This guide provides a step-by-step walkthrough for building a robust system to handle Sinch SMS delivery report (DLR) callbacks using Fastify, Node.js, and Prisma. You'll learn how to receive, process, and store real-time updates on the status of messages you send via the Sinch SMS API.

By the end of this tutorial, you will have a functional webhook endpoint capable of receiving DLRs from Sinch, processing them, and persisting the status updates to a database. This enables real-time tracking of message delivery, crucial for applications requiring reliable communication and status monitoring.

Project Goals:

• Set up a Node.js project using the Fastify framework.
• Create a secure webhook endpoint to receive HTTP POST requests from Sinch.
• Parse and validate incoming delivery report payloads.
• Store message status updates in a database (using Prisma and PostgreSQL/SQLite).
• Implement basic security, logging, and error handling.
• Demonstrate how to send an SMS via Sinch API configured to trigger these callbacks.
• Provide instructions for local testing using ngrok.

Technologies Used:

• Node.js: JavaScript runtime environment.
• Fastify: High-performance, low-overhead web framework for Node.js. Chosen for its speed, extensibility, and developer-friendly features.
• Sinch SMS API: Used for sending SMS messages and providing delivery status updates via webhooks.
• Prisma: Next-generation ORM for Node.js and TypeScript. Used for database schema management and interaction.
• dotenv: Module to load environment variables from a .env file.
• axios: Promise-based HTTP client for making requests to the Sinch API.
• ngrok: Utility to expose local development servers to the internet for webhook testing.

System Architecture:

graph TD
    subgraph Your Application
        A[Fastify Server] --> B{Webhook Endpoint (/webhooks/sinch/dlr)};
        B --> C[DLR Processing Logic];
        C --> D[(Database / Prisma)];
        E[Send SMS Logic] --> F{Sinch SMS API};
    end

    subgraph External Services
        G[User/Trigger] --> E;
        F -- Sends SMS --> H[End User Device];
        F -- Sends DLR Callback --> B;
    end

    style F fill:#f9f,stroke:#333,stroke-width:2px
    style B fill:#ccf,stroke:#333,stroke-width:2px

Prerequisites:

• Node.js (LTS version recommended, e.g., v18 or v20) and npm/yarn.
• A Sinch account with API credentials (Service Plan ID and API Token). You can find these on your Sinch Customer Dashboard.
• ngrok installed for local testing (ngrok Installation Guide).
• Basic understanding of Node.js, APIs, and webhooks.
• Access to a database (PostgreSQL is used here, but SQLite can be substituted for simpler local development).

1. Setting up the Project

Let's initialize the Node.js project and install the necessary dependencies.

1. Create Project Directory: Open your terminal and create a new directory for the project.

   bash

   mkdir sinch-fastify-dlr
   cd sinch-fastify-dlr
   

2. Initialize Node.js Project: Initialize the project using npm (or yarn).

   bash

   npm init -y
   

   This creates a package.json file.

3. Install Dependencies: Install Fastify, dotenv (for environment variables), axios (for sending SMS), and Prisma (for database interaction).

   bash

   npm install fastify dotenv axios @prisma/client
   

4. Install Development Dependencies: Install Prisma CLI as a development dependency.

   bash

   npm install -D prisma
   

5. Initialize Prisma: Set up Prisma in your project. We'll use PostgreSQL as an example, but you can change the provider if needed (e.g., sqlite).

   bash

   npx prisma init --datasource-provider postgresql
   

   This creates:

   • A prisma directory with a schema.prisma file for defining your database schema.
   • A .env file (if it doesn't exist) for environment variables, including the DATABASE_URL.

6. Configure .gitignore: Create a .gitignore file in the root of your project to avoid committing sensitive information and unnecessary files.

   # .gitignore
   
   # Node
   node_modules/
   npm-debug.log*
   yarn-debug.log*
   yarn-error.log*
   pids/
   *.pid
   *.seed
   *.log
   *.csv
   *.dat
   *.out
   
   # Environment variables
   .env
   .env*.local
   .env*.development
   .env*.production
   
   # Prisma
   prisma/migrations/*/*.sql
   # If using SQLite, ignore the database file:
   prisma/dev.db*
   

7. Project Structure: Your basic project structure should look like this:

   plaintext

   sinch-fastify-dlr/
   ├── prisma/
   │   └── schema.prisma
   ├── node_modules/
   ├── .env
   ├── .gitignore
   ├── package.json
   ├── package-lock.json
   └── server.js  (We will create this next)
   

2. Environment Configuration

Securely manage your API keys and other configuration settings using environment variables.

