Add support for external resources.

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Problem description

We have no way to instruct Heat to use an existing (external) physical resource id.

Use case 1:

When running a stack over a period of time a user might find it necessary to operate on a server (rebuild it) out of band. But that then left it out of sync with Heat’s perspective of it. So this is a mechanism to tell Heat “this is the new resource id, and I am now taking control of it”. At some stage later (s)he might want to tell Heat to take control of it again - by removing the “external_reference” section. This fills TripleO’s needs (and we assume many other users that have to operate an important stack for an extended time) by allowing the get_attr/get_resource to keep functioning when they are working on a resource externally and for Heat to leave it alone. Then when the user is happy with the state of it they can return it to Heat’s control.

Use case 2:

There is an existing resource that we would like to use get_attr on to retrieve useful information instead of doing this manually and passing the info in via the Parameters.

In all these cases once this is done the resource can be marked as external and any update or delete will be ignored (unless the user removes the “external” information first).

Proposed change

To achieve this the user would add the following to the template:

    type: OS::Nova::Server
    external_id: the-server-id

Note: 1. There is no place for “resource_data or Metadata” as these are used by actions that once it becomes external are not possible. There will be some resource types that will not survive going from external to normal resources because of the missing resource data/metadata. This will be documented as best as possible, and the use of resource_data should be discouraged by Heat developers.

2. Once the resource has the “external_id” attribute present the properties will be ignored (but be allowed to be present). If the “external_id” is then removed the resource will be updated with the properties.

Creating a resource with external_id.

This covers the second use case. Here we see that there is an external_id and logically do an adopt and check (to make sure the resource actually exists).

Updating a resource with external_id.

This covers the first use case. We overwrite the resource_id that Heat has previously written and ignore all the properties. Check will also be called here to make sure the resource exists. If the external_id is different to the existing physical_resource_id then the existing one will be deleted.

Removing the external_id.

To convert the resource into a “normal” resource the user must remove the “external_id” attribute from the resource and do a stack update. If the resource requires some missing resource_data or metadata that is missing (and can’t be recovered) this will fail and it will remain as external.

Deleting a stack with a resource that has external_reference.

When we have an external_reference, a deletion policy of RETAIN is assumed (it will not be deleted).


The user could use the current adopt/abandon mechanism, but it has some slightly different behaviour. Also switching physical resource_id is difficult with 2 API calls.



Primary assignee:



Target Milestone for completion:


Work Items

  • Code

  • Functional tests.

  • Documentation needs to be added to the template guide.

  • Document limitations (resources that require resource_data and metadata).