As systems grow in complexity, traditional CRUD models struggle with performance, auditability, and business flexibility. Event Sourcing and CQRS (Command Query Responsibility Segregation) are two architectural patterns that address these limitations by decoupling read and write models and storing all state changes as a sequence of events.

In this guide, we’ll explore how to implement Event Sourcing and CQRS in Spring Boot, understand their core principles, and leverage libraries like Axon Framework to bring these patterns to production.

What is Event Sourcing?

Event Sourcing is a pattern where the state of an application is derived by replaying a series of domain events rather than storing current state directly.

Benefits:

  • Complete audit history
  • Rebuild state at any point in time
  • Easy integration with analytics and projections
  • Enables temporal queries and debugging

What is CQRS?

CQRS splits the application into two sides:

  • Command Side – handles writes (create/update/delete)
  • Query Side – handles reads (optimized for performance)

This separation allows:

  • Different data models for reading and writing
  • Better scalability and performance tuning
  • Isolation of business logic vs read optimization

Key Components of a CQRS + Event Sourcing System

  1. Commands – represent intent to perform an action
  2. Events – immutable records of something that happened
  3. Aggregates – core domain entities that process commands and emit events
  4. Projections – read-optimized views of data built from events
  5. Event Store – persistent log of all domain events

Introducing Axon Framework

Axon is a mature Java framework for implementing CQRS and Event Sourcing with Spring Boot.

Add the Axon dependency:

<dependency>
<groupId>org.axonframework</groupId>
<artifactId>axon-spring-boot-starter</artifactId>
<version>4.8.0</version>
</dependency>

Configure Axon in application.yml:

axon:
eventhandling:
processors:
tracking:
mode: tracking
serializer:
general: jackson

Defining a Command and Aggregate

Create a command:

public class CreateOrderCommand {
@TargetAggregateIdentifier
private final String orderId;
private final String product;
private final int quantity;
}

Aggregate handles the command and emits an event:

@Aggregate
public class OrderAggregate {

    @AggregateIdentifier
    private String orderId;

    public OrderAggregate() {}

    @CommandHandler
    public OrderAggregate(CreateOrderCommand cmd) {
        AggregateLifecycle.apply(new OrderCreatedEvent(cmd.getOrderId(), cmd.getProduct(), cmd.getQuantity()));
    }

    @EventSourcingHandler
    public void on(OrderCreatedEvent event) {
        this.orderId = event.getOrderId();
    }
}

Event Handler and Projection

Define the event:

public class OrderCreatedEvent {
private final String orderId;
private final String product;
private final int quantity;
}

Project the event into a read model:

@Component
public class OrderProjection {

    private final OrderRepository repository;

    @EventHandler
    public void on(OrderCreatedEvent event) {
        OrderEntity entity = new OrderEntity(event.getOrderId(), event.getProduct(), event.getQuantity());
        repository.save(entity);
    }
}

This creates a materialized view optimized for queries.

Query Handling

Use @QueryHandler to respond to queries:

@QueryHandler
public List<OrderEntity> handle(GetAllOrdersQuery query) {
return repository.findAll();
}

You can now expose read models via REST without coupling them to write logic.

Benefits of Spring Boot + Axon

  • Axon integrates deeply with Spring Boot for auto-configuration
  • Works with JPA, MongoDB, or custom stores
  • Enables sagas and distributed command buses for microservices
  • Provides Axon Server as an optional event store and routing mechanism

Testing Aggregates and Events

Use Axon’s test fixture for behavior-driven tests:

AggregateTestFixture<OrderAggregate> fixture = new AggregateTestFixture<>(OrderAggregate.class);

@Test
void shouldEmitEventOnCommand() {
fixture.givenNoPriorActivity()
.when(new CreateOrderCommand("123", "Laptop", 2))
.expectEvents(new OrderCreatedEvent("123", "Laptop", 2));
}

This ensures your command logic produces correct events.

Use Cases for CQRS and Event Sourcing

  • Financial applications needing complete audit history
  • E-commerce systems with high read/write asymmetry
  • Collaborative platforms requiring change tracking
  • Complex domains benefiting from Domain-Driven Design (DDD)

Best Practices

  • Use immutable events with clear naming conventions
  • Keep aggregates focused and small
  • Avoid projecting too many views from one event (single responsibility)
  • Store events in append-only fashion
  • Monitor replay times and optimize indexing for projections

Conclusion

By applying CQRS and Event Sourcing in Spring Boot, you can build systems that are more scalable, audit-friendly, and aligned with complex business requirements. Tools like Axon Framework make it practical to implement these patterns with full Spring integration.

If you’re designing high-volume, domain-rich microservices, this architecture empowers you to handle complexity with clarity and maintainability.