1. Edit .env File: Open the .env file created by prisma init (or create it if it doesn't exist) and add your Sinch credentials and other configurations. You MUST replace the placeholder values below with your actual credentials and URLs.

   bash

   # .env
   
   # Database Connection (Prisma uses this)
   # Replace with your actual database connection string.
   # Example for PostgreSQL: DATABASE_URL=""postgresql://user:yourpassword@host:port/database?schema=public""
   # Example for SQLite: DATABASE_URL=""file:./prisma/dev.db""
   DATABASE_URL=""postgresql://postgres:yourpassword@localhost:5432/sinch_dlr?schema=public"" # <-- IMPORTANT: Replace with your DB connection string
   
   # Sinch API Credentials
   # Replace with your credentials from the Sinch Dashboard. Keep these secret!
   SINCH_SERVICE_PLAN_ID=""YOUR_SERVICE_PLAN_ID"" # <-- IMPORTANT: Replace with your Service Plan ID
   SINCH_API_TOKEN=""YOUR_API_TOKEN""             # <-- IMPORTANT: Replace with your API Token
   
   # Application Configuration
   PORT=3000
   # Base URL for callbacks (use ngrok URL for local testing_ public domain in production)
   # Replace with your actual ngrok forwarding URL or public server URL.
   CALLBACK_BASE_URL=""https://your-ngrok-subdomain.ngrok.io"" # <-- IMPORTANT: Replace with your ngrok or public URL
   
   # Optional: Basic Auth for Webhook (if configured in Sinch)
   # Uncomment and replace if using Basic Auth security (see Section 7)
   # WEBHOOK_USER=""your_webhook_user""
   # WEBHOOK_PASSWORD=""your_webhook_password""
   
   • DATABASE_URL: Crucial. Replace the example string with your actual database connection string. If using SQLite locally_ ensure the path is correct (e.g._ file:./prisma/dev.db). Make sure the database exists if using PostgreSQL.
   • SINCH_SERVICE_PLAN_ID: Required. Find this on your Sinch Dashboard under API Credentials.
   • SINCH_API_TOKEN: Required. Also found on your Sinch Dashboard. Keep this secret!
   • PORT: The port your Fastify server will listen on (default is 3000).
   • CALLBACK_BASE_URL: Required. This is the publicly accessible base URL where Sinch will send callbacks. When testing locally with ngrok_ you'll replace the placeholder with your ngrok forwarding URL (the https://... one). In production_ this will be your server's public domain/IP.
   • WEBHOOK_USER/WEBHOOK_PASSWORD: (Optional) Only needed if you configure Basic Authentication for your callback URL in the Sinch Dashboard and implement the check in Section 7.

2. Load Environment Variables: We'll load these variables into our application using dotenv in the server.js file. Ensure require('dotenv').config(); is called early in your application startup.

3. Creating the Database Schema

Define the database schema using Prisma to store message status information.

1. Define the Schema (prisma/schema.prisma): Open prisma/schema.prisma and define a model to store the delivery status updates. We add a unique constraint on batchId and recipient to facilitate the upsert operation correctly.

   sql

   // prisma/schema.prisma
   
   generator client {
     provider = "prisma-client-js"
   }
   
   datasource db {
     provider = "postgresql" // Or "sqlite"_ etc. match your .env
     url      = env("DATABASE_URL")
   }
   
   model MessageStatus {
     id             String   @id @default(cuid()) // Unique record ID (auto-generated CUID)
     batchId        String   // Sinch's batch ID
     recipient      String   // Phone number (MSISDN) of the recipient
     status         String   // Sinch delivery status (e.g._ Delivered_ Failed)
     statusCode     Int      // Sinch status code (e.g._ 0_ 401)
     clientReference String?  // Optional client reference sent with the batch
     sinchTimestamp DateTime // Timestamp from the Sinch DLR payload ('at' field)
     receivedAt     DateTime @default(now()) // Timestamp when the callback was received by our server
     updatedAt      DateTime @updatedAt // Timestamp when the record was last updated
   
     // Unique constraint combining batchId and recipient.
     // This allows upsert to find the correct record to update based on these two fields.
     @@unique([batchId_ recipient])
   
     // Index for efficient querying (optional_ as @@unique often creates an index)
     @@index([batchId_ recipient])
   }
   
   • We use a CUID for the primary id_ automatically generated by Prisma.
   • The @@unique([batchId_ recipient]) constraint is key. It ensures that for any given message batch_ each recipient can only have one status record. This constraint is used by the upsert operation in the code.
   • receivedAt defaults to the time the record is created in our database.
   • updatedAt automatically tracks the last modification time.

