Two-factor authentication (2FA), often using One-Time Passwords (OTP) sent via SMS, adds a critical layer of security to user verification processes. This guide provides a complete walkthrough for building a robust SMS OTP verification system using Node.js, the Fastify framework, and the Vonage Verify API.

We will build a backend API with two primary endpoints: one to request an OTP sent to a user's phone number and another to verify the OTP entered by the user. This guide covers everything from project setup to deployment considerations, focusing on production readiness.

Project Overview and Goals

What We're Building:

A backend API service built with Node.js and Fastify that:

1. Accepts a user's phone number.
2. Uses the Vonage Verify API to send an SMS containing a unique OTP to that number.
3. Provides an endpoint to verify the OTP submitted by the user against the one sent by Vonage.

Problem Solved:

This implementation addresses the need for a secure, out-of-band verification method commonly used during registration, login, or sensitive actions within an application. It mitigates risks associated with compromised passwords by requiring possession of the user's registered phone.

Technologies Used:

• Node.js: A JavaScript runtime environment for building server-side applications.
• Fastify: A high-performance, low-overhead web framework for Node.js, chosen for its speed, extensibility, and developer-friendly features like built-in validation and logging.
• Vonage Verify API: A service that handles the complexities of OTP generation, delivery (SMS, voice), and verification logic. We use this to avoid building and maintaining our own OTP system.
• dotenv: A module to load environment variables from a .env file into process.env.
• @vonage/server-sdk: The official Vonage Node.js SDK for interacting with Vonage APIs.

System Architecture:

The system involves a user interacting with a client application, which communicates with our Fastify API. The Fastify API, in turn, uses the Vonage Verify API to handle SMS OTP sending and verification. The typical flow is: User provides phone number -> Client App sends number to Fastify API -> Fastify API asks Vonage to send OTP -> Vonage sends SMS to User -> User enters OTP in Client App -> Client App sends OTP and request ID to Fastify API -> Fastify API asks Vonage to verify -> Vonage confirms/denies -> Fastify API informs Client App -> Client App shows result to User.

Prerequisites:

• Node.js and npm (or yarn): Ensure Node.js (v16 or later recommended) and npm/yarn are installed. Download Node.js
• Vonage API Account: Required to get API credentials. Sign up for Vonage.
• Vonage API Key and Secret: Found on your Vonage API Dashboard after signing up.
• (Optional) Vonage CLI: Useful for managing Vonage applications and numbers. Install via npm install -g @vonage/cli.
• Basic understanding of Node.js, APIs, and asynchronous JavaScript.

Expected Outcome:

A functional Fastify API service capable of sending and verifying SMS OTPs via Vonage, ready for integration into a larger application. The service will include basic security, error handling, and logging.

1. Setting Up the Project

Let's initialize our Node.js project and install Fastify along with necessary dependencies.

1. Create Project Directory:

Open your terminal and create a new directory for the project, then navigate into it.

mkdir fastify-vonage-otp
cd fastify-vonage-otp

2. Initialize npm Project:

npm init -y

This creates a package.json file with default settings.

3. Install Dependencies:

We need Fastify, the Vonage SDK, dotenv for environment variables, and pino-pretty for development logging.

npm install fastify @vonage/server-sdk dotenv pino-pretty

• fastify: The core web framework.
• @vonage/server-sdk: The official Vonage Node.js library.
• dotenv: Loads environment variables from a .env file.
• pino-pretty: Makes Fastify's default JSON logs human-readable during development.

4. Configure package.json for Development:

Add a dev script to package.json to run the server with readable logs using pino-pretty.

{
  ""name"": ""fastify-vonage-otp"",
  ""version"": ""1.0.0"",
  ""description"": """",
  ""main"": ""src/server.js"",
  ""scripts"": {
    ""start"": ""node src/server.js"",
    ""dev"": ""node src/server.js | pino-pretty"",
    ""test"": ""echo \""Error: no test specified\"" && exit 1""
  },
  ""keywords"": [],
  ""author"": """",
  ""license"": ""ISC"",
  ""dependencies"": {
    ""@vonage/server-sdk"": ""^..."",
    ""dotenv"": ""^..."",
    ""fastify"": ""^..."",
    ""pino-pretty"": ""^...""
  }
}

(Note: Replace ^... with actual installed versions if needed, but npm install handles this.)

