React Native Offline-First with WatermelonDB: Sync Queue and Conflict Resolution

Mobile apps live in an unreliable network world. Users open your app on the subway, at a job site without signal, or in a country with spotty coverage. If every interaction requires an API call, the app is broken for those users. Offline-first design flips this: the local database is the source of truth, changes happen instantly, and sync happens when connectivity returns.

WatermelonDB is the production choice for offline-first React Native apps. It's built on SQLite with a reactive Observables interface, lazy loading for performance, and a synchronization protocol designed for multi-device conflict handling.

Architecture

User Action
  → Write to WatermelonDB (local SQLite)
  → UI updates immediately (reactive)
  → Change queued in sync_queue table
  → NetInfo detects connectivity
  → SyncService pushes changes to API
  → API returns server changes
  → WatermelonDB merges with conflict resolution

Installation

# Core
npm install @nozbe/watermelondb
npm install @nozbe/with-observables

# Native dependencies
npm install @react-native-community/netinfo
npm install react-native-mmkv  # Fast key-value storage for metadata
npx pod-install

Add the Babel plugin:

// babel.config.js
{
  "presets": ["babel-preset-expo"],
  "plugins": [
    ["@babel/plugin-proposal-decorators", { "legacy": true }]
  ]
}

Database Schema and Models

// database/schema.ts
import { appSchema, tableSchema } from "@nozbe/watermelondb";

export const schema = appSchema({
  version: 3,
  tables: [
    tableSchema({
      name: "tasks",
      columns: [
        { name: "title", type: "string" },
        { name: "description", type: "string", isOptional: true },
        { name: "status", type: "string" },          // 'todo' | 'in_progress' | 'done'
        { name: "priority", type: "number" },
        { name: "due_date", type: "number", isOptional: true },
        { name: "project_id", type: "string", isIndexed: true },
        { name: "assignee_id", type: "string", isOptional: true },
        { name: "is_deleted", type: "boolean" },
        { name: "server_id", type: "string", isOptional: true, isIndexed: true },
        { name: "created_at", type: "number" },
        { name: "updated_at", type: "number" },
      ],
    }),
    tableSchema({
      name: "projects",
      columns: [
        { name: "name", type: "string" },
        { name: "color", type: "string" },
        { name: "workspace_id", type: "string", isIndexed: true },
        { name: "is_deleted", type: "boolean" },
        { name: "server_id", type: "string", isOptional: true, isIndexed: true },
        { name: "created_at", type: "number" },
        { name: "updated_at", type: "number" },
      ],
    }),
    tableSchema({
      name: "sync_queue",
      columns: [
        { name: "table_name", type: "string" },
        { name: "record_id", type: "string" },       // local WatermelonDB ID
        { name: "server_id", type: "string", isOptional: true },
        { name: "operation", type: "string" },        // 'create' | 'update' | 'delete'
        { name: "payload", type: "string" },          // JSON
        { name: "attempt_count", type: "number" },
        { name: "last_error", type: "string", isOptional: true },
        { name: "status", type: "string" },           // 'pending' | 'syncing' | 'failed'
        { name: "created_at", type: "number" },
        { name: "updated_at", type: "number" },
      ],
    }),
  ],
});

// database/models/Task.ts
import { Model, field, date, readonly, relation } from "@nozbe/watermelondb";
import { action } from "@nozbe/watermelondb/decorators";

export class Task extends Model {
  static table = "tasks";

  static associations = {
    projects: { type: "belongs_to" as const, key: "project_id" },
  };

  @field("title") title!: string;
  @field("description") description!: string | null;
  @field("status") status!: "todo" | "in_progress" | "done";
  @field("priority") priority!: number;
  @field("due_date") dueDate!: number | null;
  @field("project_id") projectId!: string;
  @field("assignee_id") assigneeId!: string | null;
  @field("is_deleted") isDeleted!: boolean;
  @field("server_id") serverId!: string | null;
  @readonly @date("created_at") createdAt!: Date;
  @date("updated_at") updatedAt!: Date;

  @relation("projects", "project_id") project!: Project;

  @action async markDone() {
    await this.update((task) => {
      task.status = "done";
    });
  }

  @action async softDelete() {
    await this.update((task) => {
      task.isDeleted = true;
    });
  }
}

Database Initialization

// database/index.ts
import { Database } from "@nozbe/watermelondb";
import SQLiteAdapter from "@nozbe/watermelondb/adapters/sqlite";
import { schema } from "./schema";
import { Task } from "./models/Task";
import { Project } from "./models/Project";
import { SyncQueueEntry } from "./models/SyncQueueEntry";
import { migrations } from "./migrations";

const adapter = new SQLiteAdapter({
  schema,
  migrations,
  jsi: true,  // JSI for up to 10× faster SQLite access (Expo + bare RN)
  onSetUpError: (error) => {
    console.error("WatermelonDB setup error:", error);
  },
});

export const database = new Database({
  adapter,
  modelClasses: [Task, Project, SyncQueueEntry],
});

