Add Template “Capabilities” Annotation/Resolution/Introspection

Also related to

Add an optional annotation to HOT which enables a template author to define that a template implements/provides particular capabilities.

Problem description

Currently, the environment resource_registry provides an extremely flexible but completely unconstrained interface for mapping type aliases to implementation.

This makes it difficult for those wishing to use the resource_registry for composition, in particular if you wish to offer users the choice to pick a particular implementation of a provider resource template.

For example, consider this workflow:

  1. Choose parent template.
  2. Choose a set of other templates and environments (or have this programmatically generated, for instance by pulling templates from one or more known locations/paths).
  3. Inspect that group to figure out the resource-type level capabilities/options. These are the first major choices a user will make to determine the nested stack implementations for each type.
  4. The user selects a nested stack choice for each one that has more than one choice.
  5. Reinspect given those major options for the full set of parameters such that the user may be prompted for mandatory and optional parameters, including those not exposed by the top-level parent template.
  6. The user enters in values for all of the parameters and the stack gets created.

The topic of this spec is steps 3 and 4 above. discusses step 5. The other steps are already possible.

The discussion below focuses on the TripleO use case, since that is what is motivating this work (TripleO makes very heavy use of template composition via the resource_registry). However, the feature should be generally useful to anyone wishing to use the resource_registry to build a complex environment from a tree of interrelated templates via the resource_registry.

Here’s an example of the resource_registry mapping used for TripleO controller node implementation:

  OS::TripleO::Controller: puppet/controller-puppet.yaml
  OS::TripleO::Controller::Net::SoftwareConfig: net-config-bridge.yaml
  OS::TripleO::ControllerPostDeployment: puppet/controller-post-puppet.yaml
  OS::TripleO::ControllerConfig: puppet/controller-config.yaml
  OS::TripleO::Controller::Ports::ExternalPort: network/ports/noop.yaml
  OS::TripleO::Controller::Ports::InternalApiPort: network/ports/noop.yaml
  OS::TripleO::Controller::Ports::StoragePort: network/ports/noop.yaml
  OS::TripleO::Controller::Ports::StorageMgmtPort: network/ports/noop.yaml
  OS::TripleO::Controller::Ports::TenantPort: network/ports/noop.yaml
  OS::TripleO::Controller::CinderBackend: extraconfig/controller/noop.yaml

We can see that there are a large number of choices (and this is only a tiny subset of the full environment), with no way for a UI to determine what are valid choices for any of the mappings. It would be beneficial to describe directly in the template what valid implementations are, such that they may be discovered by UI/CLI tools and constrained at validation time.

Taking the above as a worked example, there are multiple choices to be made:

  • Configuration Tool type (all puppet/*.yaml resource)
  • NIC configuration (physical network, e.g bridged, bonded, etc)
  • Port assignement (overlay network, where ports/noop.yaml assigns all ports to a common network)
  • Choice of (potentially multiple CinderBackend implementations)

For simplicity, the examples below will consider only the choice between puppet and some other implementation.

Proposed change

The proposed change covers three areas:

  • Annotation - How to convey the relationship between a resource type and templates that may potentially fufill the type.
  • Resolution - How Heat can use user settings on the stack to choose the most applicable template to fulfill a resource type from a list of options.
  • Introspection - How a UI can programmatically use the annotations to present the user with an interface in which to select an option for each eligible resource type.

The remainder of this change is broken up into those three sections. It should be noted that all three are not necessary for a minimal viable feature. It is possible that the implementation for mitaka only covers, for example, the annotations and introspection APIs necessary for the user to understand the “schema” (for lack of a better word) around selecting template implementations for a stack.


Add an optional template annotation, inspired by the TOSCA “substitution_mappings” interface [1], which allows an optional new block in HOT templates where template authors may declare that a template provides a particular set of capabilities.

There are two slightly different uses of the capabilities annotation being proposed.


