Firebase Realtime Database

Minimum viable example

import database from "@react-native-firebase/database";

// Write
await database().ref(`messages/${roomId}`).push({
  text: "hi",
  uid: auth().currentUser?.uid,
  ts: database.ServerValue.TIMESTAMP,
});

// Listen
const ref = database().ref(`messages/${roomId}`).limitToLast(50);
const unsub = ref.on("value", (snap) => {
  const msgs = snap.val() ? Object.entries(snap.val()) : [];
  setMessages(msgs);
});
// cleanup: ref.off("value", unsub);

RTDB vs. Firestore — pick one

RTDB charges by bytes downloaded and concurrent connections. Firestore charges by operations. For chat with 100k messages, RTDB tends to be cheaper; for a CRUD app with mostly cold data, Firestore wins.

In a single app, use both: Firestore for the main data model, RTDB for presence and ephemeral live state.

1. Initialize

npx -y firebase-tools@latest init database

Generates database.rules.json. Pick a location at creation; cannot be changed.

{
  "rules": {
    ".read": false,
    ".write": false
  }
}

Default-deny. Open up paths explicitly.

2. Security rules — different from Firestore

Rules are a JSON tree mirroring your data tree, with .read, .write, .validate expressions at each node:

{
  "rules": {
    "users": {
      "$uid": {
        ".read":  "auth != null && auth.uid == $uid",
        ".write": "auth != null && auth.uid == $uid"
      }
    },
    "messages": {
      "$roomId": {
        ".read":  "auth != null",
        ".indexOn": ["ts"],
        "$msgId": {
          ".write": "auth != null && (!data.exists() || data.child('uid').val() == auth.uid)",
          ".validate": "newData.hasChildren(['text', 'uid', 'ts'])
                        && newData.child('text').isString()
                        && newData.child('text').val().length < 500
                        && newData.child('uid').val() == auth.uid
                        && newData.child('ts').val() == now"
        }
      }
    },
    "presence": {
      "$uid": {
        ".read":  "auth != null",
        ".write": "auth != null && auth.uid == $uid"
      }
    }
  }
}

Key differences from Firestore:

• No request.auth.uid — it's auth.uid
• data is existing, newData is the proposed value
• .read / .write cascade down — granting at a parent path grants for everything under it (in Firestore, rules don't cascade)
• .indexOn required to use orderByChild on that key, similar to Firestore composite indexes but per-path

Deploy:

npx -y firebase-tools@latest deploy --only database

3. Presence detection (the killer feature)

RTDB's onDisconnect runs server-side when the client connection drops — even on crash or network failure. Firestore has no equivalent.

import database from "@react-native-firebase/database";
import auth from "@react-native-firebase/auth";

auth().onAuthStateChanged((user) => {
  if (!user) return;
  const presenceRef = database().ref(`presence/${user.uid}`);
  const connectedRef = database().ref(".info/connected");

  connectedRef.on("value", (snap) => {
    if (snap.val() === false) return;

    // Register: when this client disconnects (gracefully or not), clear presence
    presenceRef.onDisconnect().remove().then(() => {
      presenceRef.set({
        state: "online",
        lastChanged: database.ServerValue.TIMESTAMP,
      });
    });
  });
});

The .info/connected node tracks the actual connection state. The onDisconnect().remove() is registered server-side and fires when the connection drops. Result: /presence/{uid} reliably reflects who's online without polling.

For "last seen" in addition to online state:

presenceRef.onDisconnect().set({
  state: "offline",
  lastChanged: database.ServerValue.TIMESTAMP,
});

4. Querying

RTDB queries are limited compared to Firestore — you can sort by one field and filter by one range. No compound queries.

// Recent messages
database().ref(`messages/${roomId}`)
  .orderByChild("ts")
  .limitToLast(50)
  .on("value", handler);

// Messages since timestamp
database().ref(`messages/${roomId}`)
  .orderByChild("ts")
  .startAt(sinceTs)
  .on("value", handler);

For complex filtering, fan-out write to denormalized indexes:

// On message create, also write to /user-messages/{uid}/{msgId}
const updates = {};
updates[`messages/${roomId}/${msgId}`] = msg;
updates[`user-messages/${uid}/${msgId}`] = msg;
await database().ref().update(updates);

Atomic multi-path update — both or neither.

5. Cost — minimize bytes downloaded

RTDB downloads the entire subtree you reference. ref('/messages').on('value', ...) downloads every message in every room — terrible. Always scope to the smallest path:

• ✅ ref('/messages/room-123').limitToLast(50)
• ❌ ref('/').on('value', ...)

Use .indexOn so range queries don't scan client-side.

For data that should only fetch once, use .once('value') instead of .on('value') to avoid the persistent connection cost.

6. Client SDK setup

Web

import { getDatabase, ref, push, onValue, serverTimestamp } from "firebase/database";

const db = getDatabase();
await push(ref(db, `messages/${roomId}`), { text, uid, ts: serverTimestamp() });

Expo / React Native

npx expo install @react-native-firebase/database

import database from "@react-native-firebase/database";

await database().ref(`messages/${roomId}`).push({
  text, uid, ts: database.ServerValue.TIMESTAMP,
});

7. Emulator

npx -y firebase-tools@latest emulators:start --only database

Connect:

// Web
import { connectDatabaseEmulator } from "firebase/database";
if (process.env.NODE_ENV === "development") {
  connectDatabaseEmulator(db, "127.0.0.1", 9000);
}

// RN
database().useEmulator("127.0.0.1", 9000);

8. Common mistakes

• Listening at the root. .on('value') on / downloads the entire database. Always scope to the smallest path.
• No .indexOn for orderByChild. Works locally because the emulator does in-memory sort; in production, rules reject the query.
• Forgetting to call .off() on unmount. Listeners leak; bandwidth + connection bills grow.
• Trying to do compound queries. RTDB can sort by one field, filter by one range. Use denormalized fan-out indexes for everything else.
• Storing growing arrays. RTDB has no array type — what looks like an array is {0: x, 1: y, ...}. Use push() for auto-IDs instead of array indexes.
• Forgetting onDisconnect doesn't fire on the client. It's registered server-side and runs after the connection drops. Test it by killing the device's network, not by closing the app gracefully.
• Mixing RTDB and Firestore listeners on the same data. Pick one source of truth per piece of state.