Build SMS Marketing Campaigns with Fastify and Twilio - code-examples -

This guide provides a comprehensive walkthrough for building a robust SMS marketing campaign application using the Fastify web framework for Node.js and the Twilio Messaging API. We will cover everything from project setup and core logic to database integration, security, deployment, and monitoring.

The goal is to create a system capable of managing subscriber lists, defining SMS campaigns, sending bulk messages reliably, handling delivery statuses, and managing opt-outs – forming the foundation of a scalable marketing platform. By the end, you'll have a functional backend API ready for integration with a frontend or other services.

Project Overview and Goals

What We're Building:

A backend application featuring a RESTful API to:

  1. Manage subscribers (add, view, remove, track status).
  2. Create and manage SMS marketing campaigns (define message, target audience).
  3. Initiate bulk SMS sending for campaigns.
  4. Receive and process delivery status updates from Twilio.
  5. Handle subscriber opt-outs automatically.

Problem Solved:

This system automates the often complex process of sending targeted SMS messages at scale, managing consent, and tracking delivery, enabling businesses to engage customers effectively via SMS marketing.

Technologies Used:

  • Node.js: JavaScript runtime for building the backend.
  • Fastify: A high-performance, low-overhead web framework for Node.js, chosen for its speed, extensibility, and developer experience.
  • Twilio Messaging API: Used for sending SMS messages and receiving status updates via webhooks. Provides robust infrastructure for global messaging.
  • PostgreSQL: A powerful, open-source relational database for storing subscriber, campaign, and message data.
  • Prisma: A modern database toolkit for Node.js and TypeScript, simplifying database access, migrations, and type safety.
  • dotenv: For managing environment variables securely.
  • (Optional) ngrok: A tool to expose your local development server to the internet. This is primarily useful during development for testing incoming webhooks from Twilio; production environments require a stable public IP address or domain name.

System Architecture Diagram:

graph LR
    A[User/Admin via Frontend/API Client] -- Manages Campaigns/Subscribers --> B(Fastify API);
    B -- Sends SMS via Twilio SDK --> C(Twilio Messaging API);
    C -- Sends SMS --> D(End User Phone);
    C -- Sends Status Webhook --> B;
    B -- Stores/Retrieves Data --> E(PostgreSQL Database w/ Prisma);
    D -- Sends Opt-Out Reply (e.g., STOP) --> C;
    C -- Sends Opt-Out Webhook --> B;

    style B fill:#f9f,stroke:#333,stroke-width:2px
    style C fill:#ccf,stroke:#333,stroke-width:2px
    style E fill:#cfc,stroke:#333,stroke-width:2px

(This diagram shows the basic flow: An API client interacts with the Fastify app, which uses Twilio to send SMS. Twilio sends status updates and opt-out messages back to the Fastify app via webhooks. The Fastify app uses a PostgreSQL database managed by Prisma.)

Prerequisites:

  • Node.js (v18 or later recommended).
  • npm or yarn package manager.
  • Access to a PostgreSQL database instance (local or cloud-based).
  • A Twilio account with:
    • Account SID and Auth Token.
    • A Twilio phone number capable of sending SMS (e.g., a standard long code, Toll-Free number, or Short Code configured appropriately). Consider using a Twilio Messaging Service for better scalability and features like sender ID pools and opt-out handling.
  • (Optional) ngrok installed. This is specifically useful for testing Twilio webhooks during local development by exposing your local server to the internet.
  • Basic understanding of REST APIs, Node.js, and SQL.

1. Setting up the project

Let's initialize the project, install dependencies, and configure the basic structure.

Step 1: Initialize Node.js Project

Open your terminal and create a project directory:

mkdir fastify-twilio-sms-campaigns
cd fastify-twilio-sms-campaigns
npm init -y
# Set package type to module for ES Module syntax
npm pkg set type=module

Step 2: Install Dependencies

Install Fastify, the Twilio SDK, Prisma, dotenv, and necessary Fastify plugins:

npm install fastify twilio prisma @prisma/client dotenv pino-pretty @fastify/formbody @fastify/rate-limit @fastify/helmet
npm install --save-dev prisma supertest jest # Or vitest
  • fastify: The core web framework.
  • twilio: Official Node.js SDK for interacting with the Twilio API.
  • prisma, @prisma/client: Prisma CLI and Client for database interactions.
  • dotenv: Loads environment variables from a .env file.
  • pino-pretty: Development dependency for nicely formatted logs.
  • @fastify/formbody: Parses x-www-form-urlencoded bodies (needed for Twilio webhooks).
  • @fastify/rate-limit: Adds rate limiting capabilities.
  • @fastify/helmet: Adds common security headers.
  • supertest, jest (or vitest): Development dependencies for testing.