For example, there may be multiple valid implementations of OS::TripleO::Controller. For the Puppet-based implementation, the capabilities annotation on the template will indicate it:

heat_template_version: 2015-10-15

   deployment: puppet

The syntax used here is similar to that defined in the TOSCA spec but the names have been adjusted to better match existing HOT conventions. The capabilities section will not be strictly validated; it will be possible to add extra key/value pairs that are not specified in the environment, such that templates may be portable.


It also may be possible to use these annotations for client-side discovery of the list of valid templates to be passed via the resource_registry by specifically referencing the name of the resource type the template may be used as a mapping for:

heat_template_version: 2015-10-15

   resource_type: OS::TripleO::Controller

This should support a list as TripleO has already seen an example of templates that can be used as either the computer or controller hooks:

heat_template_version: 2015-10-15

   resource_type: [OS::TripleO::ControllerPostDeployment,


In the environment, an optional new “requires” section will be added and support for resource_registry keys containing a list of multiple implementations. Heat will then be able to resolve the implementation that should be chosen by matching the environment requires to the list of possible templates with (hopefully matching) capabilities. A validation error will be thrown should either zero or multiple implementations be found.

For example, expanding on the examples from the previous section, take the following environment file:

   deployment: puppet

   OS::TripleO::Controller: [puppet/controller.yaml, docker/controller.yaml]

Adding annotations to the two referenced templates, we have:


heat_template_version: 2015-10-15

   deployment: puppet


heat_template_version: 2015-10-15

   deployment: docker

Putting these three files together, Heat would use the capabilities section to determine which of the two controller.yaml files to use.


The functionality described in the Annotation section provides enough information for Heat to provide a series of introspection queries to facilitate the user experience.

Specific Type Query

Given a specific resource type name, the Heat API should be able to return a list templates that claim to support that type (note: this is contingent on using the Type-Based annotation style described above).

A potential example of the output of such a query through the Heat client is below:

$ heat capabilities-find -r -c resource_type=OS::TripleO::Controller ./*

This would recurse from the current directory inspecting the capabilities in each template, returning a list of those which match the capabilities required (with the possibility of passing multiple -c options if necessary). This makes multiple implementations discoverable on the client side.

Capabilities Summary

There is also a need to have Heat analyze a series of templates and environments, returning a list of all capabilities that can be specified:

For example, given the Puppet and Docker example templates above:

$ heat template-capabilities -f puppet/controller.yaml \
                             -f docker/controller.yaml

Which would return:

  'deployment': ['puppet', 'docker']

A similar version of the call exists if the Type-Based annotation is used:

  'OS::TripleO::Controller': ['puppet/controller.yaml',

The operator or UI then knows that these are the options which may be resolved in order for the stack to be created. Note this is related to but not the same as the spec posted related to recursive validation [2], which is about exposing the parameters required for stack create, not the options related to a valid composition.


API v. Client-side

The original iteration of this spec spoke in terms of having the Heat client walk the template tree and perform the introspections described. It has since been changed to refer to the Heat API, moving the logic server-side and allowing non-Python clients access to this functionality.




The main alternative discussed (see previous revision of this patch) was adding constraints to the resource_registry, such that valid mappings may be defined inside the environment. This idea was rejected because of the desire for a more discoverable interface (e.g look for valid implementations vs a rigidly defined list of constraints).

A subsequent proposal was also rejected, which focussed on only matching a resource_type annotation in the templates, it was suggested that this was insufficiently granular and not flexible enough.


The implementation will require adding the new capabilities annotation to the Mitaka HOT version, this will be optional and if it is omitted the existing behavior will be maintained.

Then support will be added to the environment to enable lists to be passed via the resource_registry, and resolved via a new requires section.


Primary assignee(s):

  • shardy
  • jdob


Target Milestone for completion:

Work Items

Changes to Engine

  • Update HOT to support optional new capabilities annotation
  • Update environment code to allow lists for resource_registry
  • Update environment to process capabilities section to filter lists

Changes to heatclient

  • Add support to python-heatclient for parsing a tree of templates, returning a list of valid templates for a specified capability
  • Add support for passing files/environment to get required capabilities

Documentation Changes

  • Document new interfaces in template guide docs/HOT spec.