Stack lifecycle plugpoint

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A cloud provider may have a need for custom code to examine stack requests prior to performing the operations to create or update a stack. Some providers may also need code to run after operations on a stack complete. A mechanism is proposed whereby providers may easily add pre-operation calls from heat to their own code, which is called prior to performing the stack work, and post-operation calls, which are made after a stack operation completes or fails.

Problem description

There are at least two primary use cases. (1) Enabling holistic (whole-pattern) scheduling of the virtual resources in a template instance (stack) prior to creating or deleting them. This would usually include making decisions about where to host virtual resources in the physical infrastructure to satisfy policy requirements. It would also cover failing a stack create or update if the policies included with the stack create or update were not satisfiable, or other cloud provider policies being checked were not satisfiable. As an example, an application owner requires that VMs and volumes attached to them are deployed on the same rack. As another example, a cloud provider may want to enforce consultation with a license server before deploying an application. As another example, an application owner may require that their VMs be spread across a given number of racks. (2) Enabling checking of policies not related to virtual resource scheduling, with stack create or update failure if the policies would not be satisfied. As an example, a cloud provider may want to verify that compute resources for certain types of applications are deployed with certain security groups. As another example, a cloud provider may want to be warned when patterns with > 100 VMs are deployed.

Proposed change

An ordered registry of python classes which implement pre-operation and/or post-operation methods is required. This would be done through stevedore, with some addition to force a full (or partial) ordering on the classes. Pre and post operation methods should not modify the parameter stack(s). Any modifications would be considered to be a bug. A possible exception would be to allow status changes to the stack, to facilitate error handling. [The no-modifications rule could be enforced, e.g. by passing deep copies to the plugins but this might incur an unacceptable performance cost.] Both pre-operation and post-operation methods can both indicate failure, which would be treated like any other stack failure. On failure of a pre-operation call, when more than one plugin is registered, the post-op methods would be called for all the classes already processed, to indicate to each plugin that any decisions that it made with respect to the stack should be un-made.

All stack actions would need calls to either pre or post operations, or both. This includes at least create, update, delete, abandon, and adopt. In a basic design, modifications to the Stack class in are sufficient for adding the call to the pre-operation and post-operation methods found via the lifecycle plugin registry. The post-operation calls would need to be called in both the normal paths and all error paths.


No other approach was identified that would allow the operator (heat provider) to extend heat with this functionality for all stack deployments. describes an approach where heat users can optionally specify callbacks for in templates for stack and resource events. It does not provide the ubiquitous callbacks (for all stacks) that would be needed by the use cases described above, unless the heat provider tightly controls the templates that users can use.


A patch comprising a full implementation of the blueprint ( is already being reviewed, under the old pre-spec process.


Primary assignee:
William C. Arnold (barnold-8)


Target Milestone for completion:

Work Items



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