Step 3: Initialize Prisma

Set up Prisma to connect to your PostgreSQL database:

npx prisma init --datasource-provider postgresql

This creates a prisma directory with a schema.prisma file and a .env file (if one doesn't exist). Add .env to your .gitignore file!

Step 4: Configure Environment Variables

Open the .env file created by Prisma (or create one: touch .env) and add your database connection URL and Twilio credentials.

# .env

# Database Connection (replace with your actual connection string)
# Format: postgresql://USER:PASSWORD@HOST:PORT/DATABASE?schema=public
DATABASE_URL=""postgresql://user:password@localhost:5432/sms_campaigns?schema=public""

# Twilio Credentials
TWILIO_ACCOUNT_SID=""ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"" # Find on Twilio Console Dashboard
TWILIO_AUTH_TOKEN=""your_auth_token""             # Find on Twilio Console Dashboard
TWILIO_PHONE_NUMBER=""+15551234567""             # Your Twilio SMS-capable number
# OR use a Messaging Service SID for better scaling/features (Recommended)
# TWILIO_MESSAGING_SERVICE_SID=""MGxxxxxxxxxxxxxxxxxxxxxxxxxxxxx""

# Application Settings
API_BASE_URL=""http://localhost:3000"" # Base URL for webhook callbacks (use ngrok URL during dev)
PORT=3000
NODE_ENV=""development"" # or production

# Optional: For simple API Key Auth example (NOT production recommended for storing keys)
# API_KEYS=""key1,key2,key3""
# Optional: For Sentry integration
# SENTRY_DSN=""""
  • DATABASE_URL: Replace with your actual PostgreSQL connection string.
  • TWILIO_ACCOUNT_SID / TWILIO_AUTH_TOKEN: Find these on your Twilio Console dashboard. Keep these secret!
  • TWILIO_PHONE_NUMBER / TWILIO_MESSAGING_SERVICE_SID: Use either your specific Twilio number or the SID of a configured Messaging Service. Using a Messaging Service is highly recommended for production.
  • API_BASE_URL: This is crucial for Twilio webhooks. During local development, this will be your ngrok URL. In production, it's your public application URL.
  • PORT: Port the Fastify server will listen on.

Step 5: Define Project Structure

Create the following directory structure for better organization:

/fastify-twilio-sms-campaigns
|-- /prisma
|   |-- schema.prisma
|   |-- migrations/
|-- /src
|   |-- /routes         # API route definitions
|   |-- /services       # Business logic (Twilio interaction, etc.)
|   |-- /controllers    # Request handlers
|   |-- /schemas        # Request/response validation schemas
|   |-- /db             # Prisma client setup
|   |-- config.js       # Load and export configuration
|   |-- server.js       # Fastify server setup
|   |-- app.js          # Main application entry point
|-- /tests              # Integration/Unit tests
|   |-- /api            # API integration tests
|-- .env
|-- .gitignore
|-- package.json
|-- node_modules/
|-- Dockerfile          # (Added later)
|-- fly.toml            # (Added later, example)
|-- .github/workflows/  # (Added later, example)

Create these directories:

mkdir -p src/routes src/services src/controllers src/schemas src/db tests/api .github/workflows
touch src/config.js src/server.js src/app.js src/db/prisma.js Dockerfile fly.toml .github/workflows/deploy.yml .gitignore
echo "".env"" >> .gitignore
echo ""node_modules"" >> .gitignore
echo ""/dist"" >> .gitignore # If using a build step

Step 6: Configure Base Files

  • src/config.js (Load environment variables):
// src/config.js
import dotenv from 'dotenv';

dotenv.config();