2. Apply Migrations: Generate the SQL migration files based on your schema and apply them to your database.

   bash

   # Create the migration files based on schema changes
   npx prisma migrate dev --name init-message-status
   
   # (Optional but recommended) Generate Prisma Client types based on the schema
   npx prisma generate
   

   This command will:

   • Connect to the database specified in DATABASE_URL.
   • Create the necessary SQL commands based on schema.prisma (including creating the MessageStatus table and the unique constraint).
   • Apply these commands to your database.
   • Ensure your Prisma Client is up-to-date with the schema.

4. Building the Callback Endpoint

Now_ let's create the Fastify server and the endpoint to receive callbacks.

1. Create server.js: Create a file named server.js in the root of your project.

2. Implement the Server and Endpoint: Add the following code to server.js:

   javascript

   // server.js
   
   'use strict';
   
   // Load environment variables FIRST
   require('dotenv').config();
   
   // Import dependencies
   const Fastify = require('fastify');
   const { PrismaClient } = require('@prisma/client');
   const axios = require('axios'); // Import axios here if using the test route
   
   // Initialize Prisma Client
   const prisma = new PrismaClient();
   
   // Initialize Fastify
   const fastify = Fastify({
       logger: true // Enable built-in Pino logger (outputs to console)
   });
   
   // --- Function to Process Delivery Report Payload ---
   async function processDeliveryReport(payload_ log) {
       // Basic payload structure validation
       if (!payload || typeof payload !== 'object') {
           log.warn({ payload }_ 'Received invalid or non-object DLR payload. Skipping.');
           return;
       }
   
       // Check for expected webhook type (adapt if using different report types)
       const expectedType = 'recipient_delivery_report_sms'; // Common type for per-recipient reports
       if (payload.type !== expectedType) {
            log.warn({ payloadType: payload.type_ expectedType }_ 'Received DLR payload with unexpected type. Skipping.');
            // You might want different handlers for different types if needed.
            return;
       }
   
       // Validate essential fields for the expected type
       const {
           batch_id: batchId_
           recipient_
           status_
           code: statusCode_
           at: sinchTimestampStr_ // ISO 8601 format string
           client_reference: clientReference // Optional
       } = payload;
   
       if (!batchId || !recipient || !status || typeof statusCode === 'undefined' || !sinchTimestampStr) {
           log.warn({ payload }_ 'Received incomplete DLR payload (missing required fields). Skipping.');
           return; // Stop processing if essential data is missing
       }
   
       // Convert timestamp string to Date object_ handle potential errors
       const sinchTimestamp = new Date(sinchTimestampStr);
       if (isNaN(sinchTimestamp.getTime())) {
            log.warn({ payload }_ 'Invalid timestamp format in DLR payload. Skipping.');
            return;
       }
   
       log.info(`Processing status: ${status} (Code: ${statusCode}) for ${recipient} in batch ${batchId}`);
   
       try {
           // Use Prisma's upsert based on the @@unique([batchId_ recipient]) constraint
           // This will CREATE a record if none exists for this batchId/recipient combo_
           // or UPDATE the existing record if one is found.
           const messageStatus = await prisma.messageStatus.upsert({
               where: {
                   // Use the unique constraint defined in schema.prisma
                   batchId_recipient: {
                       batchId: batchId_
                       recipient: recipient_
                   }
               }_
               update: { // Data to update if record exists
                   status: status_
                   statusCode: statusCode_
                   sinchTimestamp: sinchTimestamp_
                   clientReference: clientReference_ // Update client ref if provided
                   // `updatedAt` is handled automatically by Prisma's @updatedAt
               }_
               create: { // Data to insert if record doesn't exist
                   // `id` is automatically generated (CUID)
                   batchId: batchId_
                   recipient: recipient_
                   status: status_
                   statusCode: statusCode_
                   sinchTimestamp: sinchTimestamp_
                   clientReference: clientReference_
                   // `receivedAt` is handled by @default(now())
                   // `updatedAt` is set on creation as well
               }_
           });
   
           log.info({ recordId: messageStatus.id_ batchId_ recipient }_ `Successfully saved/updated status for recipient`);
   
