Question

我正在使用需要查询PostgreSQL数据库的Django Web应用程序。使用Python threading接口实现并发时，我查询的项目出现DoesNotExist错误。当然，顺序执行查询时不会发生这些错误。

让我展示一个单元测试，我写这篇测试是为了演示出乎意料的行为：

class ThreadingTest(TestCase):
    fixtures = ['demo_city',]

    def test_sequential_requests(self):
        """
        A very simple request to database, made sequentially.

        A fixture for the cities has been loaded above. It is supposed to be
        six cities in the testing database now. We will made a request for
        each one of the cities sequentially.
        """
        for number in range(1, 7):
            c = City.objects.get(pk=number)
            self.assertEqual(c.pk, number)

    def test_threaded_requests(self):
        """
        Now, to test the threaded behavior, we will spawn a thread for
        retrieving each city from the database.
        """

        threads = []
        cities = []

        def do_requests(number):
            cities.append(City.objects.get(pk=number))

        [threads.append(threading.Thread(target=do_requests, args=(n,))) for n in range(1, 7)]

        [t.start() for t in threads]
        [t.join() for t in threads]

        self.assertNotEqual(cities, [])

正如您所看到的，第一个测试按顺序执行一些数据库请求，这确实没有问题。但是，第二个测试执行的请求完全相同，但每个请求都是在一个线程中生成的。这实际上是失败的，返回DoesNotExist例外。

执行此单元测试的输出如下：

test_sequential_requests (cesta.core.tests.threadbase.ThreadingTest) ... ok
test_threaded_requests (cesta.core.tests.threadbase.ThreadingTest) ...

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 45, in do_requests
    cities.append(City.objects.get(pk=number))
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/manager.py", line 132, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
DoesNotExist: City matching query does not exist.

...其他线程返回类似的输出......

Exception in thread Thread-6:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 45, in do_requests
    cities.append(City.objects.get(pk=number))
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/manager.py", line 132, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/home/jose/Work/cesta/trunk/parts/django/django/db/models/query.py", line 349, in get
    % self.model._meta.object_name)
DoesNotExist: City matching query does not exist.


FAIL

======================================================================
FAIL: test_threaded_requests (cesta.core.tests.threadbase.ThreadingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jose/Work/cesta/trunk/src/cesta/core/tests/threadbase.py", line 52, in test_threaded_requests
    self.assertNotEqual(cities, [])
AssertionError: [] == []

----------------------------------------------------------------------
Ran 2 tests in 0.278s

FAILED (failures=1)
Destroying test database for alias 'default' ('test_cesta')...

请记住，所有这些都发生在PostgreSQL数据库中，该数据库应该是线程安全的，而不是SQLite或类似物。测试也是使用PostgreSQL运行的。

在这一点上，我完全迷失了可能失败的东西。有什么想法或建议吗？

谢谢！

编辑：我写了一个小视图，只是为了检查它是否在测试之外。以下是视图的代码：

def get_cities(request):
    queue = Queue.Queue()

    def get_async_cities(q, n):
        city = City.objects.get(pk=n)
        q.put(city)

    threads = [threading.Thread(target=get_async_cities, args=(queue, number)) for number in range(1, 5)]

    [t.start() for t in threads]
    [t.join() for t in threads]

    cities = list()

    while not queue.empty():
        cities.append(queue.get())

    return render_to_response('async/cities.html', {'cities': cities},
        context_instance=RequestContext(request))

（请不要考虑在视图代码中编写应用程序逻辑的愚蠢。请记住，这只是一个概念证明，并不会永远不会出现在真正的应用程序中。）

结果是代码运行良好，请求在线程中成功完成，视图最终在调用URL后显示城市。

因此，我认为使用线程进行查询只会在需要测试代码时出现问题。在生产中，它将毫无问题地工作。

有成功测试此类代码的任何有用建议吗？

Answer 1

尝试使用TransactionTestCase：

class ThreadingTest(TransactionTestCase):

TestCase将数据保存在内存中，不会向数据库发出COMMIT。可能线程正在尝试直接连接到DB，而数据尚未提交。在这里看到描述： https://docs.djangoproject.com/en/dev/topics/testing/?from=olddocs#django.test.TransactionTestCase

除了方式之外，TransactionTestCase和TestCase是相同的其中数据库重置为已知状态和能力测试代码以测试提交和回滚的效果。一个 TransactionTestCase在测试运行之前重置数据库截断所有表并重新加载初始数据。一个 TransactionTestCase可以调用commit和rollback并观察这些调用对数据库的影响。

Answer 2

这听起来像是交易的问题。如果您正在当前请求（或测试）中创建元素，那么它们几乎肯定是在未提交的事务中，无法从另一个线程中的单独连接访问。您可能需要manage your transctions manually才能使其正常运行。

Answer 3

从文档的这一部分变得更加清晰

class LiveServerTestCase(TransactionTestCase):
    """
    ...
    Note that it inherits from TransactionTestCase instead of TestCase because
    the threads do not share the same transactions (unless if using in-memory
    sqlite) and each thread needs to commit all their transactions so that the
    other thread can see the changes.
    """

现在，事务尚未在TestCase中提交，因此其他线程看不到更改。

使用线程时Django中的数据库错误

3 个答案: