Real-World Use of CQRS (Not Theory)

Real-World Use of CQRS (Not Theory)

CQRS separates command (write) and query (read) models. In practice, it is most valuable when read workloads and write workloads have different scalability or latency needs.

When CQRS Is Worth It

CQRS is a good fit when:

• You have complex read queries that do not map to the write model.
• Writes are frequent, but reads need denormalized views.
• You need independent scaling of read and write paths.

A Practical CQRS Workflow

1. A command updates the write model.
2. An event is emitted via an outbox.
3. A projection updates the read model.
4. Queries read from the projection.

Spring Boot Example: Command and Query Separation

@Service
public class OrderCommandService {
    private final OrderRepository orderRepository;
    private final OutboxRepository outboxRepository;

    public OrderCommandService(OrderRepository orderRepository,
                               OutboxRepository outboxRepository) {
        this.orderRepository = orderRepository;
        this.outboxRepository = outboxRepository;
    }

    @Transactional
    public void placeOrder(OrderCommand command) {
        Order order = Order.from(command);
        orderRepository.save(order);
        outboxRepository.save(OutboxEvent.from(order));
    }
}

@Service
public class OrderQueryService {
    private final OrderReadRepository orderReadRepository;

    public OrderQueryService(OrderReadRepository orderReadRepository) {
        this.orderReadRepository = orderReadRepository;
    }

    public OrderView getOrder(String id) {
        return orderReadRepository.findById(id).orElseThrow();
    }
}

Operational Considerations

• Monitor projection lag to detect stale reads.
• Ensure idempotent projections to handle replays.
• Use separate databases or schemas for write and read models.

Summary

CQRS is not a theoretical pattern; it is a practical response to read/write asymmetry. When used with outbox and projection pipelines, it enables scalable and maintainable systems.