浅谈Django 页面缓存的cache_key是如何生成的

@cache_page(time_out, key_prefix=key_prefix)
def my_view():
 ...

class UpdateCacheMiddleware(MiddlewareMixin):
 def process_response(self, request, response):
  """Sets the cache, if needed."""
  ...
  if timeout and response.status_code == 200:
   # 根据请求和响应参数、设定的key_prefix生成页面缓存的key
   cache_key = learn_cache_key(request, response, timeout, self.key_prefix, cache=self.cache)
   self.cache.set(cache_key, response, timeout)
  return response

# django/middleware/cache.py
class FetchFromCacheMiddleware(MiddlewareMixin):
 def process_request(self, request):
  """
  Checks whether the page is already cached and returns the cached
  version if available.
  """
  # 只对方法为 GET 或 HEAD 的请求获取缓存
  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
  # 这里会根据请求的信息、缓存键前缀生成一个cache_key。默认情况下，访问同一个接口其cache_key应该相同
  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.
  # 如果获取到response，则直接返回缓存的response，那么实际的视图就不会被执行
  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:
   # 如果没有获取到缓存，将返回None，则会执行到实际的视图，并且重建缓存
   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

# django/utils/cache.py
def learn_cache_key(request, response, cache_timeout=None, key_prefix=None, cache=None):
 # 见下文，这个cache_key由 request的完整url 以及 key_prefix 唯一确定
 cache_key = _generate_cache_header_key(key_prefix, request)
 if cache is None:
  # cache 是一个缓存实例
  cache = caches[settings.CACHE_MIDDLEWARE_ALIAS]
 # Vary 是一个HTTP响应头字段。其内容是一个或多个http头部名称
 # 比如 `Vary: User-Agent` 表示此响应根据请求头 `User-Agent` 的值有所不同
 # 只有当下一个请求的 `User-Agent` 值与当前请求相同时，才会使用当前响应的缓存
 if response.has_header('Vary'):
  headerlist = []
  for header in cc_delim_re.split(response['Vary']):
   # 将 Vary 中出现的 http头部名称 加到 headerlist 中去
   header = header.upper().replace('-', '_')
   headerlist.append('HTTP_' + header)
  headerlist.sort()
  # 当前 cache_key 实际上是 cache_header_key，它存的是响应头中Vary字段的值
  cache.set(cache_key, headerlist, cache_timeout)
  # 这里返回的才是页面内容对应的 cache_key，它由 
  # 出现在Vary字段中的request请求头字段的值(有序拼在一起)、request的完整url、request的method、key_prefix 唯一确定
  return _generate_cache_key(request, request.method, headerlist, key_prefix)
 else:
  # if there is no Vary header, we still need a cache key
  # for the request.build_absolute_uri()
  cache.set(cache_key, [], cache_timeout)
  return _generate_cache_key(request, request.method, [], key_prefix)

def _generate_cache_header_key(key_prefix, request):
 """Returns a cache key for the header cache."""
 # request.build_absolute_uri()返回的是完整的请求URL。如 http://127.0.0.1:8000/api/leaflet/filterList?a=1
 # 因此，请求同一个接口，但是接口参数不同，会生成两个cache_key
 url = hashlib.md5(force_bytes(iri_to_uri(request.build_absolute_uri())))
 cache_key = 'views.decorators.cache.cache_header.%s.%s' % (
  key_prefix, url.hexdigest())
 return _i18n_cache_key_suffix(request, cache_key)

def _generate_cache_key(request, method, headerlist, key_prefix):
 """Returns a cache key from the headers given in the header list."""
 ctx = hashlib.md5()
 # headerlist是响应头中Vary字段的值
 for header in headerlist:
  # 出现在Vary字段中的request请求头字段的值
  value = request.META.get(header)
  if value is not None:
   ctx.update(force_bytes(value))
 url = hashlib.md5(force_bytes(iri_to_uri(request.build_absolute_uri())))
 cache_key = 'views.decorators.cache.cache_page.%s.%s.%s.%s' % (
  key_prefix, method, url.hexdigest(), ctx.hexdigest())
 return _i18n_cache_key_suffix(request, cache_key)
​ 再看获取缓存的get_cache_key方法

def get_cache_key(request, key_prefix=None, method='GET', cache=None):
 # 由 request的完整url 以及 key_prefix 生成 cache_header_key
 cache_key = _generate_cache_header_key(key_prefix, request)
 # headerlist是之前缓存的 与当前请求具有相同cache_header_key 的请求的响应的响应头中Vary字段的值
 headerlist = cache.get(cache_key)
 # 即使响应头没有Vary字段，还是会针对当前 cache_header_key 存一个空数组
 # 因此如果headerlist为None，表示当前请求没有缓存
 if headerlist is not None:
  # 根据 出现在Vary字段中的request请求头字段的值(有序拼在一起)、request的完整url、request的method、key_prefix 生成 cache_key
  return _generate_cache_key(request, method, headerlist, key_prefix)
 else:
  return None