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Make Heat aware of reality and use the reality as basis for comparison before updating the resource. This spec is proposal to add a get_reality/collect_reality to resource class.
Due to external factors in cloud, a provisioned resource from stack may diverge from its original state. Since heat is unaware of this change, it cannot act on it to bring it to desired state. Also, when user tries to update it, the resource might end-up in error state since it is not aware of its state and cannot carry-out any corrective action. Heat needs to detect the changes in reality and compare with the new properties and update the resource.
A solution discussed at L summit meet-up was to implement get_reality and handle_get_reality. The idea is to get the resource properties from the reality and compare it with the current properties from template + environment, instead of comparing with old template. When a user issues an update, the resource plugins get the current resource properties from reality and compare with the current template. If there are any changes, the resource is updated.
The enable observer config option should be used to decide whether to get the reality before updating the resource. This feature is enabled only when the option is set in config.
This is first part of convergence observer specification. The observer will depend on get_reality and handle_get_reality APIs. The scope of this change is limited to implementing mechanisms to find the changes from reality and compare before updating a resource. The bigger effort involves continuously monitoring the changes and continuously acting on them.
The changes will mostly reside in heat resource class and plugins if required.
1. Implement get_reality/collect_reality in resource plugins. The existing show_resource can be used to get the reality, in addition to any other queries to reality.
2. Use the existing update/handle_update to take necessary action on resource based on output of get_reality.