Mistral Custom Actions API

The Mistral community has been discussing improving the developer experience for creating custom actions for nearly a year.  As my first experience with Mistral has been as a consumer via TripleO, the custom actions experience has been important to me and I’ve been fortunate enough to be a part of the planning and implementation.  Mistral allows developers to implement new actions and plug them into Mistral via a mechanism based on Stevedore.  The ability to create custom actions is important as some things are easier to implementor more easily readable in Python instead of YAML/YAQL.  Developers wanting to integrate with Mistral may already have code they want to reuse or integrate with other products and systems that Mistral doesn’t provide support for directly.

According to the spec, one of the primary motivations for the new custom actions effort is to make it clear what parts of Mistral are publicly callable.  This was originally a source of confusion when creating custom actions for TripleO.  We decided to use a utility class for getting information from Keystone and it wasn’t initially clear in the review process on whether or not we were accessing private internals or not.  unclear on what parts of the Mistral code base you can or should be importing into your own actions and what can be used without fear of change.  This was a good indicator that Mistral needs to give developers an explicit public interface.

As the discussion has continued, the efforts to improve custom action development have also spawned additional ideas that will likely have a positive impact on the overall Mistral project ecosystem.  At the recent OpenStack Project Teams Gathering in Atlanta, GA, the attendees from the Mistral developer community decided to move forward on the following changes:

All of the common code between the Mistral ecosystem of projects and general 3rd party integration code to include custom actions API and custom yaql functions API will be contained in the mistral-lib project.

All of the OpenStack specific integration code and examples will reside in the mistral-extra project.

There is already a patch up for review, but it will likely see some additional changes to land and I’ll write a more detailed example of creating custom actions once it lands.


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