Built on the JVM. Plugs into your stack.

@AuditAction(
    action = "USER_PROFILE_UPDATED",
    description = "#{#user.email} updated by #{#actor}",
    payload = "#{#user}"
)
public User updateProfile(User user, String actor) {
    // your business logic — audit happens AFTER_COMMIT
    return userRepository.save(user);
}

@NucleusListener(
    topic = "nucleus.payment.completed",
    group = "loyalty-points"
)
public void awardLoyaltyPoints(PaymentEvent event) {
    // runs whether broker is Kafka, Solace,
    // RabbitMQ, SQS, or Chronicle Queue
    pointsService.award(event.userId(), event.amount());
}

@WorkflowStep(name = "VALIDATE_INVOICE")
public StepResult validate(Invoice inv) {
    if (inv.amount().isNegative()) {
        return StepResult.abort("NEGATIVE_AMOUNT");
    }
    if (inv.requiresApproval()) {
        return StepResult.delegate("APPROVAL");
    }
    return StepResult.handled(inv);
}

@SpringBootApplication
@EnableGatewaySecurity   // that's it
public class PaymentsServiceApp {
    public static void main(String[] args) {
        SpringApplication.run(PaymentsServiceApp.class, args);
    }
}
// service self-registers with the gateway,
// rejects any traffic missing the marker token