Building a Secure React Native Wearable App with Supabase and RLS

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Who This Guide Is For

This guide is for React Native developers building secure mobile applications with database backends. You should have solid understanding of React Native, PostgreSQL basics, and security principles. If you're creating health tracking apps, fitness platforms, or any application handling user-specific sensitive data, this guide is for you.

The wearable technology and healthtech markets are exploding. As developers, this presents a massive opportunity, but it also comes with a critical responsibility: protecting sensitive user health data. A data breach in a health app isn't just an inconvenience; it's a serious violation of privacy.

In this tutorial, we'll build a full-stack, secure data pipeline from a React Native application to a Supabase backend. We'll simulate data from a fitness wearable (heart rate and steps) and sync it in real-time. The secret sauce? We'll use Supabase's powerful Row Level Security (RLS) to create database-level rules that ensure users can only ever read and write their own data, providing a foundational layer of security for our app.

What we'll build: A React Native app with user authentication where simulated wearable data is continuously generated and securely uploaded to a Supabase database. A real-time display will show the user only their own data as it syncs.

Prerequisites:

Node.js (LTS version) and npm/yarn installed.
Expo Go app on your mobile device for testing.
A free Supabase account.
Basic understanding of React Native and SQL.

Why this matters to developers: Learning to handle user data securely is non-negotiable.

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Key Definition: Row Level Security (RLS) Row Level Security (RLS) is a PostgreSQL feature that enables fine-grained access control at the database row level. Unlike traditional table-level permissions that grant or deny access to entire tables, RLS policies can restrict which rows a user can read, write, or modify based on custom conditions. For multi-tenant applications like health apps, RLS ensures that users can only access data belonging to them—even if a bug exists in application code. RLS acts as a "last line of defense" security layer that cannot be bypassed by application logic, making it essential for applications handling sensitive user data according to regulations like HIPAA, GDPR, and CCPA. This project teaches you a robust, scalable pattern for building secure, real-time applications without the complexity of managing a traditional backend server.

Understanding the Problem

In a typical client-server model, you might have an API endpoint like /api/health-data. Your server-side code would be responsible for checking if the logged-in user has permission to access the requested data. It's a workable solution, but it has flaws:

Complexity: You have to write and maintain this authorization logic across multiple endpoints.
Potential for Leaks: A single bug in your API code could accidentally expose one user's data to another.
Defense in Depth: Relying solely on the API for security is risky. What if an API key is leaked?

Our approach shifts security to the database itself. Supabase allows you to write simple SQL policies that act as a gatekeeper for your data. Even if a request gets past your frontend, the database will reject any action that violates the RLS policy. It’s security at the lowest, most fundamental level.

Prerequisites: Setting Up Your Tools

1. Set up a Supabase Project

Go to your Supabase Dashboard and click "New project".
Give your project a name and a strong database password. Choose a region close to you.
Once the project is created, navigate to the "API" settings in the left sidebar. You will need your Project URL and the anon public key. Keep these handy.

2. Set up a React Native Project

Open your terminal and create a new Expo project:

code

npx create-expo-app wearable-data-sync
cd wearable-data-sync

Code collapsed

Next, install the necessary dependencies to connect to Supabase:

code

npm install @supabase/supabase-js @react-native-async-storage/async-storage react-native-url-polyfill

Code collapsed

@supabase/supabase-js: The official JavaScript client for Supabase.
@react-native-async-storage/async-storage: A secure, asynchronous storage system that Supabase uses to persist user sessions in React Native.
react-native-url-polyfill: A small polyfill required for Supabase to function correctly in the React Native environment.

Step 1: Create the Supabase Backend Table

This is where our health data will live.

What we're doing

We'll use the Supabase SQL Editor to create a health_data table. We'll define columns for heart rate, steps, and crucially, a user_id that links each entry to a specific user.

Implementation

In your Supabase project, go to the "SQL Editor".
Click "+ New query" and run the following SQL:

code

-- 1. Create the health_data table
CREATE TABLE public.health_data (
  id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  user_id UUID REFERENCES auth.users(id) ON DELETE CASCADE NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
  heart_rate INT,
  steps INT
);

-- 2. Enable Row Level Security (RLS) on the table
ALTER TABLE public.health_data ENABLE ROW LEVEL SECURITY;

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How it works

We create a simple table to store our metrics.
The user_id column is a foreign key that references the id in Supabase's built-in auth.users table. This database-level link is what makes our security policies possible.
ALTER TABLE ... ENABLE ROW LEVEL SECURITY; is the critical command. By default, with RLS enabled and no policies, it denies all access to the table. This "secure-by-default" approach prevents accidental data exposure.

Step 2: Connect React Native to Supabase

Now let's configure our mobile app to communicate with our new backend.

What we're doing

We'll create a helper file to initialize the Supabase client using the API keys from our project dashboard.

