缓存后端在开发服务器上有效但在mod_wsgi上无效

Question

我正在使用一个自定义的缓存后端，目的是为了在内置的缓存后端上加上一层，这样我就可以把当前的站点ID加到所有的缓存键里（这对于使用单个memcached实例的多站点功能很有用）。

不过不幸的是，这在Django自带的开发服务器上运行得很好，但在我尝试在使用mod_wsgi的实际服务器上运行时，却出现了很糟糕的错误。

这是我错误日志中的追踪信息：

[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1] mod_wsgi (pid=22933): Exception occurred processing WSGI script '/home/jiaaro/webapps/op_wsgi/myProject/deploy/myProject.wsgi'.
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1] Traceback (most recent call last):
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/lib/python2.5/django/core/handlers/wsgi.py", line 230, in __call__
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     self.load_middleware()
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/lib/python2.5/django/core/handlers/base.py", line 40, in load_middleware
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     mod = import_module(mw_module)
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/lib/python2.5/django/utils/importlib.py", line 35, in import_module
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     __import__(name)
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/myProject/apps/site_settings/__init__.py", line 6, in <module>
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     import cache_wrapper
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/myProject/apps/site_settings/cache_wrapper.py", line 4, in <module>
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     from django.core.cache.backends.base import BaseCache
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/lib/python2.5/django/core/cache/__init__.py", line 73, in <module>
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     cache = get_cache(settings.CACHE_BACKEND)
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]   File "/home/jiaaro/webapps/op_wsgi/lib/python2.5/django/core/cache/__init__.py", line 68, in get_cache
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1]     return getattr(module, 'CacheClass')(host, params)
[Wed Feb 17 00:39:34 2010] [error] [client 127.0.0.1] AttributeError: 'module' object has no attribute 'CacheClass'

如果我在同一台机器上（也就是实际服务器）运行开发服务器，它就能正常工作……我可以毫无问题地做到这一点：

$ cd /home/jiaaro/webapps/op_wsgi/myProject
$ python2.5 manage.py runserver

而且在一个单独的SSH会话中……

$ wget 127.0.0.1:8000

页面能够正确显示，使用的是memcached和实际的数据库。请问mod_wsgi在导入模块的方式上有什么不同吗？我应该知道吗？

也许是开发服务器的单进程、单线程特性导致的？

我已经为这个问题挣扎了好几天，任何帮助都将不胜感激。

额外信息：

Webfaction共享主机（centos）
Apache 2
Python 2.5
Django 1.1.1
mod_wsgi 2.5
MySql 5.0

更多细节：

我已经设置了缓存后端（并且它在追踪信息中显示正确的模块被导入，说明它是有效的）
模块中有一个叫“CacheClass”的类：

site_settings/cache_wrapper.py :

from django.conf import settings
CACHE_BACKEND = getattr(settings, 'CUSTOM_CACHE_BACKEND')

from django.core.cache.backends.base import BaseCache

class CacheClass(BaseCache):
    from decorators import accept_site

    def __init__(self, *args):
        from django.core.cache import get_cache
        self.WRAPPED_CACHE = get_cache(CACHE_BACKEND)

    @accept_site
    def add(self, site, key, *args):
        return self.WRAPPED_CACHE.add(self._key(site, key),*args)

    @accept_site
    def get(self, site, key, *args):
        return self.WRAPPED_CACHE.get(self._key(site, key),*args)

    ... (all the rest of the wrapped methods)

    def _key(self, site, key):
        from exceptions import NoCurrentSite
        if not site:
            raise NoCurrentSite
        return "%s|%s" % (site.id, key)

accept_site装饰器与一些中间件一起工作，以确定当前的站点。它们如下：

decorators.py:

def accept_site(fn):
    def decorator(self, *args, **kwargs):
        site = kwargs.get('site', None)
        try: 
            del kwargs['site']
        except KeyError: 
            pass

        from .middleware import get_current_site
        site = site or get_current_site()

        if not site:
            raise NoCurrentSite("The current site is not available via thread locals, please specify a site with the 'site' keyword argument")

        return fn(self, site, *args, **kwargs)

    return decorator

以及middleware.py

try:
    from threading import local
except ImportError:
    from django.utils._threading_local import local

from django.conf import settings
from django.contrib.sites.models import Site

DEFAULT_SITE_ID = 1

_thread_locals = local()
def get_current_site():
    return getattr(_thread_locals, 'site', None)

def set_current_site(site):
    setattr(_thread_locals, 'site', site)

class SiteSettings(object):
    """Middleware that gets various objects from the
    request object and saves them in thread local storage."""
    def process_request(self, request):
        if settings.DEBUG:
            site_id = request.GET.get('site_id', DEFAULT_SITE_ID)
        else:
            site_id = DEFAULT_SITE_ID

        current_site_domain = request.META["HTTP_HOST"]
        try:
            current_site = Site.objects.get(domain__iexact=current_site_domain())
        except:
            current_site = Site.objects.get(id=site_id)
        set_current_site(current_site)

所有这些在开发服务器上都能正常工作，使用的设置与wsgi服务器相同，并且配置了相同的Python路径（就我所看到的情况而言）。

此时我希望我在某个地方创建了一个导入循环（尽管这不太合理，因为它只在开发服务器上发生）。

编辑：我在Django文档中找到了一份关于开发服务器和Apache之间差异的列表，上面提到：

开发服务器会将已安装的应用添加到 sys.path，而Apache则不会。

也许这和问题有关？

django apache mod_wsgi 开发服务器缓存错误日志多站点 memcached

缓存后端在开发服务器上有效但在mod_wsgi上无效

2 个回答

撰写回答