I’ve been building systems for years, and I keep coming back to Event Sourcing when I need robust audit trails and temporal capabilities. Last month, I worked on a financial application where we needed to track every state change precisely. Traditional databases made this cumbersome, but Event Sourcing with EventStore and NestJS transformed how we handle data. Let me walk you through my approach.

Event Sourcing stores every change as an immutable event, rather than just the current state. This means you can reconstruct any past state by replaying events. Have you ever needed to debug exactly what happened in your system three months ago? With Event Sourcing, it’s straightforward. Combined with CQRS, which separates reads from writes, it creates a scalable architecture perfect for complex domains.

First, let’s set up the environment. I use Docker to run EventStore locally. Here’s a simple docker-compose.yml:

services:
  eventstore:
    image: eventstore/eventstore:latest
    environment:
      - EVENTSTORE_INSECURE=true
    ports:
      - "2113:2113"

After starting EventStore, I initialize a NestJS project. The structure is crucial. I organize code into domains, infrastructure, and application layers. This separation keeps business logic clean and testable.

Now, let’s define a base event. Every event in the system extends this class:

export abstract class BaseEvent {
  constructor(
    public readonly aggregateId: string,
    public readonly data: any,
    public readonly occurredAt: Date = new Date()
  ) {}
}

Why is immutability important here? Because events are facts that should never change. Once stored, they tell the true story of what happened.

Next, I implement aggregates. An aggregate is a cluster of domain objects that can be treated as a single unit. For example, a BankAccount aggregate might handle deposits and withdrawals. It processes commands and emits events.

Here’s a snippet of an aggregate root:

export class BankAccount extends AggregateRoot {
  private balance: number = 0;

  constructor(id: string) {
    super(id);
  }

  deposit(amount: number) {
    this.apply(new DepositEvent(this.id, { amount }));
  }

  private onDepositEvent(event: DepositEvent) {
    this.balance += event.data.amount;
  }
}

Notice how the state changes are handled in private methods. This keeps the logic contained and predictable.

Projections are where things get interesting. They transform events into read-optimized views. If you’ve ever needed to display data in multiple formats, projections make it efficient. I use EventStore’s built-in projections or write custom ones in NestJS.

For instance, to create a current balance view:

@Projection({ name: 'account_balance' })
export class AccountBalanceProjection {
  constructor(private readonly repository: ReadRepository) {}

  @EventHandler(DepositEvent)
  async handleDeposit(event: DepositEvent) {
    const account = await this.repository.findById(event.aggregateId);
    account.balance += event.data.amount;
    await this.repository.save(account);
  }
}

What happens when business requirements change and you need to modify event structures? That’s where versioning comes in. I use upcasters to transform old events into new formats during replay. This ensures backward compatibility without losing history.

Performance can be a concern with large event streams. Snapshots help by storing the current state at intervals, so you don’t always replay from the beginning. I implement snapshots for aggregates with frequent changes.

Error handling is critical. I use dead-letter queues for events that fail processing and implement retry mechanisms. How do you ensure your system remains resilient under load? Monitoring and logging every step help me catch issues early.

Testing event-sourced systems requires a different mindset. I focus on behavior rather than state. Here’s a simple test for the deposit command:

it('should emit DepositEvent when depositing', async () => {
  const account = new BankAccount('123');
  account.deposit(100);
  expect(account.getUncommittedEvents()).toContainEqual(
    expect.objectContaining({ eventType: 'DepositEvent' })
  );
});

Deployment involves ensuring EventStore is clustered for high availability and configuring proper backups. I use environment variables for connection strings and enable secure communication in production.

I hope this guide gives you a solid foundation. Event Sourcing might seem complex at first, but its benefits in auditability and flexibility are worth the effort. What challenges have you faced with traditional data storage? Share your thoughts in the comments below—I’d love to hear from you. If this helped, please like and share it with others who might benefit.