The Realtime Database provides support for offline environments. By default, data will be stored locally on your device and automatically managed by the Firebase SDKs.

Persistence is disabled by default when using Realtime Database, however it can be changed to be enabled by default in the firebase.json. You can also enable persistence programmatically, by calling setPersistenceEnabled as early on in your application code as possible:

// index.js
import { AppRegistry } from 'react-native';
import { getDatabase, setPersistenceEnabled } from '@react-native-firebase/database';

setPersistenceEnabled(getDatabase(), true);

AppRegistry.registerComponent('app', () => App);

The API provides a goOffline function to force the Realtime Database SDK to go offline, which can be useful for testing.

import { getDatabase, goOffline } from '@react-native-firebase/database';

goOffline(getDatabase());

Once offline, all operations will continue to execute, instead using a local instance of your database to perform writes to. For example, we write to a record which has a listener whilst offline allowing the listener to be called with the updated data:

import React, { useEffect } from 'react';
import { getDatabase, ref, onValue, goOffline, set } from '@react-native-firebase/database';

function App() {
  useEffect(() => {
    const db = getDatabase();
    const userAgeRef = ref(db, '/users/123/age');

    onValue(userAgeRef, snapshot => {
      console.log('Users age: ', snapshot.val());
    });

    goOffline(db);
    set(userAgeRef, 32).then(() => {
      console.log('User updated whilst offline.');
    });
  }, []);
}

The above code will first execute the onValue listener with data from the remote database.

Once offline, set on the reference node will locally be set to a new value.

The onValue listener however will now subscribe to the local database and provide the new value.

This provides the ability to write code which works in both an online and offline environment without worrying about data synchronization.

The goOnline function re-connects the Realtime Database with the remote database. Any locally written changes performed whilst offline will be automatically synchronized with the remote database.

import { getDatabase, goOnline } from '@react-native-firebase/database';

goOnline(getDatabase());

By default Firebase Database will use up to 10MB of disk space to cache data. If the cache grows beyond this size, Firebase Database will start removing data that hasn't been recently used. If you find that your application caches too little or too much data, call setPersistenceCacheSizeBytes to update the default cache size:

import { getDatabase, setPersistenceCacheSizeBytes } from '@react-native-firebase/database';

setPersistenceCacheSizeBytes(getDatabase(), 2000000); // 2MB