// Basic check for essential variables
if (!process.env.DATABASE_URL || !process.env.TWILIO_ACCOUNT_SID || !process.env.TWILIO_AUTH_TOKEN) {
    console.error(""FATAL ERROR: Required environment variables are missing (DATABASE_URL, TWILIO_ACCOUNT_SID, TWILIO_AUTH_TOKEN)."");
    process.exit(1);
}
if (!process.env.TWILIO_PHONE_NUMBER && !process.env.TWILIO_MESSAGING_SERVICE_SID) {
     console.error(""FATAL ERROR: Either TWILIO_PHONE_NUMBER or TWILIO_MESSAGING_SERVICE_SID must be set."");
    process.exit(1);
}


export default {
    port: parseInt(process.env.PORT || '3000', 10),
    host: process.env.HOST || '0.0.0.0',
    nodeEnv: process.env.NODE_ENV || 'development',
    databaseUrl: process.env.DATABASE_URL,
    twilio: {
        accountSid: process.env.TWILIO_ACCOUNT_SID,
        authToken: process.env.TWILIO_AUTH_TOKEN,
        phoneNumber: process.env.TWILIO_PHONE_NUMBER,
        messagingServiceSid: process.env.TWILIO_MESSAGING_SERVICE_SID,
    },
    apiBaseUrl: process.env.API_BASE_URL || `http://localhost:${process.env.PORT || 3000}`,
    // Simple API Key Auth Example (Insecure storage - use secrets manager in prod)
    apiKeys: new Set((process.env.API_KEYS || '').split(',').filter(Boolean)),
    // Sentry DSN
    sentryDsn: process.env.SENTRY_DSN,
    // Rate Limit Config
    rateLimit: {
        max: parseInt(process.env.RATE_LIMIT_MAX || '100', 10),
        timeWindow: process.env.RATE_LIMIT_WINDOW || '1 minute',
    }
};
  • src/db/prisma.js (Initialize Prisma Client):
// src/db/prisma.js
import { PrismaClient } from '@prisma/client';
import config from '../config.js'; // Import config to access NODE_ENV

const prisma = new PrismaClient({
    log: config.nodeEnv === 'development' ? ['query', 'info', 'warn', 'error'] : ['warn', 'error'],
});

export default prisma;
  • src/server.js (Fastify Server Setup):
// src/server.js
import Fastify from 'fastify';
import formbody from '@fastify/formbody';
import rateLimit from '@fastify/rate-limit';
import helmet from '@fastify/helmet';
import * as Sentry from '@sentry/node';
import { ProfilingIntegration } from '@sentry/profiling-node';
import config from './config.js';
import prisma from './db/prisma.js'; // Import prisma for health check

// Import routes
import campaignRoutes from './routes/campaignRoutes.js';
import subscriberRoutes from './routes/subscriberRoutes.js';
import twilioWebhookRoutes from './routes/twilioWebhookRoutes.js';

// Initialize Sentry (if DSN is provided and in production)
if (config.sentryDsn && config.nodeEnv === 'production') {
    Sentry.init({
        dsn: config.sentryDsn,
        integrations: [new ProfilingIntegration()],
        tracesSampleRate: 1.0, // Adjust in production
        profilesSampleRate: 1.0, // Adjust in production
        environment: config.nodeEnv,
        // release: 'my-project-name@1.0.0', // Optional: Set release version
    });
    console.log('Sentry initialized for production.');
}

export function buildServer(options = {}) {
    const fastify = Fastify({
        logger: config.nodeEnv === 'development'
            ? {
                transport: {
                    target: 'pino-pretty',
                    options: {
                        translateTime: 'HH:MM:ss Z',
                        ignore: 'pid,hostname',
                    },
                },
            }
            : { level: 'info' }, // Use default JSON logger in production
        ...options,
    });

    // Register Sentry Fastify plugin (must be done early)
    if (config.sentryDsn && config.nodeEnv === 'production') {
        fastify.register(import('@sentry/fastify')).after(() => {
            // Custom error handler integrated with Sentry
            fastify.setErrorHandler(async (error, request, reply) => {
                // Log error locally regardless
                 request.log.error(error);
                // Send error to Sentry
                Sentry.captureException(error);
                 // Use Fastify's default handling or customize response
                if (!reply.sent) {
                    const statusCode = error.statusCode && error.statusCode >= 400 ? error.statusCode : 500;
                    const message = statusCode >= 500 ? 'Internal Server Error' : error.message;
                    reply.code(statusCode).send({ message });
                }
            });
        });
    }


    // Register essential plugins
    fastify.register(helmet); // Security headers
    fastify.register(formbody); // Parse form bodies (for Twilio webhooks)
    fastify.register(rateLimit, { // Rate limiting
        max: config.rateLimit.max,
        timeWindow: config.rateLimit.timeWindow,
    });

