设计购票系统时，需要考虑到不同的功能模块，如用户、订单、支付和票管理等。以下是相关数据表的设计及关键Java伪代码实现，并发处理和数据一致性方面的考虑。

数据库表设计

1. 用户表 (users)

   • user_id (主键)
   • username
   • balance

2. 票表 (tickets)

   • ticket_id (主键)
   • status （例如 available, sold）

3. 订单表 (orders)

   • order_id (主键)
   • user_id (外键, 参照 users.user_id)
   • ticket_id (外键, 参照 tickets.ticket_id, 可为空)
   • amount
   • status (如 pending, success, failed, withdrawn)

4. 队列表 (queue)

   • queue_id (主键)
   • user_id (外键, 参照 users.user_id)
   • status (如 in_queue, withdrawn)

核心伪代码实现

class TicketingSystem {

    private final PaymentService paymentService;
    private final TicketService ticketService;
    private final OrderService orderService;
    private final QueueService queueService;

    public TicketingSystem() {
        // Initialize services
        paymentService = new PaymentService();
        ticketService = new TicketService();
        orderService = new OrderService();
        queueService = new QueueService();
    }

    // 用户提交购票申请
    public void submitTicketRequest(int userId, double prePaymentAmount) {
        synchronize (this) {
            if (queueService.isUserInQueue(userId)) {
                throw new IllegalStateException("User already in queue");
            }
            // 创建订单
            int orderId = orderService.createOrder(userId, prePaymentAmount, OrderStatus.PENDING);
            // 将用户加入队列
            queueService.addUserToQueue(userId);
        }
    }

    // 查询排队人数
    public int getQueuePosition(int userId) {
        return queueService.getQueuePosition(userId);
    }

    // 后台处理购票请求
    public void processTicketRequests() {
        while (true) {
            int userId = queueService.getNextUserInQueue();
            if (userId == -1) break;
            
            Order order = orderService.getPendingOrderByUserId(userId);
            if (order == null) continue;

            synchronize (this) {
                if (paymentService.freezeAmount(userId, order.getAmount())) {
                    int ticketId = ticketService.allocateTicket(order.getOrderId());
                    if (ticketId != -1) {
                        // 扣款并更新订单状态
                        paymentService.debitAmount(userId, order.getAmount());
                        orderService.updateOrderStatus(order.getOrderId(), OrderStatus.SUCCESS);
                        orderService.updateOrderTicket(order.getOrderId(), ticketId);
                        queueService.removeUserFromQueue(userId);
                    } else {
                        // 出票失败，解冻用户金额并更新订单状态
                        paymentService.unfreezeAmount(userId, order.getAmount());
                        orderService.updateOrderStatus(order.getOrderId(), OrderStatus.FAILED);
                    }
                } else {
                    orderService.updateOrderStatus(order.getOrderId(), OrderStatus.FAILED);
                }
            }
        }
    }

    // 用户撤回购票申请
    public void withdrawTicketRequest(int userId) {
        synchronize (this) {
            if (!queueService.isUserInQueue(userId)) {
                throw new IllegalStateException("User is not in the queue");
            }
            queueService.removeUserFromQueue(userId);
            Order order = orderService.getPendingOrderByUserId(userId);
            if (order != null && order.getStatus() == OrderStatus.PENDING) {
                orderService.updateOrderStatus(order.getOrderId(), OrderStatus.WITHDRAWN);
                paymentService.unfreezeAmount(userId, order.getAmount());
            }
        }
    }

}

class PaymentService {
    // 预付款冻结资金
    public boolean freezeAmount(int userId, double amount) {
        // 逻辑实现
    }
  
    // 扣款
    public void debitAmount(int userId, double amount) {
        // 逻辑实现
    }

    // 解冻金额
    public void unfreezeAmount(int userId, double amount) {
        // 逻辑实现
    }
}

class TicketService {
    // 分配票
    public int allocateTicket(int orderId) {
        // 逻辑实现（并发时注意减小锁粒度）
    }
}

class OrderService {
    // 创建订单
    public int createOrder(int userId, double amount, OrderStatus status)