5. Create Project Structure:

Organize the project for clarity:

mkdir src
mkdir src/routes
touch src/server.js
touch .env
touch .gitignore

• src/: Contains the main application code.
• src/routes/: Will hold our API route definitions.
• src/server.js: The main entry point for the Fastify application.
• .env: Stores sensitive information like API keys (DO NOT commit this file).
• .gitignore: Specifies intentionally untracked files that Git should ignore.

6. Configure .gitignore:

Add node_modules and .env to your .gitignore file to prevent committing them.

# .gitignore

node_modules/
.env
*.log

7. Set Up Environment Variables:

Open the .env file and add your Vonage API credentials and a brand name for the SMS message.

# .env

VONAGE_API_KEY=YOUR_API_KEY
VONAGE_API_SECRET=YOUR_API_SECRET
VONAGE_BRAND_NAME=""YourAppName"" # Keep it short, SMS length limits apply
PORT=3000 # Optional: Define the port for the server

Replace YOUR_API_KEY and YOUR_API_SECRET with the actual credentials from your Vonage API Dashboard.

8. Basic Fastify Server Setup:

Create the initial server configuration in src/server.js.

// src/server.js

// Load environment variables early
require('dotenv').config();

const fastify = require('fastify')({
  logger: true // Enable built-in Pino logger
});

// --- Plugin Registration ---
// Example: Register formbody parser if needed later for simpler forms
// fastify.register(require('@fastify/formbody'));

// --- Vonage Client Setup (Placeholder) ---
// We will add Vonage initialization here later

// --- Route Registration (Placeholder) ---
// We will register our OTP routes here

// --- Error Handling (Placeholder) ---
// We will add custom error handling here

// --- Start Server ---
const start = async () => {
  try {
    const port = process.env.PORT || 3000;
    await fastify.listen({ port: port, host: '0.0.0.0' }); // Listen on all available network interfaces
    fastify.log.info(`Server listening on port ${fastify.server.address().port}`);
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
};

start();

Explanation:

• require('dotenv').config();: Loads variables from .env into process.env. Crucially done before other modules might need them.
• fastify({ logger: true }): Initializes Fastify with logging enabled. Pino is used by default.
• fastify.listen(): Starts the server. We listen on 0.0.0.0 to make it accessible within Docker containers or VMs if needed later.

You can now run npm run dev in your terminal. You should see log output indicating the server is running, likely on port 3000. Stop the server with Ctrl+C.

2. Integrating with Vonage

Now, let's initialize the Vonage SDK client and make it available within our Fastify application.

1. Initialize Vonage Client:

Update src/server.js to create and configure the Vonage client using the environment variables. We'll use Fastify's decorate utility to make the client accessible within route handlers via request.vonage or fastify.vonage.

// src/server.js
require('dotenv').config();

const fastify = require('fastify')({
  logger: true
});
const { Vonage } = require('@vonage/server-sdk'); // Import Vonage

// --- Plugin Registration ---
// fastify.register(require('@fastify/formbody'));

// --- Vonage Client Setup ---
const vonage = new Vonage({
  apiKey: process.env.VONAGE_API_KEY,
  apiSecret: process.env.VONAGE_API_SECRET
});

// Decorate Fastify instance/request with the Vonage client
fastify.decorate('vonage', vonage);

// --- Route Registration (Placeholder) ---
// We will register our OTP routes here

// --- Error Handling (Placeholder) ---
// We will add custom error handling here

// --- Start Server ---
const start = async () => {
  // ... (rest of the start function remains the same)
  try {
    const port = process.env.PORT || 3000;
    await fastify.listen({ port: port, host: '0.0.0.0' }); // Listen on all available network interfaces
    fastify.log.info(`Server listening on port ${fastify.server.address().port}`);
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
};

start();

Explanation:

• We import the Vonage class from the SDK.
• We instantiate Vonage using the API key and secret loaded from .env.
• fastify.decorate('vonage', vonage) adds the vonage instance to Fastify's application context, making it easily accessible in routes and plugins.

