Automated reminders and scheduled notifications are crucial for user engagement, appointment management, and timely communication. Building a reliable system to handle this requires careful consideration of scheduling, third-party integration, error handling, and scalability.

This guide provides a complete walkthrough for building a robust SMS scheduling and reminder application using the NestJS framework and the Plivo communications API. We will cover everything from initial project setup to deployment and monitoring, equipping you with the knowledge to build a production-grade solution.

What We'll Build:

• A NestJS application capable of accepting requests to schedule SMS messages for a future date and time.
• A reliable scheduling mechanism using @nestjs/schedule to check for due messages.
• Integration with the Plivo API to send SMS messages.
• A REST API for creating and managing reminders.
• Database persistence for storing reminder details using TypeORM and PostgreSQL.
• Robust error handling, logging, and basic security measures.

Technologies Used:

• Node.js: The JavaScript runtime environment.
• NestJS: A progressive Node.js framework for building efficient, reliable, and scalable server-side applications. Chosen for its modular architecture, dependency injection, and built-in support for features like task scheduling and configuration management.
• Plivo: A cloud communications platform providing SMS and Voice APIs. Chosen for its reliable SMS delivery and developer-friendly Node.js SDK.
• @nestjs/schedule: A NestJS module for handling cron jobs, timeouts, and intervals declaratively. Ideal for periodically checking for due reminders.
• TypeORM: An ORM (Object-Relational Mapper) for TypeScript and JavaScript. Used for database interaction with PostgreSQL.
• PostgreSQL: A powerful, open-source object-relational database system.
• @nestjs/config: For managing environment variables securely and efficiently.
• class-validator & class-transformer: For robust request payload validation.

Prerequisites:

• Node.js (LTS version recommended) and npm/yarn installed.
• A Plivo account (Sign up for free credits).
• Access to a PostgreSQL database (local or cloud-hosted).
• Basic understanding of TypeScript, NestJS concepts, and REST APIs.
• Docker (optional, for containerized development and deployment).

System Architecture:

graph TD
    subgraph ""User Interaction""
        Client[Client Application / Postman] -->|POST /reminders| API{NestJS API Layer};
    end

    subgraph ""NestJS Application""
        API -->|Create Reminder| RemindersService[Reminders Service];
        RemindersService -->|Save Reminder| DB[(PostgreSQL Database)];
        Scheduler[@nestjs/schedule Cron Job] -->|Check Due Reminders| RemindersService;
        RemindersService -->|Fetch Due Reminders| DB;
        RemindersService -->|Send SMS Request| PlivoService[Plivo Service];
        PlivoService -->|Send SMS API Call| PlivoAPI[Plivo Cloud API];
        API -->|Manage Reminders (GET, DELETE)| RemindersService;
        RemindersService -->|Interact with DB| DB;

        %% Internal Modules
        API -- Uses --> ValidationPipe[Validation Pipe];
        RemindersService -- Uses --> Logger[Logging Service];
        PlivoService -- Uses --> ConfigService[Config Service for API Keys];
        Scheduler -- Uses --> Logger;
    end

    subgraph ""External Services""
        PlivoAPI -->|Sends SMS| UserPhone[User's Phone];
    end

    %% Styling
    classDef nest fill:#e0234e,stroke:#a21b3b,color:#fff;
    classDef db fill:#336791,stroke:#2a5274,color:#fff;
    classDef ext fill:#f96d00,stroke:#c75600,color:#fff;
    class API,RemindersService,PlivoService,Scheduler,ValidationPipe,Logger,ConfigService nest;
    class DB db;
    class PlivoAPI,UserPhone ext;
    class Client client;

This diagram illustrates the flow: a client sends a request to the NestJS API to create a reminder. The RemindersService validates and saves it to the database. A separate scheduled job (@nestjs/schedule) periodically queries the database via the RemindersService for due reminders. If found, it uses the PlivoService (which securely accesses API keys via ConfigService) to call the Plivo API, sending the SMS to the user's phone.

