Implement custom resource

Implement a custom resource, that can be managed with stack life cycle methods.

1. Add new resources(external resources)

2. Interact with heat workflow.

3. Inject Dynamic Data

4. Extending exiting resources

Example Use-Case: Mount cinder volume
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If mounting of a cinder volume to an instance is handled in the 'user-data' script, there is no way stack would get to know if the mounting has been successful or not. Before detaching the volume, it should be unmounted. However, there is no way it can be done before the instance is deleted.

Example Use-Case: Manage DNS records for nova instances
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Add and remove DNS records for stack managed instances

If we implement it inline with AWS custom resource, the flow could be as mentioned below.

Heat ---->Marconi Notification Topic ----> Marconi Queue---->Custom Resource Implementation.