django cache middleware

django cache middleware

"""

缓存中间件。如果可能，每一个django页面将会基于URL缓存，这种能够缓存中间件的方法设置为updatedCacheMiddleware为第一个中间件，FetchFromCacheMiddleware为最后一个
MIDDLEWARE = 
    [    'django.middleware.cache.UpdateCacheMiddleware',
        ...
        'django.middleware.cache.FetchFromCacheMiddleware'
    ]

在响应阶段，UpdateCacheMiddleware需要最后执行，处理中间件需要自顶向上，在请求阶段，FetchFromCacheMiddleware需要最后执行，处理中间件自定向上
简单的类CacheMiddleware被一些简单的网站使用，然而，如果任何其他的一个缓存中间件需要影响缓存键，你将需要两部分UpdateCacheMiddleware`` 和
``FetchFromCacheMiddleware`，当你使用djanog的LocaleMiddleware这个经常发生。

"""

from django.conf import settings
from django.core.cache import DEFAULT_CACHE_ALIAS, caches
from django.utils.cache import (
    get_cache_key, get_max_age, has_vary_header, learn_cache_key,
    patch_response_headers,
)
from django.utils.deprecation import MiddlewareMixin

class UpdateCacheMiddleware(MiddlewareMixin):
  """ 如果响应被缓存，响应阶段的缓存中间件更新，两部分更新和抓取缓存中间件必须有一部分被使用。
    UpdateCacheMiddleware在MIDDLEWARE中必须成为中间件的第一部分
    在响应阶段它必须最后被获得"""
    def __init__(self, get_response=None):
        self.cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
        self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
        self.cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
        self.cache = caches[self.cache_alias]
        self.get_response = get_response

    def _should_update_cache(self, request, response):
        return hasattr(request, '_cache_update_cache') and request._cache_update_cache

    def process_response(self, request, response):
        """Set the cache, if needed."""
        if not self._should_update_cache(request, response):
            # We don't need to update the cache, just return.
            return response

        if response.streaming or response.status_code not in (200, 304):
            return response

        # Don't cache responses that set a user-specific (and maybe security
        # sensitive) cookie in response to a cookie-less request.
        if not request.COOKIES and response.cookies and has_vary_header(response, 'Cookie'):
            return response

        # Don't cache a response with 'Cache-Control: private'
        if 'private' in response.get('Cache-Control', ()):
            return response

        # Try to get the timeout from the "max-age" section of the "Cache-
        # Control" header before reverting to using the default cache_timeout
        # length.
        timeout = get_max_age(response)
        if timeout is None:
            timeout = self.cache_timeout
        elif timeout == 0:
            # max-age was set to 0, don't bother caching.
            return response
        patch_response_headers(response, timeout)
        if timeout and response.status_code == 200:
            cache_key = learn_cache_key(request, response, timeout, self.key_prefix, cache=self.cache)
            if hasattr(response, 'render') and callable(response.render):
                response.add_post_render_callback(
                    lambda r: self.cache.set(cache_key, r, timeout)
                )
            else:
                self.cache.set(cache_key, response, timeout)
        return response

class FetchFromCacheMiddleware(MiddlewareMixin):
    """
   请求阶段的缓存中间件从缓存中抓取一个页面。

   如果请求被缓存，请求阶段的缓存中间件更新，两部分更新和抓取缓存中间件必须有一部分被使用。
    UpdateCacheMiddleware在MIDDLEWARE中必须成为中间件的第一部分
    在请求阶段它必须最后被获得。

    Must be used as part of the two-part update/fetch cache middleware.
    FetchFromCacheMiddleware must be the last piece of middleware in MIDDLEWARE
    so that it'll get called last during the request phase.
    """
    def __init__(self, get_response=None):
        self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
        self.cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
        self.cache = caches[self.cache_alias]
        self.get_response = get_response

    def process_request(self, request):
        """
        Check whether the page is already cached and return the cached
        version if available.
        """
        if request.method not in ('GET', 'HEAD'):
            request._cache_update_cache = False
            return None  # Don't bother checking the cache.

        # try and get the cached GET response
        cache_key = get_cache_key(request, self.key_prefix, 'GET', cache=self.cache)
        if cache_key is None:
            request._cache_update_cache = True
            return None  # No cache information available, need to rebuild.
        response = self.cache.get(cache_key)
        # if it wasn't found and we are looking for a HEAD, try looking just for that
        if response is None and request.method == 'HEAD':
            cache_key = get_cache_key(request, self.key_prefix, 'HEAD', cache=self.cache)
            response = self.cache.get(cache_key)

        if response is None:
            request._cache_update_cache = True
            return None  # No cache information available, need to rebuild.

        # hit, return cached response
        request._cache_update_cache = False
        return response

class CacheMiddleware(UpdateCacheMiddleware, FetchFromCacheMiddleware):
    """
    缓存中间件对于许多简单的网站提供基本的行为
    对于缓存装饰器作为钩子点使用，缓存装饰器使用中间件装饰器产生。
    """
    def __init__(self, get_response=None, cache_timeout=None, **kwargs):
        self.get_response = get_response
        # We need to differentiate between "provided, but using default value",
        # and "not provided". If the value is provided using a default, then
        # we fall back to system defaults. If it is not provided at all,
        # we need to use middleware defaults.

        try:
            key_prefix = kwargs['key_prefix']
            if key_prefix is None:
                key_prefix = ''
        except KeyError:
            key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
        self.key_prefix = key_prefix

        try:
            cache_alias = kwargs['cache_alias']
            if cache_alias is None:
                cache_alias = DEFAULT_CACHE_ALIAS
        except KeyError:
            cache_alias = settings.CACHE_MIDDLEWARE_ALIAS
        self.cache_alias = cache_alias

        if cache_timeout is None:
            cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
        self.cache_timeout = cache_timeout
        self.cache = caches[self.cache_alias]