1. Setting up the Project

Let's initialize our NestJS project and install the necessary dependencies.

1. Create a new NestJS project:

   bash

   npm install -g @nestjs/cli
   nest new nestjs-plivo-scheduler
   cd nestjs-plivo-scheduler
   

2. Install Dependencies: We need modules for scheduling, configuration, Plivo integration, database interaction (TypeORM, PostgreSQL driver), and validation.

   bash

   npm install @nestjs/schedule @nestjs/config plivo typeorm @nestjs/typeorm pg class-validator class-transformer
   npm install --save-dev @types/cron
   
   • @nestjs/schedule: For scheduling tasks.
   • @nestjs/config: For managing environment variables.
   • plivo: The official Plivo Node.js SDK.
   • typeorm, @nestjs/typeorm, pg: For database interaction with PostgreSQL using TypeORM.
   • class-validator, class-transformer: For validating incoming request data.
   • @types/cron: Type definitions for the underlying cron library used by @nestjs/schedule.

3. Environment Variables Setup: Create a .env file in the project root for storing sensitive information and configuration. Never commit this file to version control. Add it to your .gitignore file if it's not already there.

   bash

   # .env
   
   # Application Port
   PORT=3000
   
   # Plivo Credentials
   PLIVO_AUTH_ID=YOUR_PLIVO_AUTH_ID
   PLIVO_AUTH_TOKEN=YOUR_PLIVO_AUTH_TOKEN
   PLIVO_SENDER_NUMBER=YOUR_PLIVO_PHONE_NUMBER # Must be a Plivo number in E.164 format (e.g., +14155552671)
   
   # Database Connection (PostgreSQL)
   DB_HOST=localhost
   DB_PORT=5432
   DB_USERNAME=your_db_user
   DB_PASSWORD=your_db_password
   DB_DATABASE=scheduler_db
   
   # Cron Schedule (e.g., every minute)
   # See https://crontab.guru/ for syntax help
   REMINDER_CHECK_CRON_TIME=* * * * *
   
   • How to get Plivo Credentials:
     1. Log in to your Plivo Console.
     2. Your Auth ID and Auth Token are displayed prominently on the dashboard homepage.
     3. Navigate to Messaging -> Phone Numbers -> Numbers in the left sidebar. Purchase or use an existing Plivo number. Copy the full number including the country code (e.g., +12025550144). This is your PLIVO_SENDER_NUMBER.
   • Database Credentials: Replace the placeholders with your actual PostgreSQL connection details. Important: Ensure the database specified (scheduler_db in this example) exists on your PostgreSQL server before running the application or migrations. You may need to create it manually (e.g., using CREATE DATABASE scheduler_db;).

4. Configure Modules: Import the necessary modules into your main application module (src/app.module.ts).

   typescript

   // src/app.module.ts
   import { Module, ValidationPipe } from '@nestjs/common';
   import { ConfigModule, ConfigService } from '@nestjs/config';
   import { ScheduleModule } from '@nestjs/schedule';
   import { TypeOrmModule } from '@nestjs/typeorm';
   import { AppController } from './app.controller';
   import { AppService } from './app.service';
   import { RemindersModule } from './reminders/reminders.module';
   import { PlivoModule } from './plivo/plivo.module';
   import { DatabaseModule } from './database/database.module'; // Optional
   import { Reminder } from './reminders/entities/reminder.entity'; // Import Reminder entity
   import { APP_PIPE } from '@nestjs/core';
   
