Open Stack

Heat devs,

Liang pointed out a race-condition in the current multi-engine
implementation that will be difficult to fix without a DB lock.  I've
discussed the multi-engine design with my teammates and written up a few
alternative designs here:
https://etherpad.openstack.org/vJKcZcQOU9

Every design has its own downsides, so I was hoping to get some feedback
from the core devs as to which one is preferable.

Feel free to add comments in-line.  Please don't click "Clear Authorship
Colors" ;)

Thanks,
Jason