All, 

I am taking over work on https://review.openstack.org/#/c/154521/ from 
Johanness and have spent some time today discussing the current patchset and 
what is remaining with Dan and Johannes.

I wanted to broach the topic of the remaining development with the lists to 
make sure we go in the right direction with it as it is not an easy  problem to 
solve. 

The remaining work is to have a way of preventing database contracts from 
running until data migrations that the column interacts with are complete, this 
is not an easy problem to solve as the goal of the online migrations is to do 
pure model based schema migrations and there is no way of currently identifying 
when a data migration has taken place and a contract is safe to perform.

The first thought I had was to default to a requirement of a data migration 
removing data from a column and have the default check of the contract be one 
of verifying that the column is currently empty. This has the issue of not 
working in all cases, especially around Foreign Keys.

At this point I would like to open this discussion up to all in order to make 
sure the best solution is chosen for this problem..

-Ph
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