       } catch (dbError) {
            log.error({ err: dbError_ batchId_ recipient }_ 'Database error while saving DLR status');
            // Re-throw the error to be caught by the caller (the webhook route handler)
            // This allows the main handler to log it and potentially implement retry/DLQ logic.
            throw dbError;
       }
   }
   
   // --- Webhook Endpoint for Sinch Delivery Reports ---
   // Path: /webhooks/sinch/dlr (matches common webhook patterns)
   // Define the handler function separately for clarity
   const webhookHandler = async (request_ reply) => {
       const webhookPayload = request.body;
   
       // 1. Log the incoming payload for debugging
       request.log.info({ payload: webhookPayload }, 'Received Sinch DLR callback');
   
       // 2. Immediately acknowledge receipt to Sinch (essential!)
       // Sinch expects a 2xx response quickly. Process the data asynchronously.
       reply.code(200).send({ status: 'OK', message: 'Callback received' });
   
       // 3. Asynchronous Processing (does not delay the 200 OK response)
       // Use setImmediate or process.nextTick for truly non-blocking behavior if needed,
       // but awaiting the async function here is generally fine for typical loads.
       try {
           await processDeliveryReport(webhookPayload, request.log);
       } catch (error) {
           // Log processing errors, but the 200 response has already been sent.
           request.log.error({ err: error, payload: webhookPayload }, 'Error processing DLR payload after acknowledging');
           // Consider adding failed payloads to a dead-letter queue here for retry/inspection.
       }
   };
   
   // Register the webhook route (will be potentially modified by Basic Auth logic later)
   fastify.post('/webhooks/sinch/dlr', webhookHandler);
   
   // --- Health Check Endpoint (Good Practice) ---
   fastify.get('/health', async (request, reply) => {
       try {
           // Optional: Check database connection is alive
           await prisma.$queryRaw`SELECT 1`;
           reply.code(200).send({ status: 'OK', timestamp: new Date().toISOString() });
       } catch (error) {
           request.log.error({ err: error }, 'Health check failed (database connection issue?)');
           reply.code(503).send({ status: 'Service Unavailable' });
       }
   });
   
   // --- Test SMS Sending Route (Add this before start()) ---
   fastify.get('/send-test-sms', async (request, reply) => {
       // Validate 'to' number query parameter
       const to = request.query.to;
       if (!to || !/^\+?[1-9]\d{1,14}$/.test(to)) { // Basic E.164 format check
           return reply.code(400).send({ error: 'Missing or invalid required query parameter: to (must be E.164 format, e.g., +15551234567)' });
       }
       const recipientNumber = to.startsWith('+') ? to : `+${to}`; // Ensure '+' prefix
   
       // Get other optional parameters
       const from = request.query.from || 'SinchTest'; // Optional Sender ID
       const body = request.query.body || `Test SMS from Fastify @ ${new Date().toLocaleTimeString()}`;
   
       const servicePlanId = process.env.SINCH_SERVICE_PLAN_ID;
       const apiToken = process.env.SINCH_API_TOKEN;
       const callbackBaseUrl = process.env.CALLBACK_BASE_URL;
   
       if (!servicePlanId || !apiToken || !callbackBaseUrl) {
           request.log.error('Missing SINCH_SERVICE_PLAN_ID, SINCH_API_TOKEN, or CALLBACK_BASE_URL in .env');
           return reply.code(500).send({ error: 'Server configuration error: Missing required environment variables.' });
       }
   
       // Construct the full callback URL for Sinch
       const callbackUrl = `${callbackBaseUrl}/webhooks/sinch/dlr`;
   