3. Implementing the OTP Flow (API Layer)

We'll create two API endpoints within a dedicated route file:

• POST /request-otp: Initiates the OTP process.
• POST /verify-otp: Verifies the submitted OTP.

1. Create Route File:

Create a new file src/routes/otp.js.

touch src/routes/otp.js

2. Define Routes:

Add the route logic to src/routes/otp.js. We'll use Fastify's schema validation for request bodies.

// src/routes/otp.js

async function otpRoutes(fastify, options) {

  // Schema for the /request-otp endpoint body
  const requestOtpSchema = {
    body: {
      type: 'object',
      required: ['phoneNumber'],
      properties: {
        phoneNumber: { type: 'string', description: 'User phone number in E.164 format (e.g., +14155552671)' }
      },
      additionalProperties: false // Disallow extra fields
    }
  };

  // Schema for the /verify-otp endpoint body
  const verifyOtpSchema = {
    body: {
      type: 'object',
      required: ['requestId', 'code'],
      properties: {
        requestId: { type: 'string', description: 'The request ID received from /request-otp' },
        code: { type: 'string', minLength: 4, maxLength: 6, description: 'The OTP code entered by the user' }
      },
      additionalProperties: false // Disallow extra fields
    }
  };

  // --- Endpoint: Request OTP ---
  fastify.post('/request-otp', { schema: requestOtpSchema }, async (request, reply) => {
    const { phoneNumber } = request.body;
    const vonage = fastify.vonage; // Access Vonage client via decorator
    const brand = process.env.VONAGE_BRAND_NAME || 'MyApp'; // Use brand from .env or default

    request.log.info(`Requesting OTP for phone number: ${phoneNumber}`);

    try {
      const response = await vonage.verify.start({
        number: phoneNumber,
        brand: brand,
        // code_length: '6' // Optional: Uncomment to request a 6-digit code (default is 4)
        // workflow_id: 6 // Optional: Specify workflow (e.g., 6 for SMS -> TTS -> TTS)
      });

      if (response.status === '0') {
        request.log.info(`OTP request successful, requestId: ${response.request_id}`);
        return reply.send({ success: true, requestId: response.request_id });
      } else {
        // Non-zero status indicates an error from Vonage before sending
        request.log.error(`Vonage verify start error for ${phoneNumber}: Status ${response.status} - ${response.error_text}`);
        // Use Fastify's error handling (we'll improve this later)
        return reply.status(500).send({ success: false, message: response.error_text || 'Failed to initiate OTP verification.' });
      }

    } catch (error) {
      request.log.error({ err: error, phoneNumber }, 'Error calling Vonage verify start API');
      // Handle network errors or SDK issues
      return reply.status(500).send({ success: false, message: 'An internal server error occurred while requesting OTP.' });
    }
  });

  // --- Endpoint: Verify OTP ---
  fastify.post('/verify-otp', { schema: verifyOtpSchema }, async (request, reply) => {
    const { requestId, code } = request.body;
    const vonage = fastify.vonage; // Access Vonage client

    request.log.info(`Verifying OTP for requestId: ${requestId}`);

    try {
      const response = await vonage.verify.check(requestId, code);

      if (response.status === '0') {
        // Status '0' means successful verification
        request.log.info(`OTP verification successful for requestId: ${requestId}`);
        return reply.send({ success: true, message: 'OTP verified successfully.' });
      } else {
        // Handle specific Vonage error statuses
        request.log.warn(`OTP verification failed for requestId ${requestId}: Status ${response.status} - ${response.error_text}`);
        let userMessage = 'OTP verification failed.';
        let statusCode = 400; // Bad Request by default for verification failures