   @Module({
     imports: [
       ConfigModule.forRoot({
         isGlobal: true, // Make ConfigModule available globally
         envFilePath: '.env',
       }),
       ScheduleModule.forRoot(), // Initialize the scheduler
       TypeOrmModule.forRootAsync({
         imports: [ConfigModule],
         useFactory: (configService: ConfigService) => ({
           type: 'postgres',
           host: configService.get<string&gt;('DB_HOST'),
           port: configService.get<number&gt;('DB_PORT'),
           username: configService.get<string&gt;('DB_USERNAME'),
           password: configService.get<string&gt;('DB_PASSWORD'),
           database: configService.get<string&gt;('DB_DATABASE'),
           entities: [Reminder], // Register the Reminder entity
           synchronize: true, // WARNING: Set to false in production! Use migrations instead.
           autoLoadEntities: true, // Automatically load entities registered via forFeature()
         }),
         inject: [ConfigService],
       }),
       // DatabaseModule, // Optional: If you created a separate DB module
       RemindersModule, // Feature module for reminders
       PlivoModule,     // Feature module for Plivo integration
     ],
     controllers: [AppController],
     providers: [
       AppService,
       // Apply validation pipe globally
       {
         provide: APP_PIPE,
         useValue: new ValidationPipe({
           whitelist: true, // Strip properties not defined in DTO
           transform: true, // Automatically transform payloads to DTO instances
           forbidNonWhitelisted: true, // Throw error if non-whitelisted properties are present
         }),
       },
     ],
   })
   export class AppModule {}
   
   • We initialize ConfigModule.forRoot globally to access .env variables anywhere.
   • ScheduleModule.forRoot enables task scheduling.
   • TypeOrmModule.forRootAsync sets up the database connection using credentials from ConfigService.
     • Important: synchronize: true is convenient for development as it automatically updates the schema. In production, disable it (synchronize: false) and use TypeORM Migrations to manage schema changes safely.
   • We register our future RemindersModule and PlivoModule.
   • A global ValidationPipe is configured to automatically validate incoming request bodies based on DTOs.

5. Update main.ts: Enable shutdown hooks for graceful termination and set the application port from the environment variable.

   typescript

   // src/main.ts
   import { NestFactory } from '@nestjs/core';
   import { AppModule } from './app.module';
   import { ConfigService } from '@nestjs/config';
   import { Logger, ValidationPipe } from '@nestjs/common'; // Import Logger
   
   async function bootstrap() {
     const app = await NestFactory.create(AppModule);
     const configService = app.get(ConfigService);
     const port = configService.get<number&gt;('PORT', 3000); // Default to 3000 if not set
   
     // Global validation pipe is already configured in AppModule providers
     // app.useGlobalPipes(new ValidationPipe({ ... })); // This would be redundant
   
     app.enableShutdownHooks(); // Enable graceful shutdown
   
     await app.listen(port);
     Logger.log(`Application running on: http://localhost:${port}`, 'Bootstrap');
   }
   bootstrap();
   

2. Creating a Database Schema and Data Layer

We need a way to store the reminders. We'll use TypeORM to define an entity and interact with the PostgreSQL database.

1. Define the Reminder Entity: Create the entity file that maps to our database table.

   typescript

   // src/reminders/entities/reminder.entity.ts
   import { Entity, PrimaryGeneratedColumn, Column, CreateDateColumn, UpdateDateColumn, Index } from 'typeorm';
   
   export enum ReminderStatus {
     PENDING = 'PENDING',
     SENT = 'SENT',
     FAILED = 'FAILED',
     PROCESSING = 'PROCESSING', // Optional: To lock record during processing
   }
   
   @Entity('reminders') // Specifies the table name
   export class Reminder {
     @PrimaryGeneratedColumn('uuid')
     id: string;
   
     @Column({ type: 'varchar', length: 20 }) // Store E.164 format
     recipientPhoneNumber: string;
   
     @Column({ type: 'text' })
     message: string;
   
     @Index() // Index this column for faster querying
     @Column({ type: 'timestamp with time zone' }) // Store with timezone awareness
     sendAt: Date;
   
     @Index() // Index this column for faster querying
     @Column({
       type: 'enum',
       enum: ReminderStatus,
       default: ReminderStatus.PENDING,
     })
     status: ReminderStatus;
   
