Prior to this update, Heat did not update resource definitions incrementally. If a stack update failed after updating a resource and then deleting a resource that the updated resource previously depended on, the stored template could contain a reference to a non-existent resource, leaving Heat unable to make changes to the stack. With this update, Heat updates resource definitions incrementally during the update of a non-convergence stack, which allows Heat to make changes to the stack after a failed stack update. Now, if an update fails, the stored template contains only valid references.