Firebase Realtime Database

Firebase offers two cloud-based, client-accessible database solutions that support realtime data syncing:

• Cloud Firestore is Firebase's newest database for mobile app development. It builds on the successes of the Realtime Database with a new, more intuitive data model. Cloud Firestore also features richer, faster queries and scales further than the Realtime Database.
• Realtime Database is Firebase's original database. It's an efficient, low-latency solution for mobile apps that require synced states across clients in realtime.

let me talk about Firebase Realtime Database service:

The Firebase Realtime Database is a cloud-hosted database. Data is stored as JSON and synchronized in Realtime to every connected client. When you build cross-platform apps with our Apple platforms, Android, and JavaScript SDKs, all of your clients share one Realtime Database instance and automatically receive updates with the newest data.

Structure Data:

{
  "users": {
    "alovelace": {
      "name": "Ada Lovelace",
      "contacts": { "ghopper": true },
    },
    "ghopper": { ... },
    "eclarke": { ... }
  }
}        

How does it work?

The Firebase Realtime Database lets you build rich, collaborative applications by allowing secure access to the database directly from client-side code. Data is persisted locally, and even while offline, realtime events continue to fire, giving the end user a responsive experience. When the device regains connection, the Realtime Database synchronizes the local data changes with the remote updates that occurred while the client was offline, merging any conflicts automatically.

The Realtime Database provides a flexible, expression-based rules language, called Firebase Realtime Database Security Rules, to define 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 and as such has different optimisations and functionality compared to a relational database. The Realtime Database API is designed to only allow operations that can be executed quickly. This enables you to build a great realtime experience that can serve millions of users without compromising on responsiveness.

Add the Realtime Database SDK to your app:

dependencies {
    // Import the BoM for the Firebase platform
    implementation(platform("com.google.firebase:firebase-bom:32.2.3"))

    // Add the dependency for the Realtime Database library
    // When using the BoM, you don't specify versions in Firebase library dependencies
    implementation("com.google.firebase:firebase-database")
}        

Write to your database:

// Create database
FirebaseDatabase database = FirebaseDatabase.getInstance();

//Table(Parent) Name (User)
DatabaseReference myRef = database.getReference("User");

// Child value
myRef.setValue("Buthaina");        

Read from your database:

// Read from the database
myRef.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        // This method is called once with the initial value and again
        // whenever data at this location is updated.
        String value = dataSnapshot.getValue(String.class);
        Log.d(TAG, "Value is: " + value);
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        // Failed to read value
        Log.w(TAG, "Failed to read value.", error.toException());
    }
});        

Using Cloud Firestore and Realtime Database:

You can use both databases within the same Firebase app or project. Both NoSQL databases can store the same types of data and the client libraries work in a similar manner. Keep in mind the differences outlined above if you decide to use both databases in your app.

for more information please visit this link: Realtime Database