Merge "[spec] Improve and make pluggable type conversion"
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doc/specs/in-progress/pluggable-types.rst
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doc/specs/in-progress/pluggable-types.rst
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..
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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=============================
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Make Resource Types Pluggable
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=============================
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Rally's current type resolution subsystem (``rally.task.types``) isn't
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easily pluggable, is tied to OpenStack, and cannot handle resources
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that must be resolved by the context of each iteration rather than the
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context of the subtask. This spec aims to solve all three problems.
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Problem description
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===================
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The Rally ``types.set()`` decorator is used to convert resource names
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or regular expressions to resource objects. For instance, in a task
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file a user can specify:
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.. code-block:: yaml
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image:
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regex: cirros-.*-uec
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flavor:
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name: m1.tiny
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Rally will convert those into the matching image ID and flavor ID. As
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it currently exists, this process has several shortcomings and bugs:
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* Although it is technically pluggable, the resource type classes do
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not call ``rally.common.plugin.configure`` and the code is not
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patterned as a plugin, with code in ``rally.plugins``. Technically,
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a user could implement a subclass of
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``rally.task.types.ResourceType`` in a plugin and use it, but this
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is not obvious from the code or documentation, and it would not be
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registered as a plugin. Moreover, OpenStack-specific resources are
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in the ``rally.task.types`` module instead of the OpenStack plugin.
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* It is tied to OpenStack. ``rally.task.types.preprocess()`` loads an
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OpenStack Clients object and passes it to the resource type objects.
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* In some cases, resources must be loaded by the client context
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created for each iteration, not by the admin context. For instance,
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when Glance images are created by the ``images`` context they are
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created as private images in each tenant; trying to load the image
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with the admin context fails. We need to be able to support this use
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case, without taking on a significant or universal performance
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penalty.
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Proposed change
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===============
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This change is very involved and is broken into a number of distinct
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sections.
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Create ``types.convert()``
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--------------------------
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First, we will add a new function, ``types.convert()``, to replace
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``types.set()``. ``types.convert()`` will accept arguments differently
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than ``types.set()``. For instance, this:
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.. code-block:: python
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@types.set(image=types.ImageResourceType,
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flavor=types.FlavorResourceType)
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...will change to:
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.. code-block:: python
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@types.convert(image={"type": "glance_image"},
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flavor={"type": "nova_flavor"})
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This has a number of advantages:
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* Resource type classes can be named or removed, or the interface
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changed, without breaking the public API.
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* Users will not have to import types in the code. Currently this is
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only a single module, but this spec proposes to change that.
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* Plugins are loaded automatically, rather than users having to import
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them explicitly.
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* We can use the existing plugin deprecation mechanisms.
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* By passing a dict to ``types.convert()`` instead of a class, we
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could in theory pass arguments to the types. Nothing in this spec
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requires that functionality, but it is provided for the future.
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* ``set`` is a reserved keyword, so by renaming the function we
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eliminate a bit of code that is in violation of the OpenStack Style
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Guidelines.
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Convert ``ResourceType`` to plugin
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----------------------------------
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Next, the code will be rearranged to make it obviously pluggable,
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and a ``types.configure()`` call will be added to register the
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``ResourceType`` subclasses as plugins. OpenStack resources will be
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moved into the OpenStack plugin space, and documentation will be added
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to make it clear that ``ResourceType`` can be subclassed by other
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plugins. The old resource type classes will be left in place, but
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deprecated. ``types.set()`` will also be deprecated at this point.
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Switch scenarios to ``types.convert()`` and new type plugins
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------------------------------------------------------------
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After resource type plugins are created, all existing scenarios will
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be changed over to ``types.convert()``. This will allow us to make the
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changes below that affect the type conversion API without having to
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make further changes to the scenarios.
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Change type preprocessing signature
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-----------------------------------
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The arguments with which each preprocessor is called will be
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changed. Instead of:
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.. code-block:: python
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def transform(cls, clients, resource_config):
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...it will be:
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.. code-block:: python
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def preprocess(self, resource_config, context=None, clients=None):
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Within the types subsystem proper, only ``context`` will be passed;
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``clients`` will remain for compatibility with the validation
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subsystem, which does not have a context object yet, and remains tied
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to OpenStack.
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If ``clients`` is not passed to ``transform()``, the responsibility
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for creating OpenStack clients (or doing anything else with the
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subtask context) will lie with the ``ResourceType`` subclass
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itself. This entails a small performance penalty, but it's necessary
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to divorce the types subsystem from OpenStack. If ``clients`` is
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passed, then a deprecation warning will be logged. When the validation
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subsystem is made independent from OpenStack, the ``clients`` keyword
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argument should be removed.
