About

NestJS module for scheduling events and ensure only 1 event is emitted using mongodb.

Dependencies

• @nestjs/mongoose
• @nestjs/event-emitter
• @nestjs/schedule

How to use

Name your events (this is optional but recommended):

// if you prefer enums...
export enum MyEvents {
  EventNameFoo = 'EventNameFoo',
}

// if you dont like enums use constants:
export const MyEventNameFoo = 'EventNameFoo';

// if you dont like constants use object literals:
export const MyEvents = {
  EventNameFoo: 'EventNameFoo',
}

Register the module within the module you want to use it:

// ... inside module imports array
    ScheduleEventModule.register([
      {
        eventName: MyEvents.EventName, // the event name as you decided to define it
        cronExpression: CronExpression.EVERY_DAY_AT_MIDNIGHT, // frequency to schedule
      },
    ])
// ...

Create the appropriate event handler:

@Injectable()
export class MyServiceThatDoesThings {
  
  @OnEvent(MyEvents.EventName)
  async handleMyEventName(eventData: ScheduleEventData) {
    // ... do stuff
  }
}

In you main app module import both, ScheduleModule and EventEmitterModule:

@Module({
  imports: [
    EventEmitterModule.forRoot(),
    ScheduleModule.forRoot(),
//...
})
export class AppModule{}

What's the purpose?

This repository provides an opinionated solution for executing scheduled events in a distributed environment where multiple instances of an application are running simultaneously. It uses MongoDB as a semaphore to ensure that scheduled events are executed exactly once, preventing duplicate executions across different instances.

The module integrates with NestJS's scheduling and event emitter systems, allowing you to:

• Define scheduled events using cron expressions
• Ensure single execution across multiple application instances
• Track event execution history in MongoDB
• Handle scheduled events using event-driven patterns

TODO

Create unit tests.