How do you handle offline support and data synchronization?

Question

Accepted Answer

Mobile apps often need to work **offline** (no/poor connectivity) and **sync** data when connectivity returns. This involves local data storage, detecting connectivity, queuing changes, and synchronizing — a challenging but important aspect of robust mobile apps.

## Why offline support matters

```text
Mobile devices have UNRELIABLE connectivity (no signal, poor network, airplane mode):
  → apps that break/are useless offline frustrate users
  → OFFLINE-FIRST apps work regardless of connectivity → better, more robust UX
→ Handle: reading cached data offline, making changes offline, syncing when back online.
```

## Key elements of offline support

```text
✓ LOCAL STORAGE — cache data locally (SQLite, WatermelonDB, AsyncStorage/MMKV) so the
  app works with local data offline
✓ DETECT CONNECTIVITY — @react-native-community/netinfo (online/offline status)
✓ OFFLINE WRITES — let users make changes offline; QUEUE them locally
✓ SYNC — when back online, send queued changes to the server; pull server updates
✓ Show appropriate UI (offline indicator, pending/syncing status)
```

## Synchronization challenges

```text
⚠️ CONFLICTS — the same data changed locally AND on the server (or by another device):
  → need a conflict-resolution strategy (last-write-wins, merge, server-wins, manual)
⚠️ ORDERING and retries for queued operations; idempotency
⚠️ Keeping local and server data CONSISTENT eventually
→ Tools: WatermelonDB (offline-first DB with sync), RxDB, or custom sync logic;
  React Query (caching helps); some backends (Firebase, PouchDB/Couch) offer sync.
```

## Why it matters

Understanding offline support and data synchronization is valuable senior-level knowledge because **mobile devices have unreliable connectivity**, and handling offline scenarios well makes apps robust and user-friendly, so it's important for quality mobile apps.

Mobile connectivity is genuinely unreliable (no signal, poor networks, airplane mode), and apps that **break or become useless offline frustrate users** — so supporting offline use (reading cached data, making changes, syncing when reconnected) makes apps significantly more robust and pleasant, an important quality consideration for mobile.

Understanding the **key elements** — **local storage** (caching data so the app works offline, via SQLite/WatermelonDB/MMKV), **detecting connectivity** (NetInfo for online/offline status), **offline writes** (letting users make changes offline and queuing them locally), **synchronization** (sending queued changes and pulling updates when back online), and showing appropriate UI (offline indicators, sync status) — covers what building offline support requires.

Critically, understanding the **synchronization challenges** is what reflects depth: **conflicts** (the same data changed in multiple places, requiring a conflict-resolution strategy like last-write-wins, merge, or manual — a genuinely hard problem), operation ordering, retries, idempotency, and eventual consistency between local and server data.

These make offline sync a challenging, sophisticated problem, and understanding the challenges (not just the happy path) reflects senior-level understanding.

Knowing the tools (WatermelonDB and RxDB for offline-first sync, React Query for caching, sync-capable backends) provides practical options.

Since mobile connectivity is unreliable and robust apps handle offline scenarios (a meaningful UX and quality differentiator), and since offline support and sync involve real challenges (local storage, connectivity detection, queuing, and especially conflict resolution) that understanding well reflects senior expertise, understanding offline support and data synchronization is valuable senior-level knowledge — important for building robust mobile apps that work regardless of connectivity, addressing a genuinely challenging problem (especially sync conflicts) and reflecting the depth expected for senior roles building quality mobile experiences.