        // Customize messages based on common statuses (refer to Vonage docs for full list)
        switch (response.status) {
          case '6': // The code provided does not match the expected value
            userMessage = 'Invalid or incorrect OTP code provided.';
            break;
          case '16': // The request specified by the request_id has already been verified
            userMessage = 'This request has already been verified.';
            statusCode = 409; // Conflict
            break;
          case '17': // The wrong code was provided too many times
            userMessage = 'Too many incorrect attempts. Please request a new OTP.';
            break;
          case '101': // No response found (e.g., expired request_id)
             userMessage = 'Verification request not found or expired. Please request a new OTP.';
             statusCode = 404; // Not Found
             break;
          default:
            userMessage = response.error_text || 'OTP verification failed.';
        }
        return reply.status(statusCode).send({ success: false, message: userMessage, errorCode: response.status });
      }

    } catch (error) {
      // Handle potential SDK errors or network issues during check
      request.log.error({ err: error, requestId }, 'Error calling Vonage verify check API');
       // Check if the error is a Vonage specific error structure (might vary by SDK version)
       // Note: The structure of Vonage SDK errors (`error.body`) can change between versions. Always consult the documentation for the specific SDK version you are using.
       if (error.body && error.body.status && error.body.error_text) {
           request.log.error(`Vonage API Error during check: Status ${error.body.status} - ${error.body.error_text}`);
           return reply.status(error.statusCode || 500).send({ success: false, message: error.body.error_text || 'An internal error occurred during OTP verification.' });
       }
      return reply.status(500).send({ success: false, message: 'An internal server error occurred during OTP verification.' });
    }
  });
}

module.exports = otpRoutes;

Explanation:

• otpRoutes(fastify, options): Standard Fastify plugin structure.
• Schemas (requestOtpSchema, verifyOtpSchema): Define the expected structure and types for the request bodies. Fastify automatically validates incoming requests against these schemas and returns a 400 Bad Request error if validation fails. additionalProperties: false prevents unexpected fields.
• /request-otp:
  • Retrieves phoneNumber from the validated request.body.
  • Accesses the decorated vonage client.
  • Calls vonage.verify.start() with the phone number and brand name.
  • Crucially: Checks the response.status. A status of '0' means Vonage accepted the request and will attempt to send the SMS. Other statuses mean an error occurred before sending (e.g., invalid number format, throttling).
  • Returns the requestId on success, which the client needs for the verification step.
  • Includes basic try...catch for network/SDK errors.
• /verify-otp:
  • Retrieves requestId and code from the validated request.body.
  • Calls vonage.verify.check() with the requestId and the user-provided code.
  • Checks response.status: '0' means the code was correct.
  • Handles common non-zero statuses (invalid code, already verified, too many attempts, expired) by returning appropriate HTTP status codes (400, 409, 404) and user-friendly error messages.
  • Includes try...catch for network/SDK errors during the check.

3. Register Routes in Server:

Now, register these routes in src/server.js.

// src/server.js
require('dotenv').config();

const fastify = require('fastify')({
  logger: true
});
const { Vonage } = require('@vonage/server-sdk');
const otpRoutes = require('./routes/otp'); // Import the routes

// --- Plugin Registration ---
// fastify.register(require('@fastify/formbody'));

// --- Vonage Client Setup ---
const vonage = new Vonage({
  apiKey: process.env.VONAGE_API_KEY,
  apiSecret: process.env.VONAGE_API_SECRET
});
fastify.decorate('vonage', vonage);

// --- Route Registration ---
fastify.register(otpRoutes, { prefix: '/api/v1' }); // Register OTP routes under /api/v1 prefix

// --- Error Handling (Placeholder) ---
// We will add custom error handling here

// --- Start Server ---
const start = async () => {
 // ... (start function)
 try {
    const port = process.env.PORT || 3000;
    await fastify.listen({ port: port, host: '0.0.0.0' }); // Listen on all available network interfaces
    fastify.log.info(`Server listening on port ${fastify.server.address().port}`);
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
};

start();

Explanation:

• We import the otpRoutes function.
• fastify.register(otpRoutes, { prefix: '/api/v1' }): Registers all routes defined in otp.js under the /api/v1 path prefix (e.g., /api/v1/request-otp). Using a prefix is good practice for API versioning.

