title: Is it possible to use feature flags/toggles when feature modify database?
body: <p>This problem confuses me the most when thinking about using feature toggles in applications. Most of the features require some changes in database. How then feature flag can be implemented to be able revert database changes smoothly along with turning on/off feature toggle ? Especially when some database migration tools are used like flyway or liquibase. </p>
  
  <p>Let's use some example. For instance I have simple table Playlist with Id, Name columns. I want to add feature of adding description to Playlist in application. DB table needs to be changed. New column has been added. Column value is not mandatory. Simple so far. After some time feature is going to be turned off from production. Solution is quite easy but messy - we can leave column in db and don't use it in a code anymore. Migration tools works incrementally so i don't see any option to get back to previous db state when i turn off feature toggle (or am I wrong ?). Harder example is when field needs to be mandatory. Then i can't just leave that field in db. It will be backward incompatible. So what's then ? How to process this situation ? I think it's quite common situation.</p>
  
  <p>Additionally even with slight changes on db, my application model for that class will change too. (assuming usage of some ORM). This is not as easy as replace implementation with strategy pattern. Unless you provide a lot of abstraction to your ORM model usage. So adding such a feature flag seems to be very complicated. Can anyone help me understand it how it's possible to use feature toogles then ? Or maybe someone has full example to show ? Preferred with Java</p>