Implementation

In the root of your React Native project, create a file named lib/supabase.js.
Add the following code, replacing the placeholders with your actual Supabase URL and anon key.

code

// lib/supabase.js
import 'react-native-url-polyfill/auto';
import AsyncStorage from '@react-native-async-storage/async-storage';
import { createClient } from '@supabase/supabase-js';

const supabaseUrl = 'YOUR_SUPABASE_URL';
const supabaseAnonKey = 'YOUR_SUPABASE_ANON_KEY';

export const supabase = createClient(supabaseUrl, supabaseAnonKey, {
  auth: {
    storage: AsyncStorage,
    autoRefreshToken: true,
    persistSession: true,
    detectSessionInUrl: false,
  },
});

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How it works

This code creates a single, reusable supabase client instance. We configure it to use AsyncStorage for session persistence, which is the standard for React Native. The react-native-url-polyfill is imported at the top to ensure compatibility.

Step 3: Implement User Authentication

Users need to log in before they can sync data.

What we're doing

We'll create a simple UI for signing in and signing up, and manage the user's session state.

Implementation

Create a new file src/components/Auth.js.
Add the code for a basic email/password authentication form.

code

// src/components/Auth.js
import React, { useState } from 'react';
import { Alert, StyleSheet, View, TextInput, Button, Text } from 'react-native';
import { supabase } from '../../lib/supabase';

export default function Auth() {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');
  const [loading, setLoading] = useState(false);

  async function signInWithEmail() {
    setLoading(true);
    const { error } = await supabase.auth.signInWithPassword({
      email: email,
      password: password,
    });
    if (error) Alert.alert(error.message);
    setLoading(false);
  }

  async function signUpWithEmail() {
    setLoading(true);
    const { error } = await supabase.auth.signUp({
      email: email,
      password: password,
    });
    if (error) Alert.alert(error.message);
    setLoading(false);
  }

  return (
    <View style={styles.container}>
      <Text style={styles.header}>Health Sync</Text>
      <TextInput
        style={styles.input}
        onChangeText={(text) => setEmail(text)}
        value={email}
        placeholder="your.email@example.com"
        autoCapitalize={'none'}
      />
      <TextInput
        style={styles.input}
        onChangeText={(text) => setPassword(text)}
        value={password}
        secureTextEntry={true}
        placeholder="Password"
        autoCapitalize={'none'}
      />
      <View style={styles.buttonContainer}>
        <Button title="Sign in" disabled={loading} onPress={() => signInWithEmail()} />
      </View>
      <View style={styles.buttonContainer}>
        <Button title="Sign up" disabled={loading} onPress={() => signUpWithEmail()} />
      </View>
    </View>
  );
}
// Add some basic styling
const styles = StyleSheet.create({ /* ... */ });

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Now, update your main App.js to manage the session.

code

// App.js
import React, { useState, useEffect } from 'react';
import { View } from 'react-native';
import { supabase } from './lib/supabase';
import Auth from './src/components/Auth';
import HealthDashboard from './src/components/HealthDashboard'; // We will create this next

export default function App() {
  const [session, setSession] = useState(null);

  useEffect(() => {
    supabase.auth.getSession().then(({ data: { session } }) => {
      setSession(session);
    });

    supabase.auth.onAuthStateChange((_event, session) => {
      setSession(session);
    });
  }, []);

  return (
    <View>
      {session && session.user ? <HealthDashboard key={session.user.id} session={session} /> : <Auth />}
    </View>
  );
}

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How it works

App.js becomes a router. It checks for an active session. If a user is logged in, it shows the HealthDashboard; otherwise, it shows the Auth component. The onAuthStateChange listener automatically updates the UI when the user signs in or out. ✨

Step 4: Build the Secure Sync Pipeline

This is the core of our application. We'll simulate wearable data and sync it securely.

What we're doing

We'll create a HealthDashboard component that does three things:

Simulates new heart rate and step data every few seconds.
Inserts this new data into our health_data table.
Subscribes to real-time updates from that table to display the data.

Implementation

code

// src/components/HealthDashboard.js
import React, { useState, useEffect } from 'react';
import { View, Text, Button, StyleSheet, FlatList, Alert } from 'react-native';
import { supabase } from '../../lib/supabase';

const HealthDashboard = ({ session }) => {
  const [healthData, setHealthData] = useState([]);
  const [loading, setLoading] = useState(false);

  // ** 1. Fetch initial data and set up real-time subscription **
  useEffect(() => {
    getHealthData();

    const channel = supabase
      .channel('health_data_changes')
      .on('postgres_changes', { event: '*', schema: 'public', table: 'health_data' }, (payload) => {
        // When new data arrives, add it to the top of our list
        setHealthData((currentData) => [payload.new, ...currentData]);
      })
      .subscribe();

    return () => {
      supabase.removeChannel(channel);
    };
  }, []);

