firebase-database

name	firebase-database
description	Use when syncing real-time data, structuring JSON trees, reading/writing, creating listeners, enabling offline persistence, managing presence, sharding, or writing security rules.
license	MIT

Firebase Realtime Database Skill

This skill defines how to correctly implement Firebase Realtime Database in Flutter applications, covering data modeling, queries, real-time sync, offline support, and security rules.

When to Use

Use this skill when working with Firebase Realtime Database for simple data models, low-latency sync, or presence functionality. For rich data models requiring complex queries and high scalability, use Cloud Firestore instead.

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1. Database Selection

Choose Realtime Database when the app needs:

• Simple data models with simple lookups.
• Extremely low-latency synchronization (typical response times under 10ms).
• Deep queries that return an entire subtree by default.
• Access to data at any granularity, down to individual leaf-node values.
• Frequent state-syncing with built-in presence functionality.

Choose Cloud Firestore instead for rich data models requiring queryability, scalability, and high availability.

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2. Setup and Configuration

flutter pub add firebase_database

import 'package:firebase_database/firebase_database.dart';

// After Firebase.initializeApp():
final DatabaseReference ref = FirebaseDatabase.instance.ref();

• Select the database location closest to users.
• Enable persistence for offline capabilities:

FirebaseDatabase.instance.setPersistenceEnabled(true);
FirebaseDatabase.instance.setPersistenceCacheSizeBytes(10000000); // 10MB

Setup Checklist

1. Confirm Firebase.initializeApp() completes before accessing FirebaseDatabase.instance.
2. Set persistence before any read/write operations.
3. Verify connectivity by writing a test value and reading it back.

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3. Data Structure

• Structure data as a flattened JSON tree — avoid deep nesting.
• Maximum nesting depth is 32 levels.
• Design the data structure to support the most common queries.
• Use push IDs for unique identifiers in list-type data:

final newPostKey = FirebaseDatabase.instance.ref().child('posts').push().key;

• Denormalize data when necessary — Realtime Database does not support joins.
• Custom keys must be UTF-8 encoded, max 768 bytes, and cannot contain . $ # [ ] / or ASCII control characters 0-31 or 127.

Flattened Structure Example

// Instead of nesting chat messages inside rooms:
// rooms/roomId/messages/messageId/...

// Flatten into separate top-level paths:
// rooms/roomId: { name: "General", createdBy: "uid1" }
// room-members/roomId: { uid1: true, uid2: true }
// room-messages/roomId/messageId: { text: "Hello", sender: "uid1", timestamp: ... }

This pattern allows reading room metadata without downloading all messages.

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4. Indexing and Querying

• Queries can sort or filter on a property, but not both in the same query.
• Use .indexOn in security rules to index frequently queried fields:

{
  "rules": {
    "dinosaurs": {
      ".indexOn": ["height", "length"]
    }
  }
}

• Queries are deep by default and return the entire subtree.
• Sort with orderByChild(), orderByKey(), or orderByValue():

final query = FirebaseDatabase.instance.ref("dinosaurs").orderByChild("height");

• Limit results with limitToFirst() or limitToLast():

final query = ref.orderByChild("height").limitToFirst(10);

• Range queries using startAt(), endAt(), and equalTo():

// Find users whose name starts with "A"
final query = ref.child("users")
    .orderByChild("name")
    .startAt("A")
    .endAt("A\uf8ff");

---

5. Read and Write Operations

Read once:

final snapshot = await FirebaseDatabase.instance.ref('users/123').get();
if (snapshot.exists) {
  print(snapshot.value);
}

Real-time listener:

final subscription = FirebaseDatabase.instance
    .ref('users/123')
    .onValue
    .listen((event) {
      final data = event.snapshot.value;
      print(data);
    });

// Cancel when no longer needed:
subscription.cancel();

A DatabaseEvent fires every time data changes at the reference, including changes to children.

Write (replace):

await ref.set({
  "name": "John",
  "age": 18,
  "created_at": ServerValue.timestamp,
});

Update (partial):

await ref.update({"age": 19});

Atomic transaction:

final result = await FirebaseDatabase.instance
    .ref('posts/123/likes')
    .runTransaction((currentValue) {
      return Transaction.success((currentValue as int? ?? 0) + 1);
    });
print('Likes: ${result.snapshot.value}');

Multi-path atomic update:

final updates = <String, dynamic>{
  'posts/$postId': postData,
  'user-posts/$uid/$postId': postData,
};
await FirebaseDatabase.instance.ref().update(updates);

• Keep individual write operations under 256KB.
• Use listeners for real-time updates rather than polling.

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6. Designing for Scale

• Realtime Database scales to ~200,000 concurrent connections and 1,000 writes/second per database. For higher scale, shard data across multiple database instances.
• Avoid storing large blobs — use Firebase Storage for files.
• Use server timestamps for consistent time tracking:

await FirebaseDatabase.instance.ref('posts/123/timestamp').set(ServerValue.timestamp);

• Implement fan-out patterns for data accessed from multiple paths.
• Avoid deep nesting — it leads to performance issues when retrieving data.

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7. Offline Capabilities and Presence

FirebaseDatabase.instance.setPersistenceEnabled(true);

// Keep critical paths synced when offline
FirebaseDatabase.instance.ref('important-data').keepSynced(true);

Connection State and Presence

// Detect connection state
FirebaseDatabase.instance.ref('.info/connected').onValue.listen((event) {
  final connected = event.snapshot.value as bool? ?? false;
  if (connected) {
    // Set online status and configure onDisconnect cleanup
    final presenceRef = FirebaseDatabase.instance.ref('status/${uid}');
    presenceRef.set({'online': true, 'last_seen': ServerValue.timestamp});
    presenceRef.onDisconnect().set({
      'online': false,
      'last_seen': ServerValue.timestamp,
    });
  }
});

• Use value events (onValue) to read data and get notified of updates — optimized for online/offline transitions.
• Use get() only when data is needed once; it probes local cache if the server is unavailable.
• onDisconnect() operations are executed server-side, ensuring cleanup even if the app crashes.

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8. Security

{
  "rules": {
    "users": {
      "$uid": {
        ".read": "$uid === auth.uid",
        ".write": "$uid === auth.uid"
      }
    }
  }
}

• Use .read, .write, .validate, and .indexOn to control access and validate data.
• Read and write rules cascade in Realtime Database — a parent rule granting access cannot be revoked by a child rule.
• Use the auth variable to authenticate users in security rules.

Data Validation Example

{
  "rules": {
    "messages": {
      "$messageId": {
        ".validate": "newData.hasChildren(['text', 'sender', 'timestamp'])",
        "text": {
          ".validate": "newData.isString() && newData.val().length <= 500"
        }
      }
    }
  }
}

• Test rules thoroughly using the Firebase console's rules simulator.
• Use the Firebase Emulator Suite for local testing.

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References

• Firebase Realtime Database Flutter documentation
• Structure data
• Read and write data