Sync Service

WatermelonDB has a built-in synchronization protocol that works with a compatible server API:

// services/sync.ts
import { synchronize } from "@nozbe/watermelondb/sync";
import { database } from "@/database";
import { MMKV } from "react-native-mmkv";
import NetInfo from "@react-native-community/netinfo";

const storage = new MMKV({ id: "sync-metadata" });

const API_BASE = process.env.EXPO_PUBLIC_API_URL!;

export async function syncDatabase(): Promise<{ success: boolean; error?: string }> {
  // Check connectivity
  const netState = await NetInfo.fetch();
  if (!netState.isConnected) {
    return { success: false, error: "No network connection" };
  }

  const authToken = storage.getString("auth_token");
  if (!authToken) {
    return { success: false, error: "Not authenticated" };
  }

  try {
    await synchronize({
      database,

      // Pull: get changes from server since last sync
      pullChanges: async ({ lastPulledAt, schemaVersion, migration }) => {
        const params = new URLSearchParams({
          last_pulled_at: String(lastPulledAt ?? 0),
          schema_version: String(schemaVersion),
          migration: JSON.stringify(migration),
        });

        const response = await fetch(`${API_BASE}/api/sync/pull?${params}`, {
          headers: { Authorization: `Bearer ${authToken}` },
        });

        if (!response.ok) {
          throw new Error(`Sync pull failed: ${response.status}`);
        }

        const { changes, timestamp } = await response.json();
        return { changes, timestamp };
      },

      // Push: send local changes to server
      pushChanges: async ({ changes, lastPulledAt }) => {
        const response = await fetch(`${API_BASE}/api/sync/push`, {
          method: "POST",
          headers: {
            Authorization: `Bearer ${authToken}`,
            "Content-Type": "application/json",
          },
          body: JSON.stringify({ changes, lastPulledAt }),
        });

        if (!response.ok) {
          const error = await response.json();
          throw new Error(error.message ?? "Sync push failed");
        }
      },

      // Conflict resolution: server wins (simplest correct approach)
      conflictResolver: (tableName, local, remote) => {
        // Server always wins for most fields
        // But preserve local-only fields (like UI state)
        return {
          ...remote,
          // Keep local pending changes for fields not on server
          _status: local._status,
          _changed: local._changed,
        };
      },

      migrationsEnabledAtVersion: 1,
    });

    storage.set("last_sync_at", Date.now());
    return { success: true };
  } catch (error) {
    const message = error instanceof Error ? error.message : "Sync failed";
    console.error("Sync error:", error);
    return { success: false, error: message };
  }
}

Server-Side Sync API

// app/api/sync/pull/route.ts
import { NextRequest, NextResponse } from "next/server";
import { auth } from "@/auth";
import { prisma } from "@/lib/prisma";

export async function GET(req: NextRequest) {
  const session = await auth();
  if (!session?.user) {
    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
  }

  const { searchParams } = req.nextUrl;
  const lastPulledAt = parseInt(searchParams.get("last_pulled_at") ?? "0");
  const workspaceId = session.user.organizationId;
  const timestamp = Date.now();
  const since = new Date(lastPulledAt);

  // Fetch all changes since last pull
  const [createdTasks, updatedTasks, deletedTaskIds] = await Promise.all([
    // New records since last pull
    prisma.task.findMany({
      where: {
        workspaceId,
        createdAt: { gt: since },
        deletedAt: null,
      },
      select: {
        id: true,
        title: true,
        description: true,
        status: true,
        priority: true,
        dueDate: true,
        projectId: true,
        assigneeId: true,
        createdAt: true,
        updatedAt: true,
      },
    }),
    // Updated records since last pull
    prisma.task.findMany({
      where: {
        workspaceId,
        updatedAt: { gt: since },
        createdAt: { lte: since },
        deletedAt: null,
      },
      select: {
        id: true,
        title: true,
        description: true,
        status: true,
        priority: true,
        dueDate: true,
        projectId: true,
        assigneeId: true,
        createdAt: true,
        updatedAt: true,
      },
    }),
    // Soft-deleted records since last pull
    prisma.task
      .findMany({
        where: {
          workspaceId,
          deletedAt: { gt: since },
        },
        select: { id: true },
      })
      .then((tasks) => tasks.map((t) => t.id)),
  ]);

  const changes = {
    tasks: {
      created: createdTasks.map(toWatermelonRecord),
      updated: updatedTasks.map(toWatermelonRecord),
      deleted: deletedTaskIds,
    },
    // Repeat for other tables...
  };

  return NextResponse.json({ changes, timestamp });
}

function toWatermelonRecord(record: any) {
  return {
    id: record.id,
    title: record.title,
    description: record.description ?? null,
    status: record.status,
    priority: record.priority,
    due_date: record.dueDate ? record.dueDate.getTime() : null,
    project_id: record.projectId,
    assignee_id: record.assigneeId ?? null,
    is_deleted: false,
    server_id: record.id,
    created_at: record.createdAt.getTime(),
    updated_at: record.updatedAt.getTime(),
  };
}