    // --- Authentication Hook (Simple API Key Example) ---
    // WARNING: Storing keys directly in env vars is NOT secure for production.
    // Use a secrets manager or hashed keys in a database.
    fastify.addHook('onRequest', async (request, reply) => {
        // Exclude webhooks and health check from this simple auth
        if (request.url.startsWith('/api/v1/webhooks') || request.url === '/health') {
            return;
        }
        // Skip auth if no keys are configured (allows open access during initial dev)
        if (config.apiKeys.size === 0 && config.nodeEnv !== 'production') {
             request.log.warn('API Key authentication skipped (no keys configured)');
            return;
        }
         if (config.apiKeys.size === 0 && config.nodeEnv === 'production') {
             request.log.error('CRITICAL: API Key authentication mandatory in production, but no keys configured!');
             reply.code(500).send({ message: 'Server configuration error' });
             return reply; // Prevent further processing by returning reply
         }


        const apiKey = request.headers['x-api-key'];
        if (!apiKey || !config.apiKeys.has(apiKey)) {
            request.log.warn(`Unauthorized API access attempt. Path: ${request.url}, Key Provided: ${!!apiKey}`);
            reply.code(401).send({ message: 'Unauthorized' });
            return reply; // Stop processing by returning reply
        }
        // Optional: Log successful validation (can be noisy)
        // request.log.info(`API Key validated for request: ${request.url}`);
    });
    // --- End Authentication Hook ---


    // Register API routes
    fastify.register(campaignRoutes, { prefix: '/api/v1/campaigns' });
    fastify.register(subscriberRoutes, { prefix: '/api/v1/subscribers' });
    fastify.register(twilioWebhookRoutes, { prefix: '/api/v1/webhooks/twilio' });

    // Health check endpoint
    fastify.get('/health', async (request, reply) => {
        try {
            await prisma.$queryRaw`SELECT 1`; // Check DB connection
            return { status: 'ok', timestamp: new Date().toISOString(), checks: { database: 'ok' } };
        } catch (dbError) {
            request.log.error({ err: dbError }, 'Health check failed: Database connection error.');
            reply.code(503); // Service Unavailable
            return { status: 'error', timestamp: new Date().toISOString(), checks: { database: 'error' }, error: 'Database connection failed' };
        }
    });

    return fastify;
}
  • src/app.js (Main Entry Point):
// src/app.js
import { buildServer } from './server.js';
import config from './config.js';
import prisma from './db/prisma.js';

const server = buildServer();

const start = async () => {
    try {
        // Test DB connection on startup
        await prisma.$connect();
        server.log.info('Database connection successful.');

        await server.listen({ port: config.port, host: config.host });
        // Note: Fastify logs listening address automatically on successful listen

    } catch (err) {
        server.log.error(err, 'Failed to start server or connect to database');
        await prisma.$disconnect().catch(e => server.log.error(e, 'Failed to disconnect prisma'));
        process.exit(1);
    }
};

// Graceful shutdown
const shutdown = async (signal) => {
    server.log.info(`Received ${signal}. Shutting down gracefully...`);
    try {
        await server.close();
        server.log.info('HTTP server closed.');
        await prisma.$disconnect();
        server.log.info('Database connection closed.');
        process.exit(0);
    } catch (err) {
        server.log.error(err, 'Error during graceful shutdown');
        process.exit(1);
    }
};

// Handle termination signals
process.on('SIGINT', () => shutdown('SIGINT'));
process.on('SIGTERM', () => shutdown('SIGTERM'));

// Handle unhandled promise rejections
process.on('unhandledRejection', (reason, promise) => {
  server.log.error({ reason, promise }, 'Unhandled Rejection at Promise');
  // Consider whether to exit or let Sentry capture it
});

// Handle uncaught exceptions
process.on('uncaughtException', (error) => {
  server.log.fatal(error, 'Uncaught Exception');
  // It's generally recommended to exit after an uncaught exception
  shutdown('uncaughtException').finally(() => process.exit(1));
});


start();

Step 7: Add Start and Test Scripts

In your package.json, add scripts:

// package.json (add within ""scripts"")
""scripts"": {
  ""start"": ""node src/app.js"",
  ""dev"": ""node --watch src/app.js"",
  ""test"": ""jest"",
  ""test:watch"": ""jest --watch"",
  ""test:coverage"": ""jest --coverage"",
  ""db:migrate:dev"": ""npx prisma migrate dev"",
  ""db:migrate:deploy"": ""npx prisma migrate deploy"",
  ""db:generate"": ""npx prisma generate"",
  ""db:studio"": ""npx prisma studio""
},
// Add Jest config if needed (or use vitest.config.js)
""jest"": {
  ""testEnvironment"": ""node"",
  ""coverageProvider"": ""v8""
}

You can now run npm run dev for development or npm start to run the application.