       // Determine Sinch API region URL (adjust 'us' if your account is in another region like 'eu')
       const sinchRegion = 'us'; // Or 'eu', 'ca', 'au', etc.
       const sinchSmsUrl = `https://${sinchRegion}.sms.api.sinch.com/xms/v1/${servicePlanId}/batches`;
   
       const payload = {
           from: from,
           to: [recipientNumber], // Must be an array
           body: body,
           // ** Important: Request per-recipient DLRs directed to your webhook **
           delivery_report: 'per_recipient', // Options: 'summary', 'full', 'per_recipient', 'per_recipient_final'
           callback_url: callbackUrl,
           client_reference: `testRef_${Date.now()}` // Optional: Your custom reference ID
       };
   
       request.log.info({ url: sinchSmsUrl, recipient: recipientNumber, callback: callbackUrl }, `Attempting to send SMS via Sinch`);
   
       try {
           const response = await axios.post(sinchSmsUrl, payload, {
               headers: {
                   'Authorization': `Bearer ${apiToken}`,
                   'Content-Type': 'application/json',
                   'Accept': 'application/json'
               },
               timeout: 10000 // Add a timeout (e.g., 10 seconds)
           });
   
           request.log.info({ batchId: response.data?.id, status: response.status }, 'SMS batch submitted successfully to Sinch.');
           reply.send({
               message: 'SMS batch submission successful!',
               sinchBatchId: response.data?.id,
               callbackUrl: callbackUrl,
               recipient: recipientNumber,
               clientReference: payload.client_reference
           });
   
       } catch (error) {
           const status = error.response?.status;
           const errorData = error.response?.data;
           const errorMessage = error.message;
           request.log.error({ err: errorMessage, status, errorData, url: sinchSmsUrl }, 'Error sending SMS via Sinch API');
   
           reply.code(status || 500).send({
               error: 'Failed to send SMS via Sinch',
               details: errorData || errorMessage,
               status: status
           });
       }
   });
   
   // --- Start the Server ---
   const start = async () => {
       try {
           const port = parseInt(process.env.PORT || '3000', 10);
           const host = '0.0.0.0'; // Listen on all available network interfaces
   
           await fastify.listen({ port: port, host: host });
   
           // Log after server is actually listening
           fastify.log.info(`Server listening on http://${host}:${port}`);
           fastify.log.info(`Webhook endpoint ready at /webhooks/sinch/dlr`);
           if (process.env.CALLBACK_BASE_URL) {
               fastify.log.info(`Ensure Sinch callback URL is configured to: ${process.env.CALLBACK_BASE_URL}/webhooks/sinch/dlr`);
           } else {
               fastify.log.warn('CALLBACK_BASE_URL environment variable is not set. Sinch callbacks will likely fail.');
           }
   
       } catch (err) {
           fastify.log.error(err);
           await prisma.$disconnect(); // Disconnect Prisma on startup failure
           process.exit(1);
       }
   };
   
   // Graceful Shutdown Handling
   const signals = ['SIGINT', 'SIGTERM'];
   signals.forEach(signal => {
     process.on(signal, async () => {
       fastify.log.info(`Received ${signal}, shutting down gracefully...`);
       try {
           await fastify.close(); // Stops accepting new connections and waits for pending requests
           await prisma.$disconnect(); // Closes the database connection pool
           fastify.log.info('Server and database connection closed successfully.');
           process.exit(0);
       } catch (err) {
           fastify.log.error({ err }, 'Error during graceful shutdown');
           process.exit(1);
       }
     });
   });
   
   // Call start after all routes and plugins are defined
   start();
   

   Explanation:

   • Environment Variables: dotenv loads variables from .env. Crucially, it's called first.
   • Prisma Client: An instance is created.
   • Fastify Instance: Created with logging enabled.
   • /webhooks/sinch/dlr Route:
     • Listens for POST requests using the webhookHandler.
     • Logs the incoming payload.
     • Sends 200 OK immediately. This is vital for Sinch.
     • Calls processDeliveryReport asynchronously after responding. Errors during processing are logged but don't cause Sinch retries (unless you implement a Dead Letter Queue).
   • processDeliveryReport Function:
     • Performs validation on the payload structure and type.
     • Validates required fields (batch_id, recipient, status, code, at).
     • Parses the Sinch timestamp string.
     • Uses prisma.messageStatus.upsert with the batchId_recipient unique constraint defined in the schema. This correctly finds an existing record to update or creates a new one.
     • The id field is not set in the create block; Prisma handles the CUID generation.
     • Logs success or database errors. Errors are re-thrown to be caught by the route handler.
   • /health Route: Standard health check.
   • /send-test-sms Route: (Included from Section 5 for completeness) Allows triggering an SMS for testing.
   • Server Start: Listens on the configured PORT and 0.0.0.0. Logs the expected callback URL.
   • Graceful Shutdown: Handles SIGINT (Ctrl+C) and SIGTERM to close the server and database connection cleanly.

