Firebase Realtime Database

Cloud-hosted NoSQL database: stores data as a single JSON tree and synchronizes updates across all connected clients in milliseconds.

Firebase Realtime Database is a cloud-hosted NoSQL solution storing all application data as one massive JSON tree. It uses data synchronization, not traditional HTTP requests, to push updates to all connected clients (iOS, Android, Web) in real-time—often under 100ms—making it ideal for low-latency, collaborative apps like live chat. The SDK provides built-in offline support: data persists locally and automatically syncs when the connection is reestablished, ensuring a responsive user experience. Access control and data validation are managed efficiently using declarative Firebase Security Rules.

https://firebase.google.com/products/realtime-database