Firebase Cloud Firestore Skill

This skill defines how to correctly implement Cloud Firestore in Flutter applications, covering data modeling, queries, real-time updates, security rules, and scale optimization.

When to Use

Use this skill when:

• Setting up and configuring Cloud Firestore in a Flutter project.
• Designing document and collection structure or planning subcollections.
• Performing read, write, batch, or transaction operations.
• Implementing real-time listeners or paginated queries.
• Optimizing for scale and avoiding write hotspots.
• Writing or debugging Firestore security rules.

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1. Database Selection

Choose Cloud Firestore when the app needs:

• Rich, hierarchical data models with subcollections.
• Complex queries: chaining filters, combining filtering and sorting on a property.
• Transactions that atomically read and write data from any part of the database.
• High availability (typical uptime 99.999%) or critical-level reliability.
• Automatic scaling to millions of concurrent users.

Use Realtime Database instead for simple data models requiring simple lookups and extremely low-latency synchronization (typical response times under 10ms).

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2. Setup and Configuration

flutter pub add cloud_firestore

import 'package:cloud_firestore/cloud_firestore.dart';

final db = FirebaseFirestore.instance; // after Firebase.initializeApp()

Location:

• Select the database location closest to users and compute resources.
• Use multi-region locations for critical apps (maximum availability and durability).
• Use regional locations for lower costs and lower write latency.

iOS/macOS: Consider pre-compiled frameworks to improve build times:

pod 'FirebaseFirestore',
  :git => 'https://github.com/invertase/firestore-ios-sdk-frameworks.git',
  :tag => 'IOS_SDK_VERSION'

Offline persistence is enabled by default on mobile. Configure cache size:

FirebaseFirestore.instance.settings = const Settings(
  persistenceEnabled: true,
  cacheSizeBytes: Settings.CACHE_SIZE_UNLIMITED,
);

---

3. Document Structure

• Avoid document IDs . and .. (special meaning in Firestore paths).
• Avoid forward slashes (/) in document IDs (path separators).
• Do not use monotonically increasing document IDs (e.g., Customer1, Customer2) — causes write hotspots.
• Use Firestore's automatic document IDs when possible:

final docRef = await db.collection("users").add({
  'name': 'Ada Lovelace',
  'email': 'ada@example.com',
  'created_at': FieldValue.serverTimestamp(),
});
print('Created document with ID: ${docRef.id}');

• Avoid these characters in field names (require extra escaping): . [ ] * `
• Use subcollections within documents to organize complex, hierarchical data rather than deeply nested objects.

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4. Indexing

• Firestore queries are indexed by default; query performance is proportional to the result set size, not the dataset size.
• Set collection-level index exemptions to reduce write latency and storage costs.
• Disable Descending and Array indexing for fields that do not need them.
• Exempt string fields with long values that are not used for querying.
• Exempt fields with sequential values (e.g., timestamps) from indexing if not used in queries — avoids the 500 writes/second index limit.
• Add single-field exemptions for TTL fields.
• Exempt large array or map fields not used in queries — avoids the 40,000 index entries per document limit.

---

5. Read and Write Operations

Read All Documents in a Collection

final querySnapshot = await db.collection("users").get();
for (var doc in querySnapshot.docs) {
  print("${doc.id} => ${doc.data()}");
}

Query with Filters

final query = db.collection("users")
    .where("age", isGreaterThanOrEqualTo: 18)
    .orderBy("age")
    .limit(20);

final results = await query.get();

Cursor-Based Pagination

// First page
final first = db.collection("cities").orderBy("name").limit(25);
final firstSnapshot = await first.get();

// Next page using last document as cursor
final lastDoc = firstSnapshot.docs.last;
final next = db.collection("cities")
    .orderBy("name")
    .startAfterDocument(lastDoc)
    .limit(25);

• Do not use offsets for pagination — use cursors to avoid retrieving and being billed for skipped documents.

Write with Server Timestamp

await db.collection("users").doc("user_1").set({
  'name': 'Grace Hopper',
  'updated_at': FieldValue.serverTimestamp(),
});

Batch Write (Atomic, Up to 500 Operations)

final batch = db.batch();
batch.set(db.collection("cities").doc("LA"), {'name': 'Los Angeles'});
batch.update(db.collection("cities").doc("SF"), {'population': 860000});
batch.delete(db.collection("cities").doc("OLD"));
await batch.commit();

Transaction

await db.runTransaction((transaction) async {
  final snapshot = await transaction.get(db.collection("counters").doc("visits"));
  final currentCount = snapshot.get("count") as int;
  transaction.update(snapshot.reference, {"count": currentCount + 1});
});

• Execute independent operations (e.g., a document lookup and a query) in parallel, not sequentially.
• Be aware of write rate limits: ~1 write per second per document.
• For writing a large number of documents, use a bulk writer instead of the atomic batch writer.

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6. Designing for Scale

• Avoid high read or write rates to lexicographically close documents (hotspotting).
• Avoid creating new documents with monotonically increasing fields (like timestamps) at a very high rate.
• Avoid deleting documents in a collection at a high rate.
• Gradually increase traffic when writing to the database at a high rate — ramp up over 5 minutes.
• Avoid queries that skip over recently deleted data — use start_at to find the correct start point.
• Distribute writes across different document paths to avoid contention.
• Firestore scales automatically to ~1 million concurrent connections and 10,000 writes/second.

---

7. Real-time Updates

final subscription = db.collection("messages")
    .where("room", isEqualTo: "general")
    .orderBy("timestamp", descending: true)
    .limit(50)
    .snapshots()
    .listen((querySnapshot) {
      for (var change in querySnapshot.docChanges) {
        switch (change.type) {
          case DocumentChangeType.added:
            print("New message: ${change.doc.data()}");
            break;
          case DocumentChangeType.modified:
            print("Modified: ${change.doc.data()}");
            break;
          case DocumentChangeType.removed:
            print("Removed: ${change.doc.id}");
            break;
        }
      }
    });

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

• Limit the number of simultaneous real-time listeners.
• Detach listeners when they are no longer needed to avoid memory leaks and unnecessary reads.
• Use compound queries to filter data server-side rather than filtering on the client.
• For large collections, use queries to limit the data being listened to — never listen to an entire large collection.

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8. Security

• Always use Firebase Security Rules to protect Firestore data.
• Security rules do not cascade unless a wildcard is used.
• If a query's results might contain data the user does not have access to, the entire query fails.

Example rules for user-owned documents:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, update, delete: if request.auth != null && request.auth.uid == userId;
      allow create: if request.auth != null;
    }
  }
}

• Validate user input before submitting to Firestore to prevent injection attacks.
• Use transactions for operations that require atomic updates to multiple documents.
• Implement proper error handling for all Firestore operations.
• Never store sensitive information in Firestore without proper access controls.

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References

• Cloud Firestore Flutter documentation
• Cloud Firestore best practices
• Cloud Firestore security rules