5. Sending an SMS with Callback Configuration

To test the webhook, you need to send an SMS using the Sinch API and tell Sinch where to send the delivery report callback. The code for the /send-test-sms route is included in the server.js example in Section 4.

Explanation of the Test Route:

• Takes to (required, E.164 format), from (optional), and body (optional) as query parameters.
• Validates required environment variables (SINCH_SERVICE_PLAN_ID, SINCH_API_TOKEN, CALLBACK_BASE_URL).
• Constructs the Sinch API endpoint URL (ensure the region like us is correct for your account).
• Creates the request payload, critically including:
  • to: Array containing the recipient number in E.164 format.
  • delivery_report: 'per_recipient': Tells Sinch to send a callback for each recipient's status changes. Study Sinch docs if you need summary or final reports, as the payload structure differs.
  • callback_url: The full, public URL of your webhook endpoint (/webhooks/sinch/dlr appended to CALLBACK_BASE_URL).
• Uses axios to make the POST request with the Bearer token.
• Logs success/failure and returns info including the Sinch batch_id.

6. Running and Testing Locally with ngrok

To allow Sinch's servers (on the public internet) to reach your local Fastify application during development, use ngrok.

1. Start Your Database: Ensure your PostgreSQL server is running or your SQLite file path is correct.

2. Start ngrok: Open a new, separate terminal window and run ngrok, telling it to forward HTTP traffic to the port your Fastify server runs on (default 3000).

   bash

   ngrok http 3000
   

3. Get ngrok Forwarding URL: ngrok will display output like this:

   Session Status                online
   Account                       Your Name (Plan: Free)
   ...
   Forwarding                    http://xxxxxxxx.ngrok-free.app -> http://localhost:3000
   Forwarding                    https://xxxxxxxx.ngrok-free.app -> http://localhost:3000
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                 Use this HTTPS URL
   

   Copy the HTTPS forwarding URL (e.g., https://xxxxxxxx.ngrok-free.app). This URL is now publicly accessible as long as ngrok is running. Note: Free ngrok URLs are temporary and change each time you restart ngrok.

4. Update .env: Open your .env file and replace the placeholder CALLBACK_BASE_URL with the actual HTTPS ngrok URL you just copied.

   bash

   # .env
   # ... other variables
   CALLBACK_BASE_URL=""https://xxxxxxxx.ngrok-free.app"" # <-- Paste your ACTUAL ngrok HTTPS URL here
   

   IMPORTANT: You must restart your Fastify server (node server.js) after changing the .env file for the changes to take effect.

5. Start Fastify Server: In your original terminal window (in the project directory), start the server.

   bash

   node server.js
   

   Check the logs. It should confirm it's listening and state the full callback URL it expects Sinch to use (based on your .env setting). If CALLBACK_BASE_URL was missing or incorrect, you'll see a warning.

6. Trigger Test SMS: Open your web browser or use a tool like curl to hit your server's test endpoint. Provide a valid phone number you can check (use your own mobile, in E.164 format like +15551234567).

   bash

   # Using curl (replace with your number):
   curl ""http://localhost:3000/send-test-sms?to=+15551234567""
   
   # Or open in your browser (replace with your number):
   # http://localhost:3000/send-test-sms?to=+15551234567
   

7. Observe Logs and Callbacks:

   • Fastify Terminal: Watch the logs. You should see:
     • The GET request to /send-test-sms.
     • The log indicating successful submission to Sinch (or an error if submission failed).
     • Shortly after, one or more POST requests hitting /webhooks/sinch/dlr as Sinch sends status updates (e.g., Dispatched, Delivered, Failed).
     • Logs from processDeliveryReport showing status processing and database interaction (Successfully saved/updated status...).
   • ngrok Web Interface (Optional): Open http://127.0.0.1:4040 in your browser. This is ngrok's local inspection interface. It shows all requests coming through the tunnel, which is very useful for debugging if your Fastify endpoint isn't receiving callbacks as expected (you can inspect headers, body, timing, etc.).
   • Database: Connect to your database (using psql, pgAdmin, sqlite3, or Prisma Studio: npx prisma studio) and check the MessageStatus table. You should see records being created or updated corresponding to the DLRs received for your test message.

7. Security Considerations

Securing your webhook endpoint is crucial in production.

