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POConcurrent requests in Django
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copied!<p>I have 2 models: <code>Product</code> and <code>Order</code>.</p> <p><code>Product</code> has an integer field for stock, whereas <code>Order</code> has a status and a foreign key to <code>Product</code>:</p> <pre><code>class Product(models.Model): name = models.CharField(max_length=30) stock = models.PositiveSmallIntegerField(default=1) class Order(models.Model): product = models.ForeignKey('Product') DRAFT = 'DR'; INPROGRESS = 'PR'; ABORTED = 'AB' STATUS = ((INPROGRESS, 'In progress'),(ABORTED, 'Aborted'),) status = models.CharField(max_length = 2, choices = STATUS, default = DRAFT) </code></pre> <p><strong>My goal is to have the product's stock decrease by one for each new order, and increase by one for each order cancellation.</strong> In that purpose, I've overloaded the <code>save</code> method of the <code>Order</code> model as such (inspired by <a href="https://stackoverflow.com/questions/1355150/django-when-saving-how-can-you-check-if-a-field-has-changed">Django: When saving, how can you check if a field has changed?</a>):</p> <pre><code>from django.db.models import F class Order(models.Model): product = models.ForeignKey('Product') status = models.CharField(max_length = 2, choices = STATUS, default = DRAFT) EXISTING_STATUS = set([INPROGRESS]) __original_status = None def __init__(self, *args, **kwargs): super(Order, self).__init__(*args, **kwargs) self.__original_status = self.status def save(self, *args, **kwargs): old_status = self.__original_status new_status = self.status has_changed_status = old_status != new_status if has_changed_status: product = self.product if not old_status in Order.EXISTING_STATUS and new_status in Order.EXISTING_STATUS: product.stock = F('stock') - 1 product.save(update_fields=['stock']) elif old_status in Order.EXISTING_STATUS and not new_status in Order.EXISTING_STATUS: product.stock = F('stock') + 1 product.save(update_fields=['stock']) super(Order, self).save(*args, **kwargs) self.__original_status = self.status </code></pre> <p>Using the RestFramework, I've created 2 views, one for creating new orders, one for cancelling existing orders. Both use a straightforward serializer:</p> <pre><code>class OrderSimpleSerializer(serializers.ModelSerializer): class Meta: model = Order fields = ( 'id', 'product', 'status', ) read_only_fields = ( 'status', ) class OrderList(generics.ListCreateAPIView): model = Order serializer_class = OrderSimpleSerializer def pre_save(self, obj): super(OrderList,self).pre_save(obj) product = obj.product if not product.stock > 0: raise ConflictWithAnotherRequest("Product is not available anymore.") obj.status = Order.INPROGRESS class OrderAbort(generics.RetrieveUpdateAPIView): model = Order serializer_class = OrderSimpleSerializer def pre_save(self, obj): obj.status = Order.ABORTED </code></pre> <p>Here is how to access these two views:</p> <pre><code>from myapp.views import * urlpatterns = patterns('', url(r'^order/$', OrderList.as_view(), name='order-list'), url(r'^order/(?P<pk>[0-9]+)/abort/$', OrderAbort.as_view(), name='order-abort'), ) </code></pre> <p>I am using Django 1.6b4, Python 3.3, Rest Framework 2.7.3 and PostgreSQL 9.2.</p> <h2>My problem is that concurrent requests can increase the stock of a product higher than original stock!</h2> <p>Here is the script I use to demonstrate that:</p> <pre><code>import sys import urllib.request import urllib.parse import json opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor) def create_order(): url = 'http://127.0.0.1:8000/order/' values = {'product':1} data = urllib.parse.urlencode(values).encode('utf-8') request = urllib.request.Request(url, data) response = opener.open(request) return response def cancel_order(order_id): abort_url = 'http://127.0.0.1:8000/order/{}/abort/'.format(order_id) values = {'product':1,'_method':'PUT'} data = urllib.parse.urlencode(values).encode('utf-8') request = urllib.request.Request(abort_url, data) try: response = opener.open(request) except Exception as e: if (e.code != 403): print(e) else: print(response.getcode()) def main(): response = create_order() print(response.getcode()) data = response.read().decode('utf-8') order_id = json.loads(data)['id'] time.sleep(1) for i in range(2): p = Process(target=cancel_order, args=[order_id]) p.start() if __name__ == '__main__': main() </code></pre> <p>This script gives the following output, for a product with a stock of 1:</p> <pre><code>201 # means it creates an order for Product, thus decreasing stock from 1 to 0 200 # means it cancels the order for Product, thus increasing stock from 0 to 1 200 # means it cancels the order for Product, thus increasing stock from 1 to 2 (shouldn't happen) </code></pre> <h1>EDIT</h1> <p>I've added a sample project to reproduce the bug: <a href="https://github.com/ThinkerR/django-concurrency-demo" rel="nofollow noreferrer">https://github.com/ThinkerR/django-concurrency-demo</a></p>

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