At this point, you have the core API functionality. You can test it using curl or a tool like Postman after starting the server (npm run dev).

Testing with curl:

Replace +1XXXXXXXXXX with a real phone number you can receive SMS on (use E.164 format).

1. Request OTP:

   bash

   curl -X POST http://localhost:3000/api/v1/request-otp \
     -H 'Content-Type: application/json' \
     -d '{""phoneNumber"": ""+1XXXXXXXXXX""}'
   

   Expected Response (Success):

   json

   {""success"":true,""requestId"":""YOUR_UNIQUE_REQUEST_ID""}
   

   You should receive an SMS with a 4-digit code (by default).

2. Verify OTP: Replace YOUR_UNIQUE_REQUEST_ID with the ID from the previous step and YOUR_OTP_CODE with the code from the SMS.

   bash

   curl -X POST http://localhost:3000/api/v1/verify-otp \
     -H 'Content-Type: application/json' \
     -d '{""requestId"": ""YOUR_UNIQUE_REQUEST_ID"", ""code"": ""YOUR_OTP_CODE""}'
   

   Expected Response (Success):

   json

   {""success"":true,""message"":""OTP verified successfully.""}
   

   Expected Response (Incorrect Code):

   json

   {""success"":false,""message"":""Invalid or incorrect OTP code provided."",""errorCode"":""6""}
   

   (Note: The HTTP status code will be 400 for incorrect code)

4. Implementing Proper Error Handling and Logging

Fastify's built-in logger (Pino) is already active. Let's add a centralized error handler to catch unhandled exceptions and format error responses consistently.

1. Add Custom Error Handler:

Update src/server.js to include setErrorHandler.

// src/server.js
require('dotenv').config();

const fastify = require('fastify')({
  logger: true
});
const { Vonage } = require('@vonage/server-sdk');
const otpRoutes = require('./routes/otp');

// --- Plugin Registration ---
// fastify.register(require('@fastify/formbody'));

// --- Vonage Client Setup ---
const vonage = new Vonage({
  apiKey: process.env.VONAGE_API_KEY,
  apiSecret: process.env.VONAGE_API_SECRET
});
fastify.decorate('vonage', vonage);

// --- Route Registration ---
fastify.register(otpRoutes, { prefix: '/api/v1' });

// --- Error Handling ---
fastify.setErrorHandler(function (error, request, reply) {
  // Log the error
  request.log.error(error);

  // Check if it's a validation error from Fastify schema check
  if (error.validation) {
    reply.status(400).send({
      success: false,
      message: 'Validation error',
      errors: error.validation, // Provides details on which fields failed
    });
    return;
  }

  // Check for specific error types you might throw or receive
  // Example: if (error instanceof MyCustomError) { ... }

  // Default fallback for other errors
  // Use the status code from the error if available (e.g., from Vonage errors handled in routes)
  const statusCode = error.statusCode || 500;
  reply.status(statusCode).send({
    success: false,
    message: error.message || 'An unexpected internal server error occurred.',
    // Avoid leaking stack traces in production environments
    ...(process.env.NODE_ENV !== 'production' && { stack: error.stack })
  });
});


// --- Start Server ---
const start = async () => {
 // ... (start function)
  try {
    const port = process.env.PORT || 3000;
    await fastify.listen({ port: port, host: '0.0.0.0' }); // Listen on all available network interfaces
    fastify.log.info(`Server listening on port ${fastify.server.address().port}`);
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
};

start();

Explanation:

• fastify.setErrorHandler: Registers a function to handle errors that occur during request processing after the initial routing but before a reply is sent, or errors explicitly passed to reply.send(error).
• It logs the full error using request.log.error(error).
• It checks if the error is a Fastify validation error (error.validation) and returns a structured 400 response.
• It provides a generic response for other unhandled errors, using the error's statusCode and message if available (falling back to 500).
• Important: It conditionally includes the stack trace only if NODE_ENV is not 'production' to avoid leaking sensitive information.

