Layers: controller, service and repository

HTTP request
   │
   ▼
Controller   ─ parse input, validate format, map to/from DTOs, pick status codes
   │
   ▼
Service      ─ business rules, transactions, calls to other services
   │
   ▼
Repository   ─ load and save entities (SQL / JPA)
   │
   ▼
Database

// Controller: HTTP only
@PostMapping("/api/bookings")
public ResponseEntity<BookingDto> book(@Valid @RequestBody BookingRequest req) {
    Booking b = bookingService.book(req.trainNo(), req.date(), req.passengerName());
    return ResponseEntity.status(HttpStatus.CREATED).body(BookingDto.from(b));
}

// Service: the rules
@Service
public class BookingService {
    private final SeatRepository seats;
    private final BookingRepository bookings;
    public BookingService(SeatRepository seats, BookingRepository bookings) { this.seats = seats; this.bookings = bookings; }

    @Transactional
    public Booking book(String trainNo, LocalDate date, String passenger) {
        if (date.isBefore(LocalDate.now())) throw new IllegalArgumentException("Cannot book a past date");
        Seat seat = seats.findFirstFree(trainNo, date)
                .orElseThrow(() -> new NoSeatsAvailableException(trainNo, date));
        seat.markBooked();
        return bookings.save(new Booking(seat, passenger));
    }
}

// Repository: data access (Spring Data generates the implementation; next topic)
public interface BookingRepository extends JpaRepository<Booking, Long> { }

public record BookingDto(long id, String trainNo, String passenger, String seat) {
    public static BookingDto from(Booking b) {
        return new BookingDto(b.getId(), b.getSeat().getTrainNo(), b.getPassenger(), b.getSeat().getLabel());
    }
}