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This also makes it so that ``ResourceType.transform()`` is no longer a
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class method, which will allow the resource classes to retain
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persistent information about a single decorated scenario
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function. ``transform`` will also be renamed to ``preprocess``, which
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will be more consistent with ``rally.task.types.preprocess`` and will
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make it easier to add a second resource type resolution hook,
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described below.
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Add ``ResourceType.map_for_scenario()``
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---------------------------------------
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A new hook will be added to the runners. In addition to
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``ResourceType.preprocess()``, which is run after contexts but before
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the scenarios start, ``ResourceType.map_for_scenario(self,
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scenario_context, resource_config)`` will run before each iteration of the
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scenario. Together with the change to make ``types.set()`` accept
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objects instead of classes, this will solve the issue of resources
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that must be resolved per-iteration.
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For instance, to resolve images, ``ImageResourceType.preprocess()``
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would resolve images for each set of credentials created for the
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subtask, as well as for the admin credentials, and cache them;
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``ImageResourceType.map_for_scenario()`` would be passed the mapped
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scenario context and the resource configuration, and would choose the
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correct image ID to pass to the scenario. Note that image listing and
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resolution is not done by ``map_for_scenario()``; we should strive to
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keep the performance profile of ``map_for_scenario()`` as small as
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possible.
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In order to simplify the type resolution workflow, only
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``map_for_scenario()`` will be able to rewrite arguments, but the
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default implementation in ``rally.task.types.ResourceType`` will
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rewrite it with the value cached in ``preprocess()``. For instance:
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.. code-block:: python
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class ResourceType(plugin.Plugin):
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@abc.abstractmethod
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def preprocess(self, context, resource_config):
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pass
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@abc.abstractmethod
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def map_for_scenario(self, scenario_context, resource_config):
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pass
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class FlavorResourceType(ResourceType):
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def preprocess(self, resource_config, context=None, clients=None):
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self._flavor_id = resource_config.get("id")
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if not self._flavor_id:
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novaclient = clients.nova()
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self._flavor_id = _id_from_name(
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resource_config=resource_config,
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resources=novaclient.flavors.list(),
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typename="flavor")
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def map_for_scenario(self, scenario_context, resource_config):
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return self._flavor_id
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class ImageResourceType(ResourceType):
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def preprocess(self, resource_config, context=None, clients=None):
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self._image_id = resource_config.get("id")
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if not self._image_id:
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self._images = {}
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all_images = clients.glance().images.list()
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for image in all_images:
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self._images.setdefault(image["owner"], []).append(image)
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def map_for_scenario(self, scenario_context, resource_config):
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if self._image_id:
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return self._image_id
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else:
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return _id_from_name(
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resource_config=resource_config,
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resources=self._images[scenario_context["user"]],
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typename="image")
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This demonstrates two different workflows.
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Flavors, which exist globally for all users and tenants, can be easily
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resolved once, at preprocessing time, and ``map_for_scenario()`` needs
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only to substitute the single, canonical flavor ID on each
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iteration. This does lead to some redundancy -- flavor arguments will
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be rewritten on each iteration, for instance -- but as it's only a
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matter of changing a few values in the argument dict, the performance
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penalty will be minimal.
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Images are more complicated, because images can exist on a per-user
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basis, and remain invisible to other users. In order to properly
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resolve image IDs, we must first find all images in ``preprocess()``,
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and then select the correct image for each iteration (and for the user
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that maps to each iteration) in ``map_for_scenario()``.
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Remove deprecated code
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----------------------
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Finally, in some future release we will remove the old, deprecated
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resource type classes and ``types.set()``.
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Alternatives
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------------
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Type resolution could be done in a single step (as opposed to the two
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step ``preprocess()``/``map_for_scenario()``) if we passed the results in
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the context object instead of rewriting scenario arguments. This is
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less straightforward, though; the scenario author would then need to
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know where to look in the context to find the resource object, even
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though for any given iteration there is exactly one resource object
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that is appropriate.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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stpierre aka Chris St. Pierre
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Work Items
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----------
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* Create ``types.convert()``.
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* Rearrange the code into plugins and add plugin
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documentation. Deprecate ``types.set()`` and the old resource type
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classes.
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* Convert existing scenarios to use ``types.convert()``.
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* Convert ``ResourceType.transform()`` to
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``ResourceType.preprocess()`` and create a new abstract intermediate
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subclass, ``OpenStackResourceType``, to which to offload OpenStack
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client creation.
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* Add the ``ResourceType.map_for_scenario()`` hook.
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* Rewrite any resource types that need to take advantage of the new
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``map_for_scenario()`` hook. This will likely be limited to
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``ImageResourceType`` and ``EC2ImageResourceType``. If there are
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obvious patterns that can be abstracted out, then add a new abstract
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intermediate subclass.
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* In the indeterminate future, remove the deprecated resource type
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classes and ``types.set()``.
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Dependencies
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============
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None.
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