Logging:

• Fastify automatically logs incoming requests and outgoing responses.
• We added specific logging within the route handlers (request.log.info, request.log.error, request.log.warn) to provide context about the OTP flow.
• During development (npm run dev), pino-pretty formats these JSON logs nicely. In production, you'd typically pipe the raw JSON logs to a log management system (e.g., Datadog, ELK stack, Splunk) for analysis and alerting.

5. Adding Security Features

Security is paramount for an authentication mechanism.

1. Rate Limiting:

Protect against brute-force attacks on both requesting and verifying OTPs.

• Install Rate Limiter Plugin:

  bash

  npm install @fastify/rate-limit
  

• Register and Configure: Add this to the ""Plugin Registration"" section in src/server.js.

  javascript

  // src/server.js
  // ... other imports
  require('dotenv').config(); // Ensure this is at the top
  
  const fastify = require('fastify')({
    logger: true
  });
  const { Vonage } = require('@vonage/server-sdk');
  const otpRoutes = require('./routes/otp');
  
  // --- Plugin Registration ---
  fastify.register(require('@fastify/rate-limit'), {
    max: 100, // Max requests per windowMs per IP (adjust globally)
    timeWindow: '1 minute',
    // Optional: customize error response
    errorResponseBuilder: function (req, context) {
      return {
        success: false,
        message: `Rate limit exceeded, please try again after ${context.after}`,
        code: 'RATE_LIMIT_EXCEEDED',
        retryAfter: context.ttl, // Time-to-live seconds for the ban
      }
    },
    // Optional: Key generator (e.g., use userId if authenticated)
    // keyGenerator: function (req) { return req.headers['x-real-ip'] || req.ip } // Default is IP
  });
  // Register Helmet here too (see step 4 below)
  // fastify.register(require('@fastify/helmet'));
  
  // --- Vonage Client Setup ---
  const vonage = new Vonage({
      apiKey: process.env.VONAGE_API_KEY,
      apiSecret: process.env.VONAGE_API_SECRET
  });
  fastify.decorate('vonage', vonage);
  
  // --- Route Registration ---
  fastify.register(otpRoutes, { prefix: '/api/v1' });
  
  // --- Error Handling ---
  // ... (setErrorHandler from Section 4)
  fastify.setErrorHandler(function (error, request, reply) {
      request.log.error(error);
      if (error.validation) {
          reply.status(400).send({ success: false, message: 'Validation error', errors: error.validation });
          return;
      }
      const statusCode = error.statusCode || 500;
      reply.status(statusCode).send({
          success: false,
          message: error.message || 'An unexpected internal server error occurred.',
          ...(process.env.NODE_ENV !== 'production' && { stack: error.stack })
      });
  });
  
  // --- Start Server ---
  const start = async () => {
      try {
          const port = process.env.PORT || 3000;
          await fastify.listen({ port: port, host: '0.0.0.0' });
          fastify.log.info(`Server listening on port ${fastify.server.address().port}`);
      } catch (err) {
          fastify.log.error(err);
          process.exit(1);
      }
  };
  
  start();
  

• Apply Specific Limits (Optional but Recommended): You can override the global settings within specific route options in src/routes/otp.js. It's wise to have stricter limits on OTP requests than general API usage.

  javascript

  // src/routes/otp.js
  
  // ... schemas ...
  
