Open Stack

Hi,

One of the steps in the direction of convergence is to enable Heat
engine to handle concurrent stack operations. The main convergence spec
talks about it. Resource versioning would be needed to handle concurrent
stack operations.

As of now, while updating a stack, a backup stack is created with a new
ID and only one update runs at a time. If we keep the raw_template
linked to it's previous completed template, i.e. have a back up of
template instead of stack, we avoid having backup of stack.

Since there won't be a backup stack and only one stack_id to be dealt
with, resources and their versions can be queried for a stack with that
single ID. The idea is to identify resources for a stack by using stack
id and version. Please let me know your thoughts.

- Anant