Merge "Custom actions API specification"
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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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Custom Actions API
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==================
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Launchpad blueprint:
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https://blueprints.launchpad.net/mistral/+spec/mistral-custom-actions-api
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This specification sets a formal basis for those Mistral users who want to
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create their own actions and make them available to use as part of Mistral
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workflows. The number one question that the spec addresses is "What is
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available in Mistral code base in order to implement custom actions.
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Problem description
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===================
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Custom actions are now possible to create and it's as simple as just
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implementing a class inherited from mistral.actions.base.Action that
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has 3 methods:
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* run() - executes main action logic, **mandatory** to implement
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* test() - execute action in test mode, related to future dry-run
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functionality, optional to implement
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* is_sync() - must return **True** if action returns its result right from
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method run() or **False** if method run() only starts action logic and
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result is supposed to be delivered later via public Mistral API
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There's also a mechanism based on stevedore library that allows to plug in
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new actions via adding new entry points in setup.cfg file.
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If a custom action doesn't require any integration neither with Mistral
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nor with OpenStack this is enough to know in order to implement it.
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However, if this action needs to leverage more advanced capabilities
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provided by Mistral and OpenStack then Action class itself doesn't
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give any knowledge about means that can be used to achieve that.
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A simple example of integration with OpenStack infrastructure is the need
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to call endpoints of OpenStack services. In this case, at minimum, action
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needs to be able to authenticate with Keystone, i.e., have access to
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Mistral security context.
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Use Cases
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---------
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Simple OpenStack actions
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^^^^^^^^^^^^^^^^^^^^^^^^
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As a user of Mistral I want to create actions that call OpenStack services.
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In this case action needs to be able to access Mistral security context
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that contains auth token to be able to pass it to a corresponding service.
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Note: This use case is generally implemented within Mistral but it needs
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to be rethought since OpenStack actions that are implemented now in Mistral
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use Mistral Python code that is not assumed to be a public API and hence
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stable.
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Complex OpenStack actions
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^^^^^^^^^^^^^^^^^^^^^^^^^
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As a user of Mistral I want to create actions that call multiple OpenStack
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services from within one action.
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For example, we may want to create action
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"create_cinder_volume_and_attach_to_vm" that creates a Cinder volume and
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attaches it to a virtual instance. In this case action needs to have access
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to Mistral security context that contains auth token so that it can pass
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that token to Cinder and Nova.
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Reusing existing actions
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^^^^^^^^^^^^^^^^^^^^^^^^
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As a user of Mistral I want to be able to reuse existing actions while
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implementing my new actions so that I don't have to reimplement similar
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functionality.
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For example, I want to create action that checks if a certain virtual
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instance exists in the tenant by calling Nova and if it does the action
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runs a number of secure shell commands to configure it. In this scenario,
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we need to call Nova and do ssh. Both already exist in Mistral as actions
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"nova.servers_get" and "std.ssh". So there should be a mechanism allowing
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to reuse those actions while create a new more complex action.
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Proposed change
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===============
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General idea
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------------
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We need to have one or more Python packages in Mistral that are designed
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and documented as a public Python API for developers that want to create
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custom actions. These packages should effectively provide a number of
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classes that can be used directly or inherited as needed. They should
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cover the following aspects of action development:
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* Base class or a number of classes that can be extended in order to build
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new Mistral actions. Currently existing **mistral.actions.base.Action**
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is an example of such class.
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* Module that provides access to security context associated with the
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current workflow that this action belongs to. Security context should
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at least include user, project/tenant, auth token.
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* Module that provides access to current Mistral execution context. That
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context should include:
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* Current workflow execution id
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* Current task execution id
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* Current action execution id
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* Package with most frequently used utils and data types used during
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custom actions development. For example, class
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mistral.workflow.utils.Result that now exists in the code base is
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needed by actions but it's not clear that it's part of Python API.
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* Module that allows to get and reuse existing actions
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Since these Python entities must be available for both engine and
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executor they should be moved to a separate subproject of Mistral, for
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example, **mistral-actions-api**.
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Existing OpenStack actions should be moved out of mistral project into
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a different Mistral subproject. The proposal is to use **mistral-extra**
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repo for this purpose because although we use it only for collecting
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Mistral examples its initial idea was also to have additional tools
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and extensions in it.
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Specific entities
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-----------------
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mistral.actions.api
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^^^^^^^^^^^^^^^^^^^
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Main Python package that contains all modules and classes which are part
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of Custom Actions API.
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mistral.actions.api.base
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^^^^^^^^^^^^^^^^^^^^^^^^
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Python module that contains base classes for custom actions. Currently
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module **mistral.actions.base** performs similar function.
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Note: Specific content of this module is out of scope of this spec and
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must be defined at implementation stage.
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mistral.actions.api.security
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Python module that contains required functions to get all required
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information related to current OpenStack security context. At minimum:
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user, project, auth token.
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Note: Specific content of this module is out of scope of this spec and
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must be defined at implementation stage.
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mistral.actions.api.types
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Python module that contains all data types that custom actions need to
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use. One candidate to go to that module that now exists is
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**mistral.workflow.utils.Result**.
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Note: Specific content of this module is out of scope of this spec and
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defined at implementation stage.
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mistral.actions.api.utils
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Python module that contains additional functions helpful for creating
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new Mistral actions. At minimum: functions to get instances of existing
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actions so that action developers could re-use functionality of existing
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actions. Return type for these actions though must be rather a wrapper
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that doesn't just call **Action.run()** method but instead uses Mistral
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action execution machinery to actually call action just like as if it
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was called as part of workflow (taking care of data transformations,
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fulfilling security and execution context etc.)
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Note: Specific content of this module is out of scope of this spec and
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must be defined at implementation stage.
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Alternatives
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------------
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None.
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Data model impact
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-----------------
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None.
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REST API impact
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---------------
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None.
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End user impact
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---------------
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REST API users
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^^^^^^^^^^^^^^
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No impact.
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Custom actions developers
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Using Custom Actions API described in this spec whereas now they
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can only use **mistral.actions.base** safely.
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Performance Impact
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------------------
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No significant impact is expected. Minor is possible.
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Deployer impact
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---------------
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Deployers will need to make sure to install a new library containing
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Custom Action API packages, modules and classes. However, this impact
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is not supposed to be severe because all dependencies must be handled
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smoothly by Pip.
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In case if there's an existing Mistral installation with installed
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actions, some DB migration might be required. Changes in DB schema are
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not expected though. If so, Mistral project should provide convenient
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tools to help make this transition to using new actions.
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Implementation
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==============
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Assignee(s)
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-----------
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To be found based on discussions around the spec.
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Work Items
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----------
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* Create a new repo containing the code of Custom Actions API (e.g.
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**mistral-lib** or **mistral-common**, particular name is to be defined)
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* Design and implement modules listed in Specific Entities section
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* Provide deprecation mechanism so that during some period of time it
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would be possible to use the old approach for implementing Mistral
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actions (with **mistral.actions.base**) and the new one
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* Fix existing action implementations so that they use new API
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* Fix Mistral Executor accordingly
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* Fix Mistral Engine accordingly
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* Revisit and restructure repo **mistral-extra**
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* Move existing OpenStack actions into **mistral-extra**
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Dependencies
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============
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No additional dependencies are required.
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Testing
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=======
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Custom Actions API can be tested on devstack based OpenStack CI gates
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such as gate-mistral-devstack-dsvm by creating and running custom
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actions that use this API.
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References
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==========
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Initial patch for TripleO/Mistral integration:
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https://review.openstack.org/#/c/282366/
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