sent by the scheduler will be ported over to
+conductor to maintain the same behaviour.
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+Some database updates that were occuring within the scheduler will be moved out
+to less performance critical sections of code.  This should speed up the
+scheduler.
+
+There may be a decrease in the amount of time to boot an instance if it needs
+to be rescheduled.  The new build_and_run_instance performs some pre-build
+checks earlier and doesn't generally deallocate and reallocate networks for a
+rescheduled instance.  It will deallocate/reallocate if the baremetal driver is
+in use as the networking there is host specific.
+
+Other deployer impact
+---------------------
+
+None.
+
+Developer impact
+----------------
+
+Developers will need to be aware of the new code path being used.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  alaski
+
+Other contributors:
+  None
+
+Work Items
+----------
+
+  * Change the conductor to query the scheduler and cast to a compute.
+
+    * Move notifications from the scheduler into the conductor.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+This is essentially a refactoring of the current spawn process.  So the current
+Tempest tests will act as good integration tests for this change since the new
+method will be used on every instance boot.
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+None