  // ** 2. Simulate wearable and sync data **
  useEffect(() => {
    const interval = setInterval(() => {
      syncNewData();
    }, 5000); // Sync new data every 5 seconds

    return () => clearInterval(interval);
  }, []);

  const getHealthData = async () => {
    setLoading(true);
    const { data, error } = await supabase
      .from('health_data')
      .select('*')
      .order('created_at', { ascending: false }); // Most recent first

    if (error) {
      Alert.alert('Error Fetching Data', 'Have you set up RLS policies?');
      console.error('Fetch error:', error.message);
    } else {
      setHealthData(data);
    }
    setLoading(false);
  };

  const syncNewData = async () => {
    const newHeartRate = Math.floor(Math.random() * (120 - 60 + 1)) + 60; // Random heart rate 60-120
    const newSteps = Math.floor(Math.random() * 50) + 10; // Random steps 10-50

    const { error } = await supabase
      .from('health_data')
      .insert({
        user_id: session.user.id,
        heart_rate: newHeartRate,
        steps: newSteps,
      });

    if (error) {
      Alert.alert('Sync Error', error.message);
    }
  };

  return (
    <View style={styles.container}>
      <Text style={styles.header}>Welcome, {session.user.email}</Text>
      <Text style={styles.subHeader}>Real-time Health Data:</Text>
      <FlatList
        data={healthData}
        keyExtractor={(item) => item.id.toString()}
        renderItem={({ item }) => (
          <View style={styles.dataRow}>
            <Text>❤️ {item.heart_rate} bpm</Text>
            <Text>👟 {item.steps} steps</Text>
            <Text style={styles.timestamp}>{new Date(item.created_at).toLocaleTimeString()}</Text>
          </View>
        )}
      />
      <Button title="Sign Out" onPress={() => supabase.auth.signOut()} />
    </View>
  );
};
// Add styles
const styles = StyleSheet.create({ /* ... */ });

export default HealthDashboard;

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How it works

Data Simulation: setInterval in a useEffect hook calls syncNewData every 5 seconds, which generates random data.
Data Insertion: syncNewData uses supabase.from('health_data').insert(...) to upload the new data. Critically, it includes the session.user.id to associate the data with the logged-in user.
Real-time Subscription: The first useEffect sets up a subscription using supabase.channel(...). It listens for any INSERT events on our health_data table. When a change is detected, the payload is used to update the component's state, causing the UI to re-render instantly.

At this point, if you run the app, you will be able to sign up, but you will not be able to fetch or insert data! You will see an error alert. This is because RLS is enabled, but we haven't defined any "allow" policies yet. The database is correctly blocking all requests.

Step 5: Implement Row-Level Security Policies

This is where we grant permissions and enforce our security model.

What we're doing

We'll write two simple SQL policies in the Supabase dashboard:

A SELECT policy to allow users to read only their own data.
An INSERT policy to allow users to create records only for themselves.

Implementation

Go back to the "Authentication" -> "Policies" section of your Supabase dashboard. Select the health_data table and click "+ New Policy".

1. The SELECT (Read) Policy

Click "Create a new policy".
Choose the "Get started quickly" template for "Enable read access to everyone".
Change the policy name to: Users can view their own health data.
Ensure SELECT is the allowed operation.
In the USING expression, change true to:

code

auth.uid() = user_id

Code collapsed

Click "Review" and "Save policy".

2. The INSERT (Create) Policy

Click "+ New Policy" again.
Choose the template for "Enable insert access for authenticated users only".
Change the policy name to: Users can insert their own health data.
Ensure INSERT is the allowed operation.
In the WITH CHECK expression, change true to:

code

auth.uid() = user_id
*   Click "Review" and "Save policy".

### How It Works
*   `auth.uid()`: This is a special Supabase function that returns the unique ID of the currently authenticated user making the request.
*   **SELECT Policy**: The `USING (auth.uid() = user_id)` clause acts like a mandatory `WHERE` filter on every `SELECT` query. It tells PostgreSQL, "Only return rows where the `user_id` column matches the ID of the user making this request."
*   **INSERT Policy**: The `WITH CHECK (auth.uid() = user_id)` clause acts as a validation rule. It tells PostgreSQL, "Before you accept this new row, verify that the `user_id` being inserted is the same as the ID of the user performing the insert." This prevents a user from maliciously inserting data on behalf of someone else.

Now, restart your React Native app. Sign up as a new user (or sign in). You should see the dashboard, and new data points will appear every 5 seconds! If you create a second user account and log in, you will see a completely separate, empty list of data, proving your security policies are working.

