Event Sourcing for Nestjs

Library that implements event sourcing using NestJS and his CQRS library.

Features

• StoreEventBus: A class that replaces Nest's EventBus to also persists events in mongodb.
• StoreEventPublisher: A class that replaces Nest's EventPublisher.
• ViewUpdaterHandler: The EventBus will also delegate the Events to his View Updaters, so you can update your read database.
• Replay: You can re-run stored events. This will only trigger the view updater handlers to reconstruct your read db.

State of the art

Getting started

npm install event-sourcing-nestjs @nestjs/cqrs --save

Usage

Importing

import { Module } from '@nestjs/common';
import { EventSourcingModule } from 'event-sourcing-nestjs';

@Module({
  imports: [
    EventSourcingModule.forRoot({
      mongoURL: 'mongodb://localhost:27017/eventstore',
    }),
  ],
})
export class ApplicationModule {}

Event emitter

Instead of using Nest's EventBus use StoreEventBus, so events will persist before their handlers are executed.

import { CommandHandler, ICommandHandler } from '@nestjs/cqrs';
import { StoreEventBus } from 'event-sourcing-nestjs';

@CommandHandler(CreateUserCommand)
export class CreateUserHandler implements ICommandHandler<CreateUserCommand> {

    constructor(
        private readonly eventBus: StoreEventBus,
    ) {}

    async execute(command: CreateUserCommand) {
        this.eventBus.publish(new UserCreatedEvent(command.name));
    }

}

Or use StoreEventPublisher if you want to dispatch event from your AggregateRoot.

View updaters

After emitting an event, use a view updater to update the read database state. This view updaters will be used to recontruct the db if needed.

import { IViewUpdater, ViewUpdater } from 'event-sourcing-nestjs';

@ViewUpdater(UserCreatedEvent)
export class UserCreatedUpdater implements IViewUpdater<UserCreatedEvent> {

    async handle(event: UserCreatedEvent) {
        // Save user into our view db
    }
}

Reconstructing the view db

Add this script to your package.json:

"reconstruct-view-db": "ts-node -r tsconfig-paths/register node_modules/event-sourcing-nestjs/src/scripts/reconstruct-view-db.ts"

And whenever you want to reconstruct your view db run:

npm run reconstruct-view-db mongodb://localhost:27017/eventstore

Example

You can find a full example here.

TODOs

• Use snapshots, so we can reconstruct the DB faster.