     @Column({ type: 'text', nullable: true }) // Store Plivo message UUID
     plivoMessageUuid?: string;
   
     @Column({ type: 'text', nullable: true }) // Store any error message
     errorMessage?: string;
   
     @CreateDateColumn()
     createdAt: Date;
   
     @UpdateDateColumn()
     updatedAt: Date;
   }
   
   • We define columns for the recipient's phone number, the message content, the scheduled time (sendAt), and the status (PENDING, SENT, FAILED, PROCESSING).
   • sendAt uses timestamp with time zone to correctly handle different time zones. Always store dates in UTC in your database.
   • Indexes are added to sendAt and status for efficient querying by the scheduler.
   • plivoMessageUuid and errorMessage are added for tracking and debugging.
   • PROCESSING status is optional but recommended to prevent a reminder from being picked up by multiple scheduler instances if the processing takes time.

2. Create the RemindersModule: Generate the module, controller, and service using the Nest CLI.

   bash

   nest generate module reminders
   nest generate controller reminders --flat # No sub-directory for controller
   nest generate service reminders --flat   # No sub-directory for service
   
   • --flat prevents the CLI from creating unnecessary subdirectories within src/reminders.

3. Register Entity and Set Up Repository: Import TypeOrmModule into RemindersModule to make the Reminder repository available for injection.

   typescript

   // src/reminders/reminders.module.ts
   import { Module } from '@nestjs/common';
   import { TypeOrmModule } from '@nestjs/typeorm';
   import { RemindersController } from './reminders.controller';
   import { RemindersService } from './reminders.service';
   import { Reminder } from './entities/reminder.entity';
   import { PlivoModule } from '../plivo/plivo.module'; // Import PlivoModule
   import { ScheduleModule } from '@nestjs/schedule'; // Import ScheduleModule if scheduling logic is here
   
   @Module({
     imports: [
       TypeOrmModule.forFeature([Reminder]), // Register Reminder entity/repository
       PlivoModule, // Make PlivoService available
       ScheduleModule, // Needed if @Cron decorator is in RemindersService
     ],
     controllers: [RemindersController],
     providers: [RemindersService],
     exports: [RemindersService], // Export if needed by other modules
   })
   export class RemindersModule {}
   

4. (Production) Setting up Migrations (Recommended): As mentioned, synchronize: true is unsafe for production. Use TypeORM migrations instead.

   • Install CLI: npm install -g typeorm (or use npx typeorm)
   • Add Migration Scripts to package.json:
     json

     // package.json
     {
       ""scripts"": {
         // ... other scripts
         ""typeorm"": ""ts-node -r tsconfig-paths/register ./node_modules/typeorm/cli.js --dataSource src/data-source.ts"",
         ""migration:generate"": ""npm run typeorm -- migration:generate src/migrations/%npm_config_name%"",
         ""migration:run"": ""npm run typeorm -- migration:run"",
         ""migration:revert"": ""npm run typeorm -- migration:revert""
       }
     }
     
   • Create Data Source File (src/data-source.ts): This file tells the TypeORM CLI how to connect to your database.
     typescript

     // src/data-source.ts
     import { DataSource, DataSourceOptions } from 'typeorm';
     import { config } from 'dotenv';
     
     config(); // Load .env variables
     
     export const dataSourceOptions: DataSourceOptions = {
       type: 'postgres',
       host: process.env.DB_HOST,
       port: parseInt(process.env.DB_PORT || '5432', 10),
       username: process.env.DB_USERNAME,
       password: process.env.DB_PASSWORD,
       database: process.env.DB_DATABASE,
       entities: ['dist/**/*.entity{.ts,.js}'], // Point to compiled entities
       migrations: ['dist/migrations/*{.ts,.js}'], // Point to compiled migrations
       synchronize: false, // IMPORTANT: Set to false for migrations
       logging: true,
     };
     
     const dataSource = new DataSource(dataSourceOptions);
     export default dataSource;
     
