Django @transaction.atomic() 防止并发创建对象

Question

我有一个票证模型，以及它的票证序列化器。工单模型有一个 bought 和一个 booked_at 字段。还有一个用于显示和座位的 unique_together 属性。

class Ticket(models.Model):
    show = models.ForeignKey(Show, on_delete=models.CASCADE)
    seat = models.ForeignKey(Seat, on_delete=models.CASCADE)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    booked_at = models.DateTimeField(default=timezone.now)
    bought = models.BooleanField(default=False)

    class Meta:
        unique_together = ('show', 'seat')

在票证序列化器上，验证时的序列化器检查是否有具有所需座位和演出的票证
- 如果有票，则检查票是否已购买。
  - 如果买了，就会报错。
  - 如果没有买，那么在5分钟内查看是否已订票。
    - 如果在 5 分钟内被预订，则报错。
    - 否则如果预订时间超过 5 分钟，则删除旧票并且 return 有效。
如果没有票则return有效

TicketSerializer:

class TicketSerializer(serializers.Serializer):
    seat = serializers.PrimaryKeyRelatedField(queryset=Seat.objects.all())
    show = serializers.PrimaryKeyRelatedField(queryset=Show.objects.all())
    user = serializers.PrimaryKeyRelatedField(queryset=User.objects.all())
    bought = serializers.BooleanField(default=False)

    def validate(self, attrs):
        if attrs['seat']:
            try:
                ticket = Ticket.objects.get(show=attrs['show'], seat=seat)
                if not ticket.bought:
                    if ticket.booked_at < timezone.now() - datetime.timedelta(minutes=5):
                        # ticket booked crossed the deadline
                        ticket.delete()
                        return attrs
                    else:
                        # ticket in 5 mins range
                        raise serializers.ValidationError("Ticket with same show and seat exists.")
                else:
                    raise serializers.ValidationError("Ticket with same show and seat exists.")
            except Ticket.DoesNotExist:
                return attrs
        else:
            raise serializers.ValidationError("No seat value provided.")

在视图中，我正在使用 @transaction.atomic() 来确保 ticket/s 仅在所有有效时才创建，或者如果无效则不创建任何票证。

@transaction.atomic()
@list_route(
    methods=['POST'],
    permission_classes=[IsAuthenticated],
    url_path='book-tickets-by-show/(?P<show_id>[0-9]+)'
)
def book_tickets_by_show(self, request, show_id=None):
    try:
        show = Show.objects.get(id=show_id)
        user = request.user
        ...
        ...
        data_list = [...]
        with transaction.atomic():
            try:
                serializer = TicketSerializer(data=data_list, many=True)
                if serializer.is_valid():
                    serializer.save()
                    ....
                return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
            except (Seat.DoesNotExist, ValueError, ConnectionError) as e:
                return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)
    except (Show.DoesNotExist, IntegrityError) as e:
        return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)

我想知道的是，它是否有助于防止调用多个请求来为同一个 seat/s 创建 ticket/s？

假设，用户A想订5,6号座位的票。用户 B 想要预订 3、6 号座位的机票，另一个用户 C 想要预订 2、3、4、5、6 号座位的机票。

上述方法是否会阻止为所有用户预订各自座位的门票，而只为一个用户（可能是第一个交易的用户）创建门票？或者，如果有更好的方法，请告诉我如何做。我希望我说得很清楚。如果不是请追问。

Answer 1

您应该使用显式分布式锁来同步请求，而不是依赖 transaction.atomic 这并不意味着是锁。

有 various ways 来实现锁，但在我们的 Django/Gunicorn 项目中，我们使用 Python 自己的 multiprocessing.Lock 来确保请求进入一个块一次编码一个。这是一个对我们有用的相对简单的解决方案。

import multiprocessing

_lock = multiprocessing.Lock()

_lock.acquire()
try:
    # Some code that needs to be accessed by one request a time
finally:
    _lock.release()

Answer 2

Will it help in preventing when more than one requests are called for creating the ticket/s for same seat/s.

是的，会的。 unique_together 约束加上 transaction.atomic() 将确保您不能为同一个 seat/show 创建两张工单。

也就是说，您当前的方法存在几个问题：

我认为没有必要包装整个视图以及在 atomic() 中进行保存的位 - 你不需要两者都做并将整个视图包装在一个事务以性能成本为代价。在交易中包装 serializer.save() 应该就足够了。

不建议在事务中捕获异常 - 请参阅 the warning in the documentation。通常最好在尽可能靠近可以生成异常的代码处捕获异常，以避免混淆。我建议将代码重构为类似这样的内容。

try:
    show = Show.objects.get(id=show_id)
# Catch this specific exception where it happens, rather than at the bottom.
except Show.DoesNotExist as e:
    return Response({'detail': str(e)}

user = request.user
...
...
data_list = [...]

try:
    serializer = TicketSerializer(data=data_list, many=True)
    if serializer.is_valid():
        try:
            # Note - this is now *inside* a try block, not outside
            with transaction.atomic():
                serializer.save()
                ....
        except IntegrityError as e:
            return Response({'detail': str(e), status=status.HTTP_400_BAD_REQUEST}

    return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
# Retained from your code - althought I am not sure how you would 
# end up with ever get a Seat.DoesNotExist or ValueError error here
# Would be better to catch them in the place they can occur.
except (Seat.DoesNotExist, ValueError, ConnectionError) as e:
    return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)

Answer 3

下面的怎么样创建间歇性 table

class showAndSeat(models.Model):
    show = models.ForeignKey(Show, on_delete=models.CASCADE)
    seat = models.ForeignKey(Seat, on_delete=models.CASCADE)
    showtime = models.DateTimeField(default=timezone.now)
    ...

    class Meta:
        unique_together = ('show', 'seat', 'showtime')

您现有的 class Ticket 将有一个用于 showAndSeat 的外键（唯一的限制是您必须使用一些 cron 创建 showAndSeat ）

将现有视图更改为

def book_tickets_by_show(self, request, show_id=None):
    ....
    ...
    ...
    try:
        with transaction.atomic():
           seat_list_from_user = [1,2,3,4] # get the list from the request
           lock_ticket = showAndSeat.objects.select_for_update(nowait=True).filter(seat__number__in=seat_list_from_user,show = selected_show_timings)
           serializer = TicketSerializer(data=data_list, many=True)
            if serializer.is_valid():
                serializer.save()
           return GOOD_Response()
    except  DatabaseError :
        # Tickets are locked by some one else 
    except (Show.DoesNotExist, IntegrityError) as e:
        return Response({'detail': str(e)}, status=status.HTTP_400_BAD_REQUEST)        
    except :
        # some other unhandled error 
        return BAD_RESPONSE()

Django @transaction.atomic() 防止并发创建对象

Django @transaction.atomic() to prevent creating objects in concurrency

django

django-database