  async function otpRoutes(fastify, options) {
  
    // --- Endpoint: Request OTP ---
    fastify.post('/request-otp', {
      schema: requestOtpSchema,
      config: { // Add rate limit config specific to this route
        rateLimit: {
          max: 5, // Allow only 5 requests per hour per key
          timeWindow: '1 hour',
           // WARNING: Using `phoneNumber` directly in `keyGenerator` can block legitimate users sharing IPs (e.g., behind NAT).
           // This approach is generally safer *only if* user authentication happens *before* the OTP request.
           // Otherwise, IP-based limiting (`req.ip`) is usually the safer default starting point. Combining IP and phone number is another option.
           // keyGenerator: function (req) { return req.body.phoneNumber || req.ip }
        }
      }
    }, async (request, reply) => {
      // ... route handler logic ...
    });
  
    // --- Endpoint: Verify OTP ---
     fastify.post('/verify-otp', {
      schema: verifyOtpSchema,
      config: { // Add rate limit config specific to this route
        rateLimit: {
          max: 10, // Allow slightly more verification attempts per key
          timeWindow: '15 minutes'
           // Keying by requestId might be useful here, combined with IP
           // keyGenerator: function (req) { return `${req.ip}-${req.body.requestId}` }
        }
      }
    }, async (request, reply) => {
       // ... route handler logic ...
     });
  }
  
  module.exports = otpRoutes;
  

Explanation:

• @fastify/rate-limit: Provides robust rate limiting based on IP address by default.
• max, timeWindow: Control how many requests are allowed within a specific timeframe.
• errorResponseBuilder: Customizes the response when the limit is hit.
• keyGenerator: Allows using factors other than IP (like phone number, requestId, or authenticated user ID) for more granular limiting. Use with caution, as poorly chosen keys can block legitimate users. IP-based is often the simplest starting point.
• Route-specific config.rateLimit overrides the global settings for finer control.

2. Input Validation and Sanitization:

• Fastify's schema validation (used in Section 3) handles basic input validation (checking types, required fields, lengths). This prevents many common injection-style attacks by ensuring data conforms to expectations before it's processed.
• For phone numbers, Vonage performs its own validation, but using a library like google-libphonenumber on the backend before sending to Vonage could provide earlier, more specific feedback to the user about formatting issues.

3. Secure Handling of Secrets:

• Environment Variables: API keys are correctly stored in .env and loaded via dotenv.
• .gitignore: The .env file is in .gitignore to prevent accidental commits.
• Production: In production environments (like Docker, PaaS, servers), use the hosting provider's mechanism for managing environment variables securely (e.g., secrets management tools, platform environment variable settings). Do not deploy .env files directly to production servers.

4. Other Considerations:

• HTTPS: Always run your API over HTTPS in production to encrypt data in transit. Use a reverse proxy like Nginx or Caddy, or platform-provided SSL termination (e.g., AWS ELB, Heroku).
• Helmet: Consider using @fastify/helmet to set various security-related HTTP headers (like X-Frame-Options, Strict-Transport-Security).
  bash

  npm install @fastify/helmet
  
  Register it in src/server.js within the ""Plugin Registration"" section:
  javascript

  // src/server.js - Ensure this is registered in the Plugin Registration section
  fastify.register(require('@fastify/helmet'));
  

6. Testing

Writing automated tests is crucial for ensuring reliability. We'll use tap, Fastify's default test runner.

1. Install tap as a Dev Dependency:

npm install --save-dev tap

2. Update test script in package.json:

{
  ""name"": ""fastify-vonage-otp"",
  ""version"": ""1.0.0"",
  ""description"": """",
  ""main"": ""src/server.js"",
  ""scripts"": {
    ""start"": ""node src/server.js"",
    ""dev"": ""node src/server.js | pino-pretty"",
    ""test"": ""tap \""test/**/*.test.js\""""
  },
  ""keywords"": [],
  ""author"": """",
  ""license"": ""ISC"",
  ""dependencies"": {
    ""@fastify/helmet"": ""^..."",
    ""@fastify/rate-limit"": ""^..."",
    ""@vonage/server-sdk"": ""^..."",
    ""dotenv"": ""^..."",
    ""fastify"": ""^...""
  },
  ""devDependencies"": {
    ""pino-pretty"": ""^..."",
    ""tap"": ""^...""
  }
}

(Note: Added @fastify/helmet, @fastify/rate-limit to dependencies and tap, pino-pretty to devDependencies based on previous steps. Ensure versions match your installation.)

