How do you implement event sourcing with NestJS microservices?

Q: How do you implement event sourcing with NestJS microservices?

// Domain event export class OrderPlacedEvent { readonly type = 'OrderPlaced'; constructor( public readonly orderId: string, public readonly userId: string, public readonly items: OrderItem[], public readonly timestamp = new Date(), ) {} } // Event store — append-only @Injectable() export class EventStore { async append(streamId: string, events: DomainEvent[]): Promise { await this.dataSource .createQueryBuilder() .insert() .into('event_store') .values(events.map(e => ({ streamId, eventType: e.type, payload: e, occurredAt: new Date(), }))) .execute(); } async load(streamId: string): Promise { const rows = await this.dataSource.query( 'SELECT payload FROM event_store WHERE stream_id = $1 ORDER BY occurred_at', [streamId], ); return rows.map(r => r.payload); } } // Aggregate — reconstructs state by replaying events export class OrderAggregate { id: string; status: string; items: OrderItem[] = []; static rehydrate(events: DomainEvent[]): OrderAggregate { const agg = new OrderAggregate(); for (const event of events) agg.apply(event); return agg; } apply(event: DomainEvent): void { if (event.type === 'OrderPlaced') { this.id = (event as OrderPlacedEvent).orderId; this.items = (event as OrderPlacedEvent).items; this.status = 'PLACED'; } } } The event store is append-only — never update or delete events; they are the permanent record. Aggregate state is derived by replaying events from the beginning of a stream. Use snapshots for aggregates with long event histories — store a checkpoint every N events. Publish events to Kafka after appending so projections and other services can subscribe. Event sourcing pairs naturally with CQRS — the write model appends events; the read model builds projections.

Difficulty: 7/10

event sourcing, NestJS microservices, CQRS

Store all domain changes as an immutable append-only sequence of events in an event store table. State is never updated in place — it is rehydrated by replaying all events for an aggregate. Aggregates apply each event to reconstruct their current state. Publish events to Kafka after appending to the store for downstream consumers.

Event store, aggregate rehydration, and append

Event sourcing design principles:

How do you implement event sourcing with NestJS microservices?

Difficulty: 7/10

event sourcing, NestJS microservices, CQRS

Store all domain changes as an immutable append-only sequence of events in an event store table. State is never updated in place — it is rehydrated by replaying all events for an aggregate. Aggregates apply each event to reconstruct their current state. Publish events to Kafka after appending to the store for downstream consumers.

Event store, aggregate rehydration, and append

Event sourcing design principles:

How do you implement event sourcing with NestJS microservices?

How do you implement event sourcing with NestJS microservices?

Scenario Questions

Follow-up Questions