Add a spec for Pluggable do actions

Change-Id: Ia9c07215609c72671ce53e6a1e8e8390f3207c23
blueprint: pluggable-do-actions

specs/pluggable-do-actions.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
+
+=================================
+Pluggable architecture for bareon
+=================================
+
+https://blueprints.launchpad.net/bareon/+spec/pluggable-do-actions
+
+At the current state bareon is monolitic tool.
+If one wants to add new drivers or do actions, then those changes should be
+landed into bareon's repo first. There's no convenient way to develop
+something as a 3rd party component or re-use actual codebase as a framework.
+That's why we need to introduce pluggable architecture.
+
+Problem description
+===================
+
+Currently, there're few flaws in current bareon architecture:
+
+  * It's purely monolitic tool. It can't be split to few tools like
+    bareon-base, bareon-provisioning, bareon-image-building,
+    bareon-partitioning. The idea is to provide the way to separate these
+    few stages like building OS root file system and putting this root file
+    system on a node. It is easy to imagine when a user wants to install OS
+    building root file system directly on a disk.
+
+  * Like it was said above, there's no convenient way to develop out of core.
+    This is the main stopper for external contributors.
+
+  * It's hard to introduce new functionality or re-use existing one.
+
+  * It glues few do_actions into single combo and prevents from running actions
+    separately. For example, run partitioning without provisioning.
+
+  * Current architecture is not expandable (maybe even maintainable). Pluggable
+    extensions is one of the ways to resolve the problem.
+
+Therefore, pluggable architecture is really necessary.
+
+Proposed change
+===============
+
+Manager's do actions should be re-introduced as pluggable extensions.
+Therefore new directory for actions will be created:
+
+  bareon/actions/
+
+Manager will be improved in order to work with these pluggable do actions.
+At least, manager should know how to check the presence of pluggable
+extensions and validate that input data already contains all necessary
+information for being processed later.
+
+`stevedore`_ will be used as a convenient extension manager.
+
+Moving towards with data-driven approach, the base class for all drivers will
+have only one method used only for initializing the data.
+Drivers still be free to implement any types of collections (schemes) of
+objects they needed.
+
+Regarding data intersections. Sometimes objects are the result of an action.
+For example when we want to use cloud-init with configdrive for provisioning,
+we should somehow tell the driver responsible for partitioning to reserve the
+space for config drive. In order to resolve that:
+
+  * All objects are shared among multiple do actions
+
+  * These objects could be modified in run-time inside any of do action
+
+  * Every do action should have a validate method for these objects, which will
+    asure us that provided data already satisfied all necessary requirements.
+    For example, configdrive action should expect special type of partition to
+    be created by partitioning action. Manager that performs partitioning
+    action, in turn, will check that configdrive action queued and create the
+    partition and reflect that in the objects.
+
+Alternatives
+------------
+
+The other ways are:
+
+  * Just make every do action independent from others, but then the node we
+    want to only provision will have to have the dependencies for e.g.
+    partitioning also. And it won't be expendable for Users.
+
+  * Create separate project for every usecase, but there will be lots of code
+    duplications and hard to maintain.
+
+We could use other plugin system like:
+
+  * `PluginBase`_
+
+  * `Yapsy`_
+
+But they're not part of openstack' ecosystem. The idea is that having all
+projects use the same approach is more important than the objections to the
+approach. Sharing code between projects is great, but by also having projects
+use the same idioms for stuff like this it makes it much easier for people to
+work on multiple projects. Therefore, stevedore is the essential choice.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  `Alexander Gordeev`_
+
+Mandatory Design Reviewers:
+  `Evgeny Li`_
+  `Vladimir Kozhukalov`_
+
+Milestones
+----------
+
+Target Milestone for completion:
+  1.0.0
+
+Work Items
+----------
+
+- Introduce pluggable extension.
+- Rework existing do actions according to the new architecture.
+
+Dependencies
+============
+
+Doesn't require new dependencies as stevedore has been already included into
+dependencies.
+
+----------
+References
+----------
+
+.. _`Alexander Gordeev`: https://launchpad.net/~a-gordeev
+.. _`Vladimir Kozhukalov`: https://launchpad.net/~kozhukalov
+.. _`Evgeny Li`: https://launchpad.net/~rustyrobot
+.. _`stevedore`: https://launchpad.net/python-stevedore
+.. _`PluginBase`: http://pluginbase.pocoo.org/
+.. _`Yapsy`: http://yapsy.sourceforge.net/