Worth noting if you're not married to mongodb it will cause you issues with trying to do things you get built-in for free with other JBDC compliant databases like and you will need to code up the difference
Another one - I don't know if this one is still true but seems like the Older Ignition had some modules to work with historian but there are none around for 8.1
Just posting for your consideration if you're still in the planning phases of what database to use. If you're 100% going with Mongo you will probably have to do a little bit more legwork for certain things. If you still have a choice, I would probably go with one Ignition is already built-in and ready to work with - MySQL, Postgres, SQL Server etc.