While the Server and Cloud domains know a myriad of vendors and database types to choose from, the same can't be said for the mobile field, where alternatives have always been scarce.
Realm is a powerful and innovative OpenSource database, that can be used in website and for develop Mobile applications for every platforms in a short time.
Off-line first approach, speed, and advanced features, such as data encryption, two-way sync and reactive architecture, will make one forget the old kind of data access models.