2. Creating a database schema and data layer

We need tables to store campaigns, subscribers, and individual message logs.

Step 1: Define Prisma Schema

Open prisma/schema.prisma and define the models:

// prisma/schema.prisma

generator client {
  provider = ""prisma-client-js""
}

datasource db {
  provider = ""postgresql""
  url      = env(""DATABASE_URL"")
}

// Note on IDs: Using CUIDs (default) which are collision-resistant unique IDs.
// UUIDs are another common alternative if preferred.

model Subscriber {
  id        String   @id @default(cuid())
  phone     String   @unique // E.164 format recommended (e.g., +15551234567)
  firstName String?
  lastName  String?
  status    String   @default(""active"") // e.g., active, unsubscribed, bounced
  createdAt DateTime @default(now())
  updatedAt DateTime @updatedAt

  // Many-to-many relationship: a subscriber can belong to multiple campaigns
  campaigns Campaign[] @relation(""CampaignSubscribers"")
  messageLogs MessageLog[] // One-to-many: a subscriber receives multiple messages
}

model Campaign {
  id          String   @id @default(cuid())
  name        String
  messageBody String   @db.Text // Use Text for potentially long messages
  status      String   @default(""draft"") // e.g., draft, scheduled, sending, sent, failed
  scheduledAt DateTime? // Optional: for scheduled campaigns
  sentAt      DateTime? // When the last message batch was initiated
  createdAt   DateTime @default(now())
  updatedAt   DateTime @updatedAt

  // Many-to-many relationship
  subscribers Subscriber[] @relation(""CampaignSubscribers"")
  messageLogs MessageLog[] // One-to-many: a campaign generates multiple messages
}

model MessageLog {
  id             String   @id @default(cuid())
  subscriberId   String
  campaignId     String?  // Can be null if message not part of campaign (e.g., direct send)
  twilioSid      String   @unique // Twilio Message SID (critical for status updates)
  status         String   // e.g., queued, sending, sent, delivered, undelivered, failed (matches Twilio statuses)
  errorCode      Int?     // Twilio error code if failed/undelivered
  errorMessage   String?  @db.Text
  sentAt         DateTime @default(now()) // When the API call was made
  lastStatusAt   DateTime @updatedAt // When the last status update was received from Twilio

  subscriber Subscriber @relation(fields: [subscriberId], references: [id], onDelete: Cascade) // If subscriber deleted, delete logs
  campaign   Campaign?  @relation(fields: [campaignId], references: [id], onDelete: SetNull) // If campaign deleted, keep logs but remove link

  // Indexes for common query patterns
  @@index([subscriberId])
  @@index([campaignId])
  @@index([status])
  @@index([sentAt])
}
  • Subscriber: Stores phone number (unique, use E.164 format), optional name details, and subscription status.
  • Campaign: Defines the campaign name, message content, status, and optional scheduling. Includes a many-to-many relation to Subscribers.
  • MessageLog: Tracks each individual message sent, linking it to a subscriber and optionally a campaign. It stores the Twilio SID (crucial for matching status updates) and the delivery status. Added indexes for performance.

Step 2: Apply Database Migrations

Generate and apply the SQL migration to create these tables in your database:

# Create a new migration file based on schema changes
# Make sure your .env file has the correct DATABASE_URL
npx prisma migrate dev --name initial_schema

# If prompted, enter a name for the migration (e.g., ""initial_schema"")

This command will:

  1. Create a new SQL migration file in prisma/migrations/.
  2. Apply the migration to your database, creating the tables.
  3. Generate the Prisma Client (@prisma/client) based on the new schema.

Your database is now ready.

3. Implementing core functionality

Let's create services to encapsulate the business logic for managing subscribers, campaigns, and sending messages via Twilio.

Step 1: Create Twilio Service

This service will handle all interactions with the Twilio API.

