With the help of the Firebase Database you can build rich, collaborative apps with secure database access directly from the client-side code. Data is persisted locally, and when offline, events continue working in a responsive way. When the device is reconnected, the Database synchronizes the local data changes with the remote updates that occurred offline.
The Database has a flexible, expression-based rules language, called Firebase Database Security Rules, that defines how your data should be structured and when data can be read from or written to. When integrated with Firebase Authentication, developers can define who has access to what data, and how they can access it.
The Realtime Database is a NoSQL database. The Realtime Database API is designed to only allow operations that can be executed quickly. This enables you to build a great real-time experience that can serve millions of users without compromising on responsiveness.