   • Generate Initial Migration: After defining your entity, run:
     bash

     npm run migration:generate --name=InitialReminderSchema
     
     This creates a migration file in src/migrations. Review it.
   • Run Migrations:
     bash

     npm run build # Compile TypeScript to JavaScript first
     npm run migration:run
     
     This applies the migration to your database.
   • Modify TypeOrmModule.forRootAsync: Update the entities and migrations paths to point to the compiled output (dist) and set synchronize: false. Note: Using __dirname relies on the compiled file structure; verify paths based on your build output.
     typescript

     // src/app.module.ts (inside TypeOrmModule.forRootAsync useFactory)
     // ... other options
     entities: [__dirname + '/../**/*.entity{.ts,.js}'], // Use dynamic path relative to compiled AppModule
     migrations: [__dirname + '/migrations/*{.ts,.js}'], // Use dynamic path relative to compiled AppModule
     synchronize: false, // Disable synchronize
     migrationsRun: true, // Optionally run migrations automatically on startup
     // ...
     

3. Integrating with Plivo

Let's create a dedicated module and service to handle interactions with the Plivo API.

1. Create the PlivoModule:

   bash

   nest generate module plivo
   nest generate service plivo --flat
   

2. Implement PlivoService: This service will initialize the Plivo client and provide a method to send SMS messages.

   typescript

   // src/plivo/plivo.service.ts
   import { Injectable, Logger, OnModuleInit } from '@nestjs/common';
   import { ConfigService } from '@nestjs/config';
   import * as plivo from 'plivo';
   
   @Injectable()
   export class PlivoService implements OnModuleInit {
     private readonly logger = new Logger(PlivoService.name);
     private client: plivo.Client;
     private senderNumber: string;
   
     constructor(private readonly configService: ConfigService) {}
   
     onModuleInit() {
       const authId = this.configService.get<string&gt;('PLIVO_AUTH_ID');
       const authToken = this.configService.get<string&gt;('PLIVO_AUTH_TOKEN');
       this.senderNumber = this.configService.get<string&gt;('PLIVO_SENDER_NUMBER');
   
       if (!authId || !authToken || !this.senderNumber) {
         this.logger.error('Plivo credentials or sender number are missing in environment variables.');
         // Consider throwing an error to prevent the application from starting incorrectly
         throw new Error('Missing Plivo configuration.');
       }
   
       try {
         this.client = new plivo.Client(authId, authToken);
         this.logger.log('Plivo client initialized successfully.');
       } catch (error) {
         this.logger.error('Failed to initialize Plivo client:', error);
         throw error; // Re-throw to indicate critical failure
       }
     }
   
     async sendSms(to: string, text: string): Promise&lt;{ messageUuid: string | null; error?: string }&gt; {
       if (!this.client) {
         this.logger.error('Plivo client not initialized. Cannot send SMS.');
         return { messageUuid: null, error: 'Plivo client not initialized.' };
       }
   
       // Basic E.164 format validation (can be more robust)
       if (!/^\+[1-9]\d{1,14}$/.test(to)) {
            this.logger.warn(`Invalid recipient phone number format: ${to}. Must be E.164.`);
            return { messageUuid: null, error: `Invalid recipient phone number format: ${to}. Must be E.164.` };
       }
        if (!/^\+[1-9]\d{1,14}$/.test(this.senderNumber)) {
            this.logger.warn(`Invalid sender phone number format: ${this.senderNumber}. Must be E.164.`);
            return { messageUuid: null, error: `Invalid sender phone number format: ${this.senderNumber}. Must be E.164.` };
        }
   
   
       this.logger.log(`Attempting to send SMS to ${to}`);
       try {
         const response = await this.client.messages.create(
           this.senderNumber, // src
           to,               // dst
           text              // text
         );
   
         this.logger.log(`SMS sent successfully to ${to}. Message UUID: ${response.messageUuid[0]}`);
         // Plivo API returns messageUuid as an array, typically with one element
         return { messageUuid: response.messageUuid[0] };
   
