Stable StackDefinition API for template plugins
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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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This template should be in ReSTructured text. The filename in the git
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repository should match the launchpad URL, for example a URL of
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https://blueprints.launchpad.net/heat/+spec/awesome-thing should be named
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awesome-thing.rst . Please do not delete any of the sections in this
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template. If you have nothing to say for a whole section, just write: None
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For help with syntax, see http://sphinx-doc.org/rest.html
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To test out your formatting, see http://www.tele3.cz/jbar/rest/rest.html
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=====================
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StackDefinition class
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=====================
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https://blueprints.launchpad.net/heat/+spec/stack-definition
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Encapsulate all data about the definition of the stack - including the
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template, parameter values, resource attributes & reference IDs - in a class
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and use that in intrinsic functions instead of the Stack object itself.
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Problem description
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===================
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A number of problems arise from the fact the we pass the
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heat.engine.stack.Stack object representing the stack to Template.parse() (and
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thence to the Function objects) in order to provide access to data needed to
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define the stack.
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The main issue is that the so-called 'lightweight stack', used in convergence
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when performing a check on a single resource, is not particularly lightweight.
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In particular, in order for the resource being checked to use an attribute or
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resource ID from another resource we create a heat.engine.resource.Resource
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object for every resource in the stack. This occurs even though the actual
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values we need are passed in over RPC and none of the Resource object's data is
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actually loaded from the database, nor any of its code used. Eliminating
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loading the data from the database reduced memory use by 10%, and further
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savings are likely from not creating O(n^2) Resource objects in the course of a
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graph traversal.
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In addition, using the Stack object in this way makes it extremely unclear what
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part of the interface is stable for the purposes of third-party Template
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plugins and their associated Functions. Developers don't know which aspects of
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the Stack interface they need to preserve across versions, and plugin
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developers don't know what they can rely on. Many of the potential operations
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that the Stack class makes possible may, in fact, be horrifically inefficient
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in the convergence architecture, or may become so in future.
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Proposed change
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===============
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Create a new StackDefinition class that encapsulates the data needed to define
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the stack. Pass this object to Template.parse() in place of the Stack object.
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When a resource is accessed via the StackDefinition, return a ResourceProxy
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object that contains the pertinent data only.
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For all intents and purposes, the initial API will comprise whatever parts of
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the Stack and Resource classes that are currently used by intrinsic functions
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in the HOT or Cfn Template plugins. Existing interfaces will be maintained, so
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that no code changes are required for any plugins using this subset of the
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functionality. This includes:
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* Template
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* Environment
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* Parameter values
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* Resources
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* Name
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* Status
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* Action
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* Attributes
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* Reference ID
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* DB ID
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* UUID
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* Parent (facade) resource
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* Metadata
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* Template
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In convergence when checking a resource, only those other resources on which
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there is a direct dependency will have a proxy available, and only attributes
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required by the current resource (as returned by the function.dep_attrs()
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function) will have values available.
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Unless mentioned specifically above, no other data will be available from the
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StackDefinition. This will potentially break third-party Template plugins, but
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it is impossible to know to what extent this is a problem due to the undefined
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surface area of the current stable API. However, anything relying on behaviour
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outside of the proposed API is quite possibly highly inefficient, and
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completely untested in upstream Heat. On balance, it's worth tolerating this
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breakage, early in a development cycle, in order to move to a guaranteed stable
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API. Third-party Template plugin developers have the opportunity to weigh in on
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this spec with requests.
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Future changes to the API (beyond Pike) will require a deprecation period.
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Alternatives
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------------
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Just put up with it the way it is?
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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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zaneb
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Milestones
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----------
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Target Milestone for completion:
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pike-1
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Work Items
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----------
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- Encapsulate in a NodeData class the data that convergence records about a
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node in the graph after traversing it.
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- Teach Resource to generate its own NodeData.
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- Create a ParentResourceProxy to encapsulate data about the facade resource
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and potentially load it independently of the parent stack.
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- Move the Stack's template and other data into a StackDefinition class and
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proxy any requests for that data to it.
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- Update the NodeData after processing each Resource in the legacy path.
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- Pass the StackDefinition in place of the Stack to Template.parse().
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Dependencies
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============
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None
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