1. Webhook Signature Verification (Ideal but Check Availability): The most secure method is verifying a signature sent by Sinch (usually HMAC), calculated using a shared secret. Currently, standard HMAC signature verification does not seem to be offered by Sinch for SMS DLR callbacks. Always consult the latest official Sinch documentation for confirmation and potential updates on security features. If signature verification becomes available, prioritize implementing it.

2. Basic Authentication (Alternative): If signature verification isn't available, you can use Basic Authentication as a layer of security. Configure a username and password in the Sinch Dashboard for your callback URL.

   • Installation (if using Basic Auth): You need the @fastify/basic-auth plugin.
     bash

     npm install @fastify/basic-auth
     
   • Implementation: Register the plugin and add a preHandler hook in server.js to check credentials before your webhook logic runs. Add this code before the fastify.post('/webhooks/sinch/dlr', ...) line and before the start() call.
   javascript

   // Add near the top of server.js, after Fastify initialization
   const basicAuth = require('@fastify/basic-auth');
   
   // Register basic auth plugin ONLY if WEBHOOK_USER and WEBHOOK_PASSWORD are set in .env
   if (process.env.WEBHOOK_USER && process.env.WEBHOOK_PASSWORD) {
       fastify.register(basicAuth, {
           validate: async function (username, password, request, reply) {
               // Compare provided credentials with environment variables
               if (username !== process.env.WEBHOOK_USER || password !== process.env.WEBHOOK_PASSWORD) {
                   request.log.warn('Webhook basic authentication failed: Invalid credentials');
                   // Return an error to trigger Fastify's default 401 Unauthorized response
                   return new Error('Authentication required.');
               }
               // No error means authentication succeeded
           },
           authenticate: { realm: 'Sinch Webhook' } // Send WWW-Authenticate header on failure
       });
   
       // Apply the basicAuth hook specifically to the webhook route
       // Define the route options object including the preHandler hook
       const webhookRouteOptions = {
           preHandler: fastify.basicAuth, // Apply authentication check
           handler: webhookHandler // Use the existing handler function
       };
   
       // Re-register the route using the options object, effectively replacing the previous registration
       // Note: Fastify route registration order matters. Ensure this happens before start().
       // It's generally safer to define all routes *then* call start().
       // If the route was already defined, this might override it depending on Fastify version/behavior.
       // A cleaner approach is to conditionally define the route *once* based on env vars.
       // (See alternative structure below if needed)
   
       // Overwrite the previous route registration with the secured one:
       fastify.route({
           method: 'POST',
           url: '/webhooks/sinch/dlr',
           ...webhookRouteOptions // Spread the options including preHandler and handler
       });
   
       fastify.log.info('Webhook endpoint /webhooks/sinch/dlr configured with Basic Authentication.');
   
   } else {
       // If no basic auth credentials in .env, the original route registration
       // fastify.post('/webhooks/sinch/dlr', webhookHandler);
       // remains active (or ensure it's registered here if structuring differently).
       fastify.log.info('Webhook endpoint /webhooks/sinch/dlr configured WITHOUT Basic Authentication.');
   }
   
   // --- Ensure the rest of your routes (like /health, /send-test-sms) are defined here ---
   
   // --- Start the Server (should be called after all routes/plugins are set up) ---
   // start(); // The start() call should be at the very end of the script execution flow
   
   • Important Structure Note: Ensure the fastify.post('/webhooks/sinch/dlr', webhookHandler); line defined earlier is either removed or placed within the else block here, so the route isn't registered twice. The code above assumes the fastify.route({...}) call replaces any previous definition for the same method/URL. Check Fastify documentation for specifics if needed. The key is that the route is registered either with the preHandler (if auth is enabled) or without it (if auth is disabled).

3. HTTPS: Always use HTTPS for your callback URL (CALLBACK_BASE_URL). ngrok provides this automatically for local testing. In production, ensure your server is behind a reverse proxy (like Nginx or Caddy) configured for SSL/TLS termination.

4. Rate Limiting: Implement rate limiting (e.g., using @fastify/rate-limit) on the webhook endpoint to prevent abuse or accidental overload.

5. Input Validation: The current code performs basic validation. Enhance it based on the exact Sinch DLR payload specification to be more robust against unexpected or malformed data.

6. Error Handling & Monitoring: Implement robust error handling. Consider a dead-letter queue (DLQ) mechanism for payloads that fail processing, allowing for later inspection and retry. Monitor logs and endpoint health.