In this page, I would like to share the information I learned from Braintree's Blog about how they handle DB schema migration while application is up and serving requests. I think this should benefits to developer who is working on HMS's schema upgrade scripts.  As for some DDL commands, they can lock out updates to a table for a long time and database operation that locks for more than a few seconds is indistinguishable from an outage for customers. Discussion here will be focused one using PostgreSQL as HMS DB and the following are the links for those useful articles: