September 10, 2019 | NestJS Paris | Sylvain Pontoreau NestJS & Event Store

Formerly Microsoft In ❤ with Co-organizer Who I am? @spontoreau

Paris TypeScript @ParisTypeScript www.typescript.paris

Domain Driven Design • DDD is not a Framework nor a Methodology… • … it's an approach to dealing with complexity • DDD focusing: • The domain representation • The domain logic • Technical aspects are secondary • Just a part that enforcing the domain

Opposite of Data Driven Development Data Driven Development Domain Driven Design != Data structure Entity Relationship CRUD Behavior Bounded Context Event

Query Just a read operation Command Data validation Business logic Persistence CQRS

CQRS GetBookById Query CreateBook Command GetAllBook Query DeleteBook Command … … Dispatcher Read DB Dispatcher QueryHandler CommandHandler Write DB Validation Plugin Monitoring Plugin Telemetry Plugin … Eventual consistency

https://martinfowler.com/eaaDev/EventSourcing.html Event Sourcing

Event Sourcing & CQRS

CQRS & Event Sourcing GetBookById Query CreateBook Command GetAllBook Query DeleteBook Command … … Dispatcher Read DB Dispatcher QueryHandler CommandHandler Event Store Projection Projection AggregateRoot State Events

Our focus in this talk GetBookById Query CreateBook Command GetAllBook Query DeleteBook Command … … Dispatcher Read DB Dispatcher QueryHandler CommandHandler Event Store Projection Projection AggregateRoot State Events

NestJS, Commands, Aggregates & Events Command Bus EventBus EventPublisher Repository CommandHandler Controller/Service Aggregate Execute Comand Execute Handler Get Aggregate Merge Aggregate Apply events Execute behaviors IEventPublisher Publish events DefaultPubSub Default implementation EventHandler EventStore Publisher

Custom EventPublisher & MessageSource import { IEvent } from './event.interface'; export interface IEventPublisher { publish(event: T); } import { Subject } from 'rxjs'; import { IEvent } from './event.interface'; export interface IMessageSource { bridgeEventsTo< T extends Ievent >(subject: Subject); } • IEventPublisher • MessageSource export class UserModule implements OnModuleInit { constructor( private readonly eventBus: EventBus, private readonly eventStorePublisher: EventStorePublisher ) {} onModuleInit() { this.eventBus.publisher = this.eventStorePublisher; } } • Module

Which database for storing events? Choose what you want! Please try me!!!

Event Store, the stream database https://eventstore.org

Demo

Resources • CQRS & Event Sourcing • CQRS pattern: https://docs.microsoft.com/en-US/azure/architecture/patterns/cqrs • Event Sourcing pattern: https://docs.microsoft.com/en-us/azure/architecture/patterns/event-sourcing • Tackle business complexity in a Microservice with DDD and CQRS: https://docs.microsoft.com/en- us/dotnet/standard/microservices-architecture/microservice-ddd-cqrs-patterns/ • NestJS CQRS module: https://docs.nestjs.com/recipes/cqrs • Event Store website: https://eventstore.org • Talk’s sample: https://github.com/spontoreau/nestjs-eventstore-demo • node-eventstore-client NPM package: https://github.com/nicdex/node-eventstore-client • gereventstore-pomise NPM package : https://github.com/RemoteMetering/geteventstore-promise • Jérôme Rouaix presentation on CQRS & ES: https://gitpitch.com/jrouaix/cqrs-es-presentation/master#/ • Arnaud Lemaire talk on CQRS, ES and DDD: https://www.youtube.com/watch?v=qBLtZN3p3FU • Greg Young, A decade of DDD, CQRS and ES: https://www.youtube.com/watch?v=LDW0QWie21s

Questions?

Thank you!