To get Firebase Realtime Database working offline, you need to turn on offline sync in your app. The Firebase SDKs make this pretty easy and reliable right out of the box.

For Android:

FirebaseDatabase.getInstance().setPersistenceEnabled(true);

For iOS (Objective-C):

[FIRDatabase database].persistenceEnabled = YES;

For iOS (Swift):

Database.database().isPersistenceEnabled = true

For JavaScript:

firebase.database().enablePersistence()

When you enable offline capabilities, the Firebase client SDK keeps a local copy of the data your app is using. Any changes you make are first updated locally, and then synced with the online database once you're back online. Your app can still read and write data even when it's offline. You can do all the usual Create, Read, Update, and Delete (CRUD) operations. The Firebase SDKs save these changes to disk right away, and other offline clients can access them too.

For Android:

DatabaseReference ref = FirebaseDatabase.getInstance().getReference("data");
// Write data offline
ref.setValue("Hello");

For JavaScript:

var ref = firebase.database().ref('data');
// Write data offline
ref.set("Hello");

Firebase has a special spot at '.info/connected' that updates whenever the client's connection state changes. You can use this to manage user presence and handle what your app does when it goes offline.

Example in JavaScript:

var connectedRef = firebase.database().ref(".info/connected");
connectedRef.on("value", (snap) => {
  if (snap.val() === true) {
    console.log("connected");
  } else {
    console.log("not connected");
  }
});

You can manage a user's presence by using '.info/connected', 'onDisconnect()', and a spot in your database to store each user's online or offline status.

Example in JavaScript:

// User's online/offline status location
var userStatusDatabaseRef = firebase.database().ref('/status/' + userId);

// Reference to the /info/connected path
var isOfflineForDatabase = {
  state: 'offline',
  last_changed: firebase.database.ServerValue.TIMESTAMP,
};

var isOnlineForDatabase = {
  state: 'online',
  last_changed: firebase.database.ServerValue.TIMESTAMP,
};

firebase.database().ref('.info/connected')
  .on('value', function(snapshot) {
    if (snapshot.val() == false) {
      // Instead of simply returning, we'll also set ourselves to 'offline' when we're disconnected.
      userStatusDatabaseRef.set(isOfflineForDatabase);
      return;
    };

    userStatusDatabaseRef.onDisconnect().set(isOfflineForDatabase).then(function() {
      userStatusDatabaseRef.set(isOnlineForDatabase);
    });
  });

A quick note about ServerValue.TIMESTAMP: The Firebase server records the time when the data value was written.