Design a Movie Ticket Booking System

enum ShowSeatStatus { AVAILABLE, HELD, BOOKED }
 
final class ShowSeat {
    private final String showId;
    private final String seatId;
    private volatile ShowSeatStatus status = ShowSeatStatus.AVAILABLE;
    private volatile String heldByBookingId;
    private volatile Instant holdExpiresAt;
 
    ShowSeat(String showId, String seatId) {
        this.showId = showId;
        this.seatId = seatId;
    }
 
    synchronized boolean tryHold(String bookingId, Duration holdDuration) {
        if (status == ShowSeatStatus.AVAILABLE
                || (status == ShowSeatStatus.HELD && Instant.now().isAfter(holdExpiresAt))) {
            status = ShowSeatStatus.HELD;
            heldByBookingId = bookingId;
            holdExpiresAt = Instant.now().plus(holdDuration);
            return true;
        }
        return false; // already HELD by someone else (unexpired) or already BOOKED
    }
 
    synchronized void confirm(String bookingId) {
        if (status != ShowSeatStatus.HELD || !bookingId.equals(heldByBookingId)) {
            throw new IllegalStateException("Cannot confirm a seat this booking does not hold");
        }
        status = ShowSeatStatus.BOOKED;
    }
 
    synchronized void release(String bookingId) {
        if (status == ShowSeatStatus.HELD && bookingId.equals(heldByBookingId)) {
            status = ShowSeatStatus.AVAILABLE;
            heldByBookingId = null;
            holdExpiresAt = null;
        }
    }
}

final class SeatLockManager {
    private final Map<String, ShowSeat> showSeats; // keyed by seatId, scoped to one Show
 
    SeatLockManager(Map<String, ShowSeat> showSeats) {
        this.showSeats = showSeats;
    }
 
    List<String> holdSeats(List<String> seatIds, String bookingId, Duration holdDuration) {
        // Sort seat IDs to establish a consistent lock ORDER across all callers —
        // this is what prevents a classic deadlock (two bookings holding seats
        // in opposite order and each waiting on the other's seat).
        List<String> sorted = seatIds.stream().sorted().toList();
        List<String> held = new ArrayList<>();
 
        for (String seatId : sorted) {
            ShowSeat seat = showSeats.get(seatId);
            if (seat.tryHold(bookingId, holdDuration)) {
                held.add(seatId);
            } else {
                // Rollback: release everything we managed to hold so far
                held.forEach(id -> showSeats.get(id).release(bookingId));
                throw new SeatUnavailableException(seatId);
            }
        }
        return held;
    }
}

final class BookingService {
    ShowSeatRepository showSeatRepo;
    SeatLockManager lockManager;
    PaymentGateway paymentGateway;
 
    Booking initiateBooking(String showId, String customerId, List<String> seatIds) {
        List<String> heldSeatIds = lockManager.holdSeats(seatIds, generateBookingId(), Duration.ofMinutes(5));
        Booking booking = new Booking(generateBookingId(), showId, customerId, heldSeatIds, BookingStatus.SEATS_HELD);
        return booking; // customer now has 5 minutes to pay
    }
 
    void onPaymentSuccess(String bookingId) {
        Booking booking = findBooking(bookingId);
        booking.getSeatIds().forEach(id -> showSeatRepo.find(id).confirm(bookingId));
        booking.setStatus(BookingStatus.CONFIRMED);
    }
 
    // Runs on a scheduled interval (e.g. every 30s)
    void expireStaleHolds() {
        showSeatRepo.findExpiredHolds(Instant.now())
            .forEach(seat -> seat.release(seat.getHeldByBookingId()));
    }
}

interface PricingStrategy {
    Money priceFor(ShowSeat seat, Show show);
}
 
final class WeekendSurchargePricing implements PricingStrategy {
    private final PricingStrategy base;
    public Money priceFor(ShowSeat seat, Show show) {
        Money price = base.priceFor(seat, show);
        boolean isWeekend = show.getStartTime().getDayOfWeek() == DayOfWeek.SATURDAY
                || show.getStartTime().getDayOfWeek() == DayOfWeek.SUNDAY;
        return isWeekend ? price.multiply(1.2) : price;
    }
}

Money calculateRefund(Booking booking, Instant now) {
    Duration untilShow = Duration.between(now, booking.getShow().getStartTime());
    if (untilShow.toHours() >= 24) return booking.getTotalAmount();               // full refund
    if (untilShow.toHours() >= 2)  return booking.getTotalAmount().multiply(0.5); // 50% refund
    return Money.ZERO;                                                            // no refund inside 2h
}