[fhunt]

Depends on the schema changes. If you (1) change/remove existing keys from existing types, you need as well a migration which runs before the app starts. If you (2) add new keys to existing types or add new types you don't need migrations. That's the beauty of NoSql or in particular Mongo which most forget, you just write to the db and Mongo creates key, collections and dbs on the fly if not there yet. But again, only do this if you have validations before in place. Otherwise you end up will all these people complaining without having a full understanding of type safety and where to deal with it.

Case 1 should be always prepared well or better avoided if possible. It doesn't matter if you have sql or nosql, they're just more complex and with a big production database some longer downtime is to be expected if the changes/migrations affect large parts.