       } catch (error) {
         this.logger.error(`Failed to send SMS to ${to}:`, error?.message || error);
         // Extract a meaningful error message if possible
         const errorMessage = error?.message || 'Unknown Plivo API error';
         return { messageUuid: null, error: errorMessage };
       }
     }
   }
   
   • It implements OnModuleInit to initialize the Plivo client once the module is ready.
   • It fetches credentials and the sender number from ConfigService.
   • Crucially, it includes error handling for missing configuration and during client initialization.
   • The sendSms method wraps the client.messages.create call, performs basic E.164 validation, logs actions, and handles potential API errors gracefully, returning the message UUID or an error message.

3. Register ConfigService in PlivoModule:

   typescript

   // src/plivo/plivo.module.ts
   import { Module } from '@nestjs/common';
   import { ConfigModule } from '@nestjs/config'; // Import ConfigModule
   import { PlivoService } from './plivo.service';
   
   @Module({
     imports: [ConfigModule], // Make ConfigService available for injection
     providers: [PlivoService],
     exports: [PlivoService], // Export the service to be used elsewhere
   })
   export class PlivoModule {}
   

4. Implementing Core Functionality (Scheduling and Sending)

Now, let's implement the service logic for creating reminders and the scheduled task to send them.

1. Implement RemindersService: Inject the Reminder repository and PlivoService. Add methods to create reminders and process due ones.

   typescript

   // src/reminders/reminders.service.ts
   import { Injectable, Logger, NotFoundException, ConflictException } from '@nestjs/common';
   import { InjectRepository } from '@nestjs/typeorm';
   import { Repository, LessThanOrEqual, In } from 'typeorm';
   import { Cron, CronExpression } from '@nestjs/schedule';
   import { ConfigService } from '@nestjs/config'; // Import ConfigService
   import { Reminder, ReminderStatus } from './entities/reminder.entity';
   import { CreateReminderDto } from './dto/create-reminder.dto';
   import { PlivoService } from '../plivo/plivo.service';
   
   @Injectable()
   export class RemindersService {
     private readonly logger = new Logger(RemindersService.name);
     private readonly reminderCheckCronTime: string;
   
     constructor(
       @InjectRepository(Reminder)
       private readonly reminderRepository: Repository<Reminder&gt;,
       private readonly plivoService: PlivoService,
       private readonly configService: ConfigService, // Inject ConfigService
     ) {
        // Get cron time from config, default to every minute if not set
        this.reminderCheckCronTime = this.configService.get<string&gt;('REMINDER_CHECK_CRON_TIME', CronExpression.EVERY_MINUTE);
        this.logger.log(`Reminder check schedule initialized with cron: ${this.reminderCheckCronTime}`);
     }
   
     async create(createReminderDto: CreateReminderDto): Promise<Reminder&gt; {
       const reminder = this.reminderRepository.create({
         ...createReminderDto,
         sendAt: new Date(createReminderDto.sendAt), // Ensure sendAt is a Date object
         status: ReminderStatus.PENDING,
       });
       this.logger.log(`Creating new reminder for ${reminder.recipientPhoneNumber} at ${reminder.sendAt}`);
       return this.reminderRepository.save(reminder);
     }
   
     // --- Scheduled Task ---
     // Note: @Cron decorator reads 'process.env' directly at load time.
     // It cannot dynamically use 'this.reminderCheckCronTime' from the constructor.
     // The constructor value is primarily for logging consistency here.
     @Cron(process.env.REMINDER_CHECK_CRON_TIME || CronExpression.EVERY_MINUTE)
     async handleCron() {
       this.logger.log(`[Cron Job] Checking for due reminders at ${new Date().toISOString()}`);
   
       const now = new Date();
       let remindersToProcess: Reminder[] = [];
       let updatedCount = 0;
   
