Handling Offline-First Sync with Couchbase Lite in Flutter

Introduction

Offline-first mobile apps provide a responsive UX regardless of network state. Couchbase Lite is a full-featured embedded NoSQL database with built-in replication, making it a strong choice for Flutter mobile development. This tutorial shows a practical approach to designing an offline-first sync pipeline with Couchbase Lite in Flutter: installation and setup, data modeling for local-first behavior, configuring continuous replication and conflict resolution, and practical error-handling and testing patterns.

Setup Couchbase Lite

Add Couchbase Lite's Flutter bindings to pubspec.yaml and configure native platforms per the official docs. Initialize the database once at app startup, and register a single Database instance per logical dataset to avoid resource contention. Use a lightweight repository layer to wrap Couchbase operations and expose typed models to the UI.

Example: open a database and create a document.

import 'package:couchbase_lite/couchbase_lite.dart';
final db = await Database.open('app_db');
final doc = MutableDocument({'type': 'task', 'title': 'Write sync guide', 'completed': false});
await db.saveDocument(doc);

// Query example
final q = QueryBuilder.select([SelectResult.all()])
  .from(DataSource.database(db));
final res = await q.execute();

Data Modeling And Local First

Design documents for eventual consistency. Use explicit metadata fields: type, createdAt, updatedAt, and a client-generated id (UUID) to enable idempotent writes across devices. Keep documents small and denormalize where reads are frequent—network bandwidth matters when replicating. Implement application-level mutability rules: add a revision vector or lastModified timestamp to choose winners during conflicts. If business logic requires causal ordering, embed a small operation log inside a document or use separate change documents.

Implementing Replication And Conflict Resolution

Use a single continuous replicator for push/pull to the Sync Gateway or Couchbase Server endpoint. Configure replication to use basic auth or certificate pinning, and set continuous to true so changes flow as soon as connectivity returns. Attach a change listener to update UI state reactively when replication status changes.

Conflict resolution must be explicit. Couchbase Lite allows both automatic and custom resolution strategies. For non-critical data, a last-write-wins (LWW) approach using updatedAt can be acceptable. For stronger correctness, register a conflict resolver that merges fields deterministically (e.g., union of array elements, max of numeric counters). Perform merges in memory quickly to avoid blocking the database thread.

Example: create and start a replicator

final target = Endpoint.url(Uri.parse('wss://sync.example.com/db'));
final config = ReplicatorConfiguration(db, target)
  ..authenticator = BasicAuthenticator('user', 'pass')
  ..continuous = true;
final replicator = Replicator(config);
replicator.addChangeListener((change) => print(change.status));
replicator.start();

Best Practices For Sync And Error Handling

Network Resilience: Detect connectivity changes and let the continuous replicator handle retry/backoff. Avoid manual stop/start loops. Provide visual feedback for sync status and last synced time.

Conflict Strategy: Choose a default safe resolver and escalate to user-driven merge for complex cases. For example, mark documents with a needsMerge flag and surface a merge UI when automatic resolution is ambiguous.

Batching And Throttling: Batch local writes that are logically related to avoid producing churn in replication. Use local transactions when writing multiple documents to ensure atomic local updates.

Testing And Observability: Simulate flaky networks in integration tests and automate conflict injection. Log replication checkpoints, error codes, and document IDs involved in conflicts. Structured logs help diagnose desyncs across clients.

Security And Data Privacy: Encrypt sensitive fields before saving to the local database if device-level encryption isn’t sufficient. Secure credentials using platform secure storage and prefer mutual TLS for server endpoints.

Performance: Keep document sizes under a few kilobytes for typical interactive data. Offload large blobs (images, attachments) to a dedicated storage service and reference them by URL or use Couchbase attachments judiciously.

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Conclusion

An offline-first sync architecture in Flutter with Couchbase Lite centers on a reliable local database, clear data modeling for eventual consistency, a continuous replicator with explicit conflict resolution, and robust error handling. Start by modeling documents with client-generated IDs and timestamps, use continuous replication with appropriate auth, and implement deterministic conflict resolution with escalation to the user when needed. With these patterns, you get responsive apps that gracefully reconcile state once connectivity is available, improving both UX and data reliability in mobile development.