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Excerpts from Henry Nash's message of 2016-09-15 00:29:44 +0100:
> Jay,
> 
> I agree with your distinction - and when I am referring to rolling upgrades for keystone I am referring to when you are running a cluster of keystones (for performance and/or redundancy), and you want to roll the upgrade across the cluster without creating downtime of the overall keystone service. Such a keystone cluster deployment will be common in large clouds - and prior to Newton, keystone did not support such a rolling upgrade (you had to take all the nodes down, upgrade the DB and then boot them all back up). In order to support such a rolling upgrade you either need to have code that can work on different DB versions (either explicitly or via versioned objects), or you hide the schema changes by “data synchronisation via Triggers”, which is where this whole thread came from.
> 

It doesn't always need to be explicit or through versioned objects. One
can often manipulate the schema and even migrate data without disturbing
old code.