       // --- Step 1: Find and Lock Pending Reminders ---
       // This transaction attempts to prevent race conditions if multiple instances run
       await this.reminderRepository.manager.transaction(async transactionalEntityManager =&gt; {
           remindersToProcess = await transactionalEntityManager.find(Reminder, {
               where: {
                   status: ReminderStatus.PENDING,
                   sendAt: LessThanOrEqual(now), // Find reminders due now or earlier
               },
               take: 100, // Process in batches to avoid overloading
               order: {
                   sendAt: 'ASC', // Process older ones first
               },
           });
   
           if (remindersToProcess.length === 0) {
               this.logger.log('[Cron Job] No pending reminders found.');
               return; // Exit transaction early if nothing to process
           }
   
           this.logger.log(`[Cron Job] Found ${remindersToProcess.length} reminders to process.`);
   
           // --- Step 2: Mark selected reminders as PROCESSING ---
           const idsToUpdate = remindersToProcess.map(r =&gt; r.id);
           const updateResult = await transactionalEntityManager.update(
               Reminder,
               { id: In(idsToUpdate), status: ReminderStatus.PENDING }, // Ensure status hasn't changed
               { status: ReminderStatus.PROCESSING, updatedAt: new Date() }
           );
   
           updatedCount = updateResult.affected || 0;
   
           if (updatedCount !== remindersToProcess.length) {
                this.logger.warn(`[Cron Job] Concurrency issue detected. Expected to lock ${remindersToProcess.length}, but locked ${updatedCount}. Some reminders might be processed by another instance.`);
                // Filter remindersToProcess to only include those successfully updated
                // For simplicity, we proceed with those originally found, assuming the DB lock prevents double processing.
                // A more robust approach might re-query based on `updatedCount`.
                remindersToProcess = remindersToProcess.slice(0, updatedCount); // Adjust based on actual locked count
           }
           this.logger.log(`[Cron Job] Locked ${updatedCount} reminders for processing.`);
       }); // Transaction ends here
   
   
       // --- Step 3: Process Locked Reminders (Outside Transaction) ---
       if (updatedCount === 0) {
           // No reminders were successfully locked, possibly due to race condition or none found
           return;
       }
   
       // Process only the reminders that were successfully locked
       const successfullyLockedReminders = remindersToProcess; // Assuming the slice/filter above worked
   
       for (const reminder of successfullyLockedReminders) {
         this.logger.log(`[Cron Job] Processing reminder ID: ${reminder.id} for ${reminder.recipientPhoneNumber}`);
         try {
           const result = await this.plivoService.sendSms(
             reminder.recipientPhoneNumber,
             reminder.message,
           );
   
           if (result.messageUuid) {
             // Update status to SENT and store message UUID
             await this.reminderRepository.update(reminder.id, {
               status: ReminderStatus.SENT,
               plivoMessageUuid: result.messageUuid,
               updatedAt: new Date(),
               errorMessage: null, // Clear any previous error
             });
             this.logger.log(`[Cron Job] Reminder ID: ${reminder.id} successfully sent. Plivo UUID: ${result.messageUuid}`);
           } else {
             // Update status to FAILED and store error message
             await this.reminderRepository.update(reminder.id, {
               status: ReminderStatus.FAILED,
               errorMessage: result.error || 'Unknown Plivo sending error',
               updatedAt: new Date(),
             });
             this.logger.error(`[Cron Job] Failed to send reminder ID: ${reminder.id}. Error: ${result.error}`);
           }
         } catch (error) {
           // Catch unexpected errors during processing or DB update
           this.logger.error(`[Cron Job] Unexpected error processing reminder ID: ${reminder.id}`, error);
           try {
             // Attempt to mark as FAILED even if an unexpected error occurred
             await this.reminderRepository.update(reminder.id, {
               status: ReminderStatus.FAILED,
               errorMessage: `Unexpected processing error: ${error.message}`,
               updatedAt: new Date(),
             });
           } catch (updateError) {
             this.logger.error(`[Cron Job] CRITICAL: Failed to update status for reminder ID: ${reminder.id} after processing error`, updateError);
             // This reminder might remain in PROCESSING state and needs manual intervention or a cleanup job.
           }
         }
       }
       this.logger.log(`[Cron Job] Finished processing batch of ${updatedCount} reminders.`);
     }
   
