Serious question to you or anyone else who uses schemaless databases. Why is the ability to change schemas on the fly a good thing? Having worked at two companies that did, it was nothing but a recipe for disaster in large groups. Code that was dependent on expecting an integer or a string and not a collection would constantly break because a developer in some other group decided to store a collection instead of a the original data type that was expected. Schemaless databases required more documentation to track changes made between groups and led to more bugs because we could never be guaranteed of what kind of data we would be receiving. I've always thought of a database schema as a contract that makes guarantees to all applications. Why would you want to be able to break that contract?

I've found a good SQL library (like Alchemy), a good migration library (like Alembic) and a DB with non-blocking migrations is much nicer to use, since it makes data migrations very easy.

How does MoSQL handle schema changes and new fields in mongo?

I'm imagining with this tool you start to need to be a bit more careful with the flexibility which initially drew you to mongo.

You should be aware however that Mongo's failover incurs downtime.

A Postgres bouncer + WAL replication achieves a similar result: There is no downtime on failover, but there is a single slave.