// app/api/sync/push/route.ts
import { NextRequest, NextResponse } from "next/server";
import { auth } from "@/auth";
import { prisma } from "@/lib/prisma";

export async function POST(req: NextRequest) {
  const session = await auth();
  if (!session?.user) {
    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
  }

  const { changes } = await req.json();
  const workspaceId = session.user.organizationId;

  // Process task changes
  const taskChanges = changes.tasks;

  await prisma.$transaction(async (tx) => {
    // Created tasks
    for (const task of taskChanges.created ?? []) {
      await tx.task.create({
        data: {
          id: task.server_id ?? undefined,  // Use server ID if provided
          title: task.title,
          description: task.description,
          status: task.status,
          priority: task.priority,
          dueDate: task.due_date ? new Date(task.due_date) : null,
          projectId: task.project_id,
          assigneeId: task.assignee_id,
          workspaceId,
          createdBy: session.user.id,
        },
      });
    }

    // Updated tasks
    for (const task of taskChanges.updated ?? []) {
      await tx.task.updateMany({
        where: {
          id: task.server_id ?? task.id,
          workspaceId,
        },
        data: {
          title: task.title,
          description: task.description,
          status: task.status,
          priority: task.priority,
          dueDate: task.due_date ? new Date(task.due_date) : null,
          updatedAt: new Date(),
        },
      });
    }

    // Deleted tasks (soft delete)
    for (const taskId of taskChanges.deleted ?? []) {
      await tx.task.updateMany({
        where: { id: taskId, workspaceId },
        data: { deletedAt: new Date() },
      });
    }
  });

  return NextResponse.json({ success: true });
}

Network State Detection and Auto-Sync

// services/sync-manager.ts
import NetInfo, { NetInfoState } from "@react-native-community/netinfo";
import { AppState, AppStateStatus } from "react-native";
import { syncDatabase } from "./sync";
import { EventEmitter } from "eventemitter3";

class SyncManager extends EventEmitter {
  private isSyncing = false;
  private syncTimer: ReturnType<typeof setInterval> | null = null;
  private unsubscribeNetInfo: (() => void) | null = null;
  private unsubscribeAppState: (() => void) | null = null;
  private lastSyncAt = 0;
  private MIN_SYNC_INTERVAL = 30_000; // 30 seconds between syncs

  start() {
    // Listen for network changes
    this.unsubscribeNetInfo = NetInfo.addEventListener(
      this.handleNetworkChange
    );

    // Listen for app foregrounding
    this.unsubscribeAppState = AppState.addEventListener(
      "change",
      this.handleAppStateChange
    ).remove;

    // Periodic background sync every 5 minutes
    this.syncTimer = setInterval(() => this.triggerSync("periodic"), 5 * 60_000);

    // Initial sync on start
    this.triggerSync("startup");
  }

  stop() {
    this.unsubscribeNetInfo?.();
    this.unsubscribeAppState?.();
    if (this.syncTimer) {
      clearInterval(this.syncTimer);
      this.syncTimer = null;
    }
  }

  private handleNetworkChange = (state: NetInfoState) => {
    if (state.isConnected && state.isInternetReachable) {
      this.triggerSync("network_restored");
    }
  };

  private handleAppStateChange = (nextState: AppStateStatus) => {
    if (nextState === "active") {
      this.triggerSync("app_foregrounded");
    }
  };

  async triggerSync(reason: string) {
    const now = Date.now();
    if (this.isSyncing || now - this.lastSyncAt < this.MIN_SYNC_INTERVAL) {
      return;
    }

    console.log(`[SyncManager] Triggering sync: ${reason}`);
    this.isSyncing = true;
    this.emit("sync:start", { reason });

    const result = await syncDatabase();
    this.lastSyncAt = Date.now();
    this.isSyncing = false;

    if (result.success) {
      this.emit("sync:success", { reason });
    } else {
      this.emit("sync:error", { reason, error: result.error });
    }
  }
}

export const syncManager = new SyncManager();