     // --- Basic CRUD Methods (for API) ---
   
     async findAll(status?: ReminderStatus): Promise<Reminder[]&gt; {
       this.logger.log(`Finding all reminders${status ? ` with status: ${status}` : ''}`);
       return this.reminderRepository.find({ where: status ? { status } : {} });
     }
   
     async findOne(id: string): Promise<Reminder&gt; {
       this.logger.log(`Finding reminder with ID: ${id}`);
       const reminder = await this.reminderRepository.findOne({ where: { id } });
       if (!reminder) {
         throw new NotFoundException(`Reminder with ID ""${id}"" not found`);
       }
       return reminder;
     }
   
     async remove(id: string): Promise<void&gt; {
       this.logger.log(`Attempting to remove reminder with ID: ${id}`);
       const result = await this.reminderRepository.delete(id);
       if (result.affected === 0) {
         throw new NotFoundException(`Reminder with ID ""${id}"" not found`);
       }
       this.logger.log(`Successfully removed reminder with ID: ${id}`);
     }
   
      // Optional: Method to retry failed reminders
      async retryFailedReminder(id: string): Promise<Reminder&gt; {
          this.logger.log(`Attempting to retry failed reminder ID: ${id}`);
          const reminder = await this.findOne(id); // findOne includes NotFoundException check
   
          if (reminder.status !== ReminderStatus.FAILED) {
              throw new ConflictException(`Reminder ID: ${id} is not in FAILED state. Current state: ${reminder.status}`);
          }
   
          // Reset status to PENDING so the cron job picks it up again
          reminder.status = ReminderStatus.PENDING;
          reminder.errorMessage = null; // Clear previous error
          reminder.updatedAt = new Date();
   
          return this.reminderRepository.save(reminder);
      }
   }
   
   • create: Takes validated DTO data, creates a Reminder entity instance (ensuring sendAt is a Date), sets the initial status to PENDING, and saves it using the repository.
   • handleCron:
     • Decorated with @Cron to run based on the REMINDER_CHECK_CRON_TIME environment variable (defaulting to every minute). Important: The @Cron decorator reads process.env directly when the application loads, not the this.reminderCheckCronTime instance variable dynamically.
     • Uses a transaction (this.reminderRepository.manager.transaction) for finding and updating reminders to prevent race conditions in multi-instance deployments.
     • Finds pending reminders (status: ReminderStatus.PENDING) whose sendAt time is less than or equal to the current time (now). It processes in batches (take: 100) and prioritizes older reminders (order: { sendAt: 'ASC' }).
     • Locks the found reminders by updating their status to PROCESSING within the transaction. It checks if the number of affected rows matches the number found to detect potential concurrency issues where another instance might have processed some reminders between the find and update steps.
     • Processes the locked reminders outside the initial transaction to avoid holding the transaction open during potentially long-running external API calls (SMS sending).
     • For each reminder, it calls plivoService.sendSms.
     • Based on the Plivo result, it updates the reminder status to SENT (with Plivo message UUID) or FAILED (with an error message).
     • Includes robust error handling for both Plivo API errors and unexpected errors during processing or database updates.
   • findAll, findOne, remove: Standard CRUD methods for managing reminders via the API, including logging and NotFoundException handling.
   • retryFailedReminder: An optional method to reset a FAILED reminder back to PENDING so the cron job can attempt to send it again. Includes checks to ensure the reminder is actually in a FAILED state.