Merge "Switch periodic tasks to the Futurist library"

specs/liberty/futurist.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
+
+==================================================
+Switch periodic tasks to the Futurist library
+==================================================
+
+https://blueprints.launchpad.net/ironic/+spec/futurist
+
+Futurist_ is a new Oslo library providing tools for writing asynchronous code.
+This spec suggests switching our periodic task implementation to Futurist_ to
+solve some long-standing problems.
+
+Problem description
+===================
+
+The main problem with our current implementation is that we run all periodic
+tasks in one thread. Any task that blocks for a while would block all other
+tasks from executing, and it happens pretty often with tasks checking power
+states via IPMI.
+
+Switch to Futurist_ will allow executing all tasks in parallel.
+
+Proposed change
+===============
+
+* Modify conductor to use Futurist_ library instead of implementation from
+  oslo incubator.
+
+  `Existing worker pool
+  <https://github.com/openstack/ironic/blob/master/ironic/conductor/manager.py#L238>`_
+  will be reused for periodic tasks. So, existing option ``workers_pool_size``
+  will set maximum number of tasks to run in parallel at every moment of time.
+
+* Switch all use cases of ``ironic.openstack.common.periodic_task`` to
+  Futurist_ decorators, and drop this module.
+
+* Switch ``ironic.drivers.base.driver_periodic_task`` to using Futurist_
+  decorators internally and deprecate it.
+
+Alternatives
+------------
+
+* We could fix existing implementation. That's not actually easier, as it
+  requires essentially rewriting it.
+
+Data model impact
+-----------------
+
+None
+
+State Machine Impact
+--------------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Client (CLI) impact
+-------------------
+
+None
+
+RPC API impact
+--------------
+
+None
+
+Driver API impact
+-----------------
+
+* Old way of creating driver periodic tasks will be deprecated, drivers should
+  eventually switch to using Futurist_ decorators.
+
+Nova driver impact
+------------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Scalability impact
+------------------
+
+* Overall positive impact on scalability expected, as every Ironic conductor
+  will be able (at least theoretically) to manage more IPMI nodes.
+
+Performance Impact
+------------------
+
+* A periodic tasks performance will no longer affect timing of other periodic
+  tasks.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  Dmitry Tantsur (irc: dtantsur, lp: divius)
+
+Work Items
+----------
+
+* Change conductor manager to use Futurist_
+
+* Modify ``driver_periodic_task`` to use Futurist_ internally
+
+Dependencies
+============
+
+The only big dependency is Futurist_ itself. At the moment of writing it
+didn't see an official release yet, but is moving pretty fast and all required
+code already landed in git master.
+
+Testing
+=======
+
+Unit tests should already cover this functionality. Specific tests ensuring
+parallelization will be added.
+
+Upgrades and Backwards Compatibility
+====================================
+
+None
+
+Documentation Impact
+====================
+
+Documentation of driver periodic tasks should be updated to mention Futurist_
+instead of ad-hoc implementation.
+
+References
+==========
+
+* `Futurist periodic task documentation
+  <http://docs.openstack.org/developer/futurist/api.html#periodics>`_
+
+.. _Futurist: https://github.com/openstack/futurist