I am currently coding the backend of a website and I have not come across an article where this is discussed. I want to store all my application data in MongoDB but I'd like to split out my sensitive user information into a MySQL database. My rational is that the MySQL database will only be accessed for authentication but I'd like to store usernames and passwords in a datbase that is ACID. The added benefit is that the user's data is physically separated from the user entity itself, which I like.
My question is, is this necessary? Should I keep all my data in MongoDB?