I’ve been thinking a lot about how modern applications handle data consistency while maintaining performance and auditability. Traditional approaches often struggle with these competing demands, which led me to explore event sourcing. This pattern fundamentally changes how we think about application state by storing every change as an immutable event rather than just the current state.

Have you ever wondered how systems maintain perfect audit trails while handling complex business workflows? Event sourcing provides exactly that capability, and when combined with Spring Boot, Axon Framework, and MongoDB, it becomes remarkably powerful.

Let me show you how to set this up. First, we need to configure our dependencies properly. The Axon Framework integrates seamlessly with Spring Boot, while MongoDB serves as our event store.

@SpringBootApplication
public class OrderApplication {
    public static void main(String[] args) {
        SpringApplication.run(OrderApplication.class, args);
    }
}

The configuration is straightforward. Here’s how we set up our application properties:

spring:
  data:
    mongodb:
      uri: mongodb://localhost:27017/order-events
axon:
  eventhandling:
    processors:
      order-processor:
        mode: tracking

Now, let’s define our domain model. Commands represent the intent to change state, while events represent what actually happened. This separation is crucial for maintaining consistency.

public class CreateOrderCommand {
    private String orderId;
    private String customerId;
    private List<OrderItem> items;
    
    // Constructors, getters, and validation
}

public class OrderCreatedEvent {
    private String orderId;
    private String customerId;
    private BigDecimal totalAmount;
    private Instant timestamp;
}

The aggregate root handles commands and produces events. This is where business rules are enforced:

@Aggregate
public class OrderAggregate {
    @AggregateIdentifier
    private String orderId;
    private OrderStatus status;
    
    @CommandHandler
    public OrderAggregate(CreateOrderCommand command) {
        apply(new OrderCreatedEvent(
            command.getOrderId(),
            command.getCustomerId(),
            calculateTotal(command.getItems()),
            Instant.now()
        ));
    }
    
    private BigDecimal calculateTotal(List<OrderItem> items) {
        return items.stream()
            .map(item -> item.getPrice().multiply(item.getQuantity()))
            .reduce(BigDecimal.ZERO, BigDecimal::add);
    }
}

What happens when we need to update our event schema? Event versioning allows us to evolve our system without breaking existing functionality. We can handle this through upcasting or event adapters.

Projections build the read model from events. This separation allows us to optimize queries without affecting the write side:

@Component
public class OrderProjection {
    @EventHandler
    public void on(OrderCreatedEvent event) {
        // Update read model in MongoDB
        mongoTemplate.save(new OrderView(
            event.getOrderId(),
            event.getCustomerId(),
            event.getTotalAmount(),
            "CREATED"
        ));
    }
}

Testing event-sourced systems requires a different approach. Axon provides excellent testing utilities:

@SpringBootTest
class OrderAggregateTest {
    @Autowired
    private FixtureConfiguration<OrderAggregate> fixture;
    
    @Test
    void testOrderCreation() {
        fixture.givenNoPriorActivity()
            .when(new CreateOrderCommand("order1", "customer1", items))
            .expectEvents(new OrderCreatedEvent("order1", "customer1", total, now()));
    }
}

Performance considerations are important. We can optimize by using snapshots for aggregates with long event histories, and we can scale read and write sides independently.

When implementing this pattern, remember that it’s particularly valuable for systems requiring strong audit capabilities, complex business workflows, or temporal querying. The initial learning curve pays dividends in maintainability and flexibility.

I’d love to hear about your experiences with event sourcing. What challenges have you faced, and how have you overcome them? Share your thoughts in the comments below, and if you found this helpful, please like and share with others who might benefit from this approach.