How does a change in contract status trigger further calculations? The status in the database has changed — what’s next?

Changing the contract status in the database typically initiates a chain of events that trigger further calculations. In a typical Java system, this is implemented through several approaches:

1. Event-driven architecture: when the status is updated in the database, an event is generated (for example, via Domain Events mechanism or a message broker). Subscribers to this event (event listeners) initiate the necessary calculations.

2. Database triggers: for simple scenarios, triggers are sometimes used to call stored procedures for calculations.

3. Observer or Callback pattern: in the service layer, after a successful status change, methods responsible for calculations are invoked.

Example in Java using events:

// Status change event
public class ContractStatusChangedEvent {
    private final Long contractId;
    private final String newStatus;

    public ContractStatusChangedEvent(Long contractId, String newStatus) {
        this.contractId = contractId;
        this.newStatus = newStatus;
    }

    // getters
}

// Service updating the status
public void updateContractStatus(Long contractId, String status) {
    contractRepository.updateStatus(contractId, status);
    eventPublisher.publish(new ContractStatusChangedEvent(contractId, status));
}

// Event handler
@EventListener
public void onContractStatusChanged(ContractStatusChangedEvent event) {
    calculationService.recalculate(event.getContractId());
}

Thus, after changing the status in the database, business logic is triggered, which performs the necessary calculations.