Override a write use case

Goal: add city-specific behaviour to a write — here, reject creating an event whose capacity exceeds a city limit, then fall back to the platform logic — reusing the platform implementation instead of re-writing it.

• Seam: CreateEventUseCase<T extends EventDto, R extends EventRequestDto>.
• Door: extends DefaultCreateEventUseCase (reuse the platform body).
• City files: one @Service.
• Applies to: both topologies.

Recipe

Extend the platform default, add your rule, then delegate to super. Because the platform default carries @Transactional and @CacheEvict on execute, and those annotations act on the called method, you must re-declare them on your override — a super.execute(…) call is a self-invocation and would not re-trigger them.

// com.modelcity.leisure.events.usecase.MyCreateEventUseCase
@Service
public class MyCreateEventUseCase extends DefaultCreateEventUseCase {

    private static final int CITY_CAPACITY_LIMIT = 5000;

    public MyCreateEventUseCase(EventStore<? extends EventView, EventRequestDto> eventStore,
                                EventWriteValidator validator,
                                StripeFacade stripeFacade,
                                SystemTrailGenerator trails) {
        super(eventStore, validator, stripeFacade, trails);
    }

    @Override
    @Transactional
    @CacheEvict(cacheNames = CacheNames.EVENTS, allEntries = true)
    public EventDto execute(String sub, EventRequestDto request, String locale) {
        if (request.getCapacity() != null && request.getCapacity() > CITY_CAPACITY_LIMIT) {
            throw new ResponseStatusException(HttpStatus.UNPROCESSABLE_ENTITY,
                    "Event capacity exceeds the city limit of " + CITY_CAPACITY_LIMIT);
        }
        return super.execute(sub, request, locale);   // platform validation, Stripe, persist, trail
    }
}

DefaultCreateEventUseCase is @ModelCityDisabledIfInherited; your subclass covers the CreateEventUseCase seam, so the default's bean definition is removed at startup and there is no duplicate.

Notes

• Two doors, pick per need. extends DefaultCreateEventUseCase reuses the whole body (best for add a guard / a side effect). implements CreateEventUseCase<…> from scratch is better when you replace the logic entirely or bind subtype DTOs (see Expose a new field).
• Always re-declare the annotations you want. Dropping @CacheEvict here would leave the events list cache stale after a create; dropping @Transactional would lose atomicity.
• Reuse the platform EventWriteValidator, StripeFacade and SystemTrailGenerator — they are ordinary beans injected through the inherited constructor.
• A thrown ResponseStatusException is turned into the right HTTP status by the platform's GlobalExceptionHandler (in commons), so you get a clean 422 without extra wiring. For a domain error with a code, throw ModelCityException instead.

Verify

POST /events with capacity > 5000 returns the validation error; a normal create still persists, evicts the list cache and writes the audit trail exactly as before.