// src/services/twilioService.js
import twilio from 'twilio';
import pRetry from 'p-retry';
import config from '../config.js';
import prisma from '../db/prisma.js';

const client = twilio(config.twilio.accountSid, config.twilio.authToken);
const sender = config.twilio.messagingServiceSid
    ? { messagingServiceSid: config.twilio.messagingServiceSid }
    : { from: config.twilio.phoneNumber };

if (!sender.messagingServiceSid && !sender.from) {
    // This check is also in config.js, but good to have redundancy here
    console.error('CRITICAL: TWILIO_PHONE_NUMBER or TWILIO_MESSAGING_SERVICE_SID must be configured.');
    // Consider throwing an error instead of exiting if this service is loaded conditionally
    process.exit(1);
}

/**
 * Sends a single SMS message using Twilio with retry logic.
 * @param {string} to - Recipient phone number (E.164 format).
 * @param {string} body - Message content.
 * @param {string} [campaignId] - Optional campaign ID for logging.
 * @param {string} subscriberId - Subscriber ID for logging.
 * @returns {Promise<object&gt;} - Twilio message object (after successful send or final retry failure).
 */
export async function sendSms(to, body, campaignId, subscriberId) {
    const statusCallbackUrl = `${config.apiBaseUrl}/api/v1/webhooks/twilio/status`;

    const runSend = async () =&gt; {
        // This code runs potentially multiple times due to pRetry
        console.log(`Attempting to send SMS via Twilio to ${to}`);
        const message = await client.messages.create({
            ...sender, // Use 'from' or 'messagingServiceSid'
            to: to,
            body: body,
            statusCallback: statusCallbackUrl, // URL Twilio posts status updates to
        });
        console.log(`SMS successfully queued via Twilio for ${to}. SID: ${message.sid}`);
        return message; // Return the successful message object
    };

    try {
        const message = await pRetry(runSend, {
            retries: 3, // Total attempts = 4 (1 initial + 3 retries)
            minTimeout: 500, // Start with 500ms delay
            factor: 2, // Double delay each time
            randomize: true, // Add jitter to avoid thundering herd
            onFailedAttempt: error =&gt; {
                console.warn(`Twilio send attempt ${error.attemptNumber} failed for ${to}. Retries left: ${error.retriesLeft}. Error: ${error.message} (Status: ${error.status}, Code: ${error.code})`);
                // Optional: Decide whether to retry based on error code
                // e.g., don't retry on 400 Bad Request (like invalid number)
                if (error.status === 400) {
                     console.error(`Permanent error sending to ${to}, stopping retries. Code: ${error.code}`);
                     throw error; // Prevent further retries for non-retryable errors
                }
            }
        });

        // Log initial 'queued' status AFTER successful API call (potentially after retries)
        await prisma.messageLog.create({
            data: {
                subscriberId: subscriberId,
                campaignId: campaignId,
                twilioSid: message.sid,
                status: message.status, // Initial status from Twilio (e.g., 'queued', 'accepted')
                errorCode: message.errorCode,
                errorMessage: message.errorMessage,
            },
        });

        return message; // Return the successful Twilio message object

    } catch (error) {
        console.error(`Failed to send SMS via Twilio to ${to} after all retries:`, error.message);
        // Log failed attempt only after all retries are exhausted
        await prisma.messageLog.create({
             data: {
                 subscriberId: subscriberId,
                 campaignId: campaignId,
                 twilioSid: `failed-${Date.now()}-${subscriberId}`, // Create a unique placeholder SID
                 status: 'failed', // Mark as failed immediately in our log
                 errorCode: error.code || null, // Twilio error code if available from last attempt
                 errorMessage: `Failed after retries: ${error.message}`,
             },
         }).catch(logError =&gt; console.error(""Failed to log final send error:"", logError));

        throw error; // Re-throw the final error to be handled by the caller (e.g., batch processor)
    }
}

/**
 * Sends messages in batches with concurrency control.
 * @param {Array&lt;{to: string_ body: string_ campaignId: string_ subscriberId: string}&gt;} messages
 * @param {number} [concurrency=5] - Number of messages to send in parallel.
 * @returns {Promise&lt;{success: Array&lt;{sid: string_ to: string}&gt;, errors: Array&lt;{to: string_ error: string}&gt;}>}
 */
export async function sendBatchSms(messages, concurrency = 5) {
    const results = { success: [], errors: [] };
    const queue = [...messages]; // Clone the array to avoid modifying the original