// hooks/useSync.ts
import { useState, useEffect } from "react";
import { syncManager } from "@/services/sync-manager";

interface SyncState {
  isSyncing: boolean;
  lastSyncAt: number | null;
  lastError: string | null;
}

export function useSync() {
  const [state, setState] = useState<SyncState>({
    isSyncing: false,
    lastSyncAt: null,
    lastError: null,
  });

  useEffect(() => {
    const onStart = () =>
      setState((s) => ({ ...s, isSyncing: true, lastError: null }));
    const onSuccess = () =>
      setState((s) => ({ ...s, isSyncing: false, lastSyncAt: Date.now() }));
    const onError = ({ error }: { error: string }) =>
      setState((s) => ({ ...s, isSyncing: false, lastError: error }));

    syncManager.on("sync:start", onStart);
    syncManager.on("sync:success", onSuccess);
    syncManager.on("sync:error", onError);

    return () => {
      syncManager.off("sync:start", onStart);
      syncManager.off("sync:success", onSuccess);
      syncManager.off("sync:error", onError);
    };
  }, []);

  const manualSync = () => syncManager.triggerSync("manual");

  return { ...state, manualSync };
}

Reactive UI with WatermelonDB

// components/TaskList.tsx
import React from "react";
import { FlatList, Text, View, StyleSheet } from "react-native";
import { withObservables } from "@nozbe/with-observables";
import { database } from "@/database";
import { Task } from "@/database/models/Task";
import { Q } from "@nozbe/watermelondb";
import { useSyncIndicator } from "@/hooks/useSync";

interface TaskListProps {
  projectId: string;
  tasks: Task[];  // Injected by withObservables
}

// Inner component receives reactive tasks array
function TaskListInner({ tasks }: TaskListProps) {
  return (
    <FlatList
      data={tasks}
      keyExtractor={(item) => item.id}
      renderItem={({ item }) => (
        <View style={styles.taskRow}>
          <Text style={[styles.title, item.status === "done" && styles.done]}>
            {item.title}
          </Text>
          <Text style={styles.status}>{item.status}</Text>
        </View>
      )}
    />
  );
}

// withObservables subscribes to database changes — UI updates automatically
const enhance = withObservables(
  ["projectId"],
  ({ projectId }: { projectId: string }) => ({
    tasks: database
      .get<Task>("tasks")
      .query(
        Q.where("project_id", projectId),
        Q.where("is_deleted", false),
        Q.sortBy("priority", Q.asc),
        Q.sortBy("created_at", Q.asc)
      )
      .observe(),
  })
);

export const TaskList = enhance(TaskListInner);

const styles = StyleSheet.create({
  taskRow: {
    flexDirection: "row",
    justifyContent: "space-between",
    alignItems: "center",
    padding: 16,
    borderBottomWidth: 1,
    borderBottomColor: "#f3f4f6",
  },
  title: { flex: 1, fontSize: 15, color: "#111827" },
  done: { textDecorationLine: "line-through", color: "#9ca3af" },
  status: { fontSize: 12, color: "#6b7280", marginLeft: 8 },
});

Creating Tasks Optimistically

// database/actions/tasks.ts
import { database } from "@/database";
import { Task } from "../models/Task";

interface CreateTaskInput {
  title: string;
  description?: string;
  priority?: number;
  projectId: string;
  assigneeId?: string;
}

export async function createTask(input: CreateTaskInput): Promise<Task> {
  return database.write(async () => {
    const task = await database.get<Task>("tasks").create((t) => {
      t.title = input.title;
      t.description = input.description ?? null;
      t.status = "todo";
      t.priority = input.priority ?? 0;
      t.projectId = input.projectId;
      t.assigneeId = input.assigneeId ?? null;
      t.isDeleted = false;
      t.serverId = null;  // Will be set after sync
    });
    return task;
  });
  // No API call here — WatermelonDB queues the change for sync
}

export async function updateTaskStatus(
  task: Task,
  status: Task["status"]
): Promise<void> {
  await database.write(async () => {
    await task.update((t) => {
      t.status = status;
    });
  });
}

export async function deleteTask(task: Task): Promise<void> {
  await database.write(async () => {
    await task.update((t) => {
      t.isDeleted = true;
    });
  });
}

Conflict Resolution Strategies

For most B2B apps, server wins is correct. The server has the authoritative state after validating business rules.

Cost and Timeline Estimates

See Also

• React Native Animations with Reanimated 3
• React Native Navigation with Expo Router
• React Native Gesture Handler: Swipe, Drag, Pinch
• React Query for Server State Management
• SaaS Audit Trail Implementation

---

Working With Viprasol

Offline-first mobile apps require rethinking every assumption about how data flows — from instantaneous local writes to multi-device sync to conflict resolution that users never see. Our team has built WatermelonDB-backed apps for field service, healthcare, and logistics customers who need their apps to work without connectivity.

What we deliver:

• WatermelonDB schema design optimized for your data model
• Server-side sync API with change tracking and soft deletes
• Conflict resolution strategy tailored to your data types
• Background sync with network state detection
• Migration system for schema evolution without data loss

Talk to our team about your offline mobile app →

Or explore our mobile and web development services.