3. Create Test File Structure:

mkdir test
mkdir test/routes
touch test/routes/otp.test.js

4. Write Basic Tests:

Here's an example testing the success path for /request-otp, mocking the Vonage API call. You would typically create a test helper (test/helper.js) to build the Fastify app instance for tests, potentially injecting mocks.

// test/routes/otp.test.js
'use strict'

const { test } = require('tap')
// Assume a helper exists at ../helper.js that builds the app instance
// const { build } = require('../helper')

// Placeholder for building the app - replace with your actual helper
async function build(t, mockVonage) {
  // This function should build your Fastify app similar to server.js
  // but allow injecting a mock Vonage client and disable the real listener.
  // It's complex to show fully without the actual helper code.
  // For now, we'll just return a mock app object for structure.
  const fastify = require('fastify')();

  // Mock the decorator if mockVonage is provided
  if (mockVonage) {
    fastify.decorate('vonage', mockVonage);
  } else {
    // Provide a default mock if none is passed, to avoid errors
    fastify.decorate('vonage', { verify: { start: async () => {}, check: async () => {} } });
  }

  // Register routes, error handlers etc. as in server.js
  const otpRoutes = require('../../src/routes/otp');
  fastify.register(otpRoutes, { prefix: '/api/v1' });
  // Add setErrorHandler from server.js here too

  await fastify.ready(); // Ensure plugins are loaded
  t.teardown(() => fastify.close()); // Close server after test
  return fastify;
}


test('POST /api/v1/request-otp - success', async (t) => {
  // Mock the Vonage verify start function
  const mockVonage = {
    verify: {
      start: async (options) => {
        t.equal(options.number, '+12345678900', 'should pass correct phone number');
        t.ok(options.brand, 'should pass a brand name');
        return { status: '0', request_id: 'mock-request-id-123' };
      }
    }
  };

  // Build the app with the mocked Vonage client
  const app = await build(t, mockVonage);

  const res = await app.inject({
    method: 'POST',
    url: '/api/v1/request-otp',
    payload: {
      phoneNumber: '+12345678900'
    }
  });

  t.equal(res.statusCode, 200, 'returns a status code of 200');
  const payload = JSON.parse(res.payload);
  t.ok(payload.success, 'success field should be true');
  t.equal(payload.requestId, 'mock-request-id-123', 'returns the correct request ID');
});

test('POST /api/v1/request-otp - validation failure (missing phone)', async (t) => {
 // Build app without specific mock (or default mock from helper)
 const app = await build(t);

 const res = await app.inject({
   method: 'POST',
   url: '/api/v1/request-otp',
   payload: { /* Missing phoneNumber */ }
 });

 t.equal(res.statusCode, 400, 'returns a status code of 400 for validation error');
 const payload = JSON.parse(res.payload);
 t.notOk(payload.success, 'success field should be false');
 // Note: Exact error message structure depends on Fastify version and schema options
 t.ok(payload.message.includes('body should have required property \'phoneNumber\''), 'error message indicates missing field');
});

// --- Add more tests ---
// - Test /request-otp Vonage error case (e.g., status != '0')
// - Test /verify-otp success case (mocking vonage.verify.check)
// - Test /verify-otp incorrect code case (mock status '6')
// - Test /verify-otp already verified case (mock status '16')
// - Test /verify-otp expired/not found case (mock status '101')
// - Test rate limiting (might require specific tap setup for timing/mocking rate limiter state)

5. Create Test Helper (Conceptual):

A file like test/helper.js would typically contain a function to build and configure the Fastify app instance for testing, allowing injection of mocks and handling teardown. Creating a full helper is beyond the scope of this rewrite, but the test file structure assumes its existence.

Run tests using npm test. Add comprehensive tests covering success paths, error paths (Vonage errors, validation errors), and security features like rate limiting.