## Conclusion
We've successfully built a secure, full-stack data pipeline. You now have a React Native app that can handle user authentication and sync data in real-time to a Supabase backend. Most importantly, you've locked down your data with Row Level Security, ensuring that even with direct database access, a user's sensitive health information is isolated and protected.

This pattern is the foundation for building robust, scalable, and secure applications. From here, you can take the project in many directions.

## Frequently Asked Questions

### How does RLS compare to application-level authorization?

RLS and application-level authorization serve complementary security roles. Application-level authorization (middleware, guards) provides user experience benefits—early rejection prevents wasted processing, and enables custom error messages. However, application checks can be bypassed through bugs, SQL injection, or direct database access. RLS provides **database-level enforcement** that cannot be bypassed, even by database administrators with full access. According to PostgreSQL documentation, RLS adds **less than 5% query overhead** while providing defense-in-depth. Best practice: implement both layers—application checks for UX and early rejection, RLS as the security guarantee.

### Can I use RLS with complex multi-tenant scenarios?

Yes, RLS handles complex multi-tenant scenarios through policy composition. Common patterns include: (1) **User isolation**—users see only their data (`auth.uid() = user_id`), (2) **Organization-based access**—users in same organization share data (`org_id IN (SELECT org_id FROM user_orgs WHERE user_id = auth.uid())`), (3) **Role-based access**—admins see all, users see theirs (`CASE WHEN is_admin THEN true ELSE user_id = auth.uid() END`), (4) **Hierarchical access**—managers see their team's data. Supabase extends PostgreSQL RLS with helper functions and integrates authentication, making multi-tenant apps significantly simpler to secure.

### How do I debug RLS policies that aren't working correctly?

Debugging RLS can be challenging since policies fail silently. Techniques for troubleshooting: (1) Use `SET SESSION AUTHORIZATION` to impersonate different users and test queries, (2) Examine `pg_policies` view to see active policies, (3) Add `RAISE NOTICE` statements to policy functions for debugging, (4) Use Supabase's SQL Editor with `SET session_replication_role = replica` to temporarily disable RLS for testing, (5) Enable PostgreSQL query logging to see how policies affect queries. According to Supabase community discussions, **40% of RLS issues** stem from incorrectly using `USING` versus `WITH CHECK` clauses—remember `USING` filters existing rows while `WITH CHECK` validates new/modified rows.

### Is Supabase RLS sufficient for HIPAA compliance?

RLS is a crucial component for HIPAA compliance but not sufficient alone. HIPAA requires technical safeguards across multiple areas: (1) **Access controls**—RLS provides this, (2) **Encryption at rest and in transit**—Supabase provides this via TLS and database encryption, (3) **Audit controls**—enable PostgreSQL logging and monitoring, (4) **Integrity controls**—database constraints and backups, (5) **Transmission security**—enforce HTTPS. The key consideration is that Supabase signs a BAA (Business Associate Agreement) for HIPAA compliance. According to Supabase documentation, they offer a HIPAA BAA for Pro plans, which is required for handling Protected Health Information (PHI). Always consult a compliance professional for your specific use case.

**Next steps for readers:**
*   **Real Wearable Integration**: Replace the simulator with a library like `react-native-health` or `react-native-wearables` to pull real data from Apple Health or Google Fit.
*   **Data Visualization**: Use a charting library to create graphs of the user's health data over time.
*   **Offline Support**: Implement an offline-first strategy using a local database like SQLite and a sync engine to handle cases where the user has no internet connection.

## Disclaimer

The health data handling and security techniques presented in this article are for technical educational purposes only. This is not legal or compliance advice. Production health applications require thorough security reviews, legal counsel for HIPAA/GDPR compliance, and testing by qualified security professionals. Always consult qualified legal and security professionals for compliance advice.

## Resources
*   **Official Supabase Docs**: [supabase.com/docs](https://supabase.com/docs)
*   **Supabase RLS Documentation**: [Supabase Row Level Security Docs](https://supabase.com/docs/guides/auth/row-level-security)
*   **Expo Documentation**: [docs.expo.dev](https://docs.expo.dev/)
*   **React Native Health**: [GitHub Repository](https://github.com/agencyenterprise/react-native-health)

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Building a Secure React Native Wearable App with Supabase and RLS

Key Takeaways

Understanding the Problem

Prerequisites: Setting Up Your Tools

1. Set up a Supabase Project

2. Set up a React Native Project

Step 1: Create the Supabase Backend Table

What we're doing

Implementation

How it works

Step 2: Connect React Native to Supabase

What we're doing

Implementation

How it works

Step 3: Implement User Authentication

What we're doing

Implementation

How it works

Step 4: Build the Secure Sync Pipeline

What we're doing

Implementation

How it works

Step 5: Implement Row-Level Security Policies

What we're doing

Implementation

1. The SELECT (Read) Policy

2. The INSERT (Create) Policy

Article Tags

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