    // Use Promise.allLimit style concurrency (simple implementation)
    const executing = [];
    while (queue.length &gt; 0 || executing.length &gt; 0) {
        while (executing.length &lt; concurrency && queue.length &gt; 0) {
            const msg = queue.shift();
            if (!msg) continue;

            const promise = sendSms(msg.to, msg.body, msg.campaignId, msg.subscriberId)
                .then(result =&gt; {
                    results.success.push({ sid: result.sid, to: msg.to });
                })
                .catch(error =&gt; {
                    // Error is already logged within sendSms after retries fail
                    results.errors.push({ to: msg.to, error: error.message });
                })
                .finally(() =&gt; {
                    // Remove the promise from the executing list when done
                    const index = executing.indexOf(promise);
                    if (index &gt; -1) {
                        executing.splice(index, 1);
                    }
                });

            executing.push(promise);
        }
        // Wait for at least one promise to settle if the queue is empty but tasks are running
        if (queue.length === 0 && executing.length &gt; 0) {
             await Promise.race(executing);
        } else if (executing.length === concurrency) {
            // Wait for one promise to settle before adding more if concurrency limit reached
            await Promise.race(executing);
        }
    }

    // Ensure all promises complete (though they remove themselves, this is a safeguard)
    await Promise.all(executing);

    console.log(`Batch send processing complete. Success attempts: ${results.success.length}, Failed attempts (after retries): ${results.errors.length}`);
    return results;
}

/**
 * Handles incoming status updates from Twilio webhook.
 * @param {object} statusData - Data from Twilio webhook request body.
 */
export async function handleStatusUpdate(statusData) {
    const { MessageSid, MessageStatus, ErrorCode, ErrorMessage } = statusData;

    if (!MessageSid) {
        console.warn('Received status update without MessageSid:', statusData);
        return; // Cannot process without SID
    }

    console.log(`Received status update for SID ${MessageSid}: ${MessageStatus}`);

    try {
        await prisma.messageLog.update({
            where: { twilioSid: MessageSid },
            data: {
                status: MessageStatus, // Update with the status from Twilio
                errorCode: ErrorCode ? parseInt(ErrorCode, 10) : null,
                errorMessage: ErrorMessage || null,
                // lastStatusAt is updated automatically by @updatedAt
            },
        });
        console.log(`Updated status for message ${MessageSid} to ${MessageStatus}`);

        // Optional: Update subscriber status based on final delivery failure codes
        if (MessageStatus === 'undelivered' || MessageStatus === 'failed') {
            const permanentFailureCodes = [
                21211, // Invalid 'To' Phone Number
                21610, // Attempt to send to unsubscribed recipient
                21614, // To number is not SMS capable
                30003, // Unreachable destination handset
                30005, // Unknown destination handset
                30006, // Landline or unreachable carrier
            ];
            if (ErrorCode && permanentFailureCodes.includes(parseInt(ErrorCode, 10))) {
                 console.warn(`Permanent failure code ${ErrorCode} for SID ${MessageSid}. Marking subscriber potentially.`);
                 // Find the subscriber associated with this message log
                 const messageLog = await prisma.messageLog.findUnique({
                     where: { twilioSid: MessageSid },
                     select: { subscriberId: true }
                 });
                 if (messageLog) {
                     await prisma.subscriber.update({
                         where: { id: messageLog.subscriberId },
                         data: { status: 'bounced' } // Or a more specific status
                     });
                     console.log(`Marked subscriber ${messageLog.subscriberId} as bounced due to error code ${ErrorCode}.`);
                 }
            }
        }

    } catch (error) {
        // Handle case where message log might not exist (e.g., race condition, data issue)
        if (error.code === 'P2025') { // Prisma code for record not found
             console.error(`MessageLog not found for Twilio SID: ${MessageSid}. Status update ignored.`);
        } else {
            console.error(`Error updating message log for SID ${MessageSid}:`, error);
            // Potentially re-queue or alert on persistent errors
        }
        // Do not throw here, as Twilio expects a 2xx response
    }
}

/**
 * Handles incoming opt-out messages (e.g., STOP) from Twilio webhook.
 * This relies on Twilio's Advanced Opt-Out feature configured on the Messaging Service.
 * @param {object} messageData - Data f