@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:
   # 根據(jù)請(qǐng)求和響應(yīng)參數(shù)、設(shè)定的key_prefix生成頁(yè)面緩存的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.
  """
  # 只對(duì)方法為 GET 或 HEAD 的請(qǐng)求獲取緩存
  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
  # 這里會(huì)根據(jù)請(qǐng)求的信息、緩存鍵前綴生成一個(gè)cache_key。默認(rèn)情況下，訪問(wèn)同一個(gè)接口其cache_key應(yīng)該相同
  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.
  # 如果獲取到response，則直接返回緩存的response，那么實(shí)際的視圖就不會(huì)被執(zhí)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:
   # 如果沒(méi)有獲取到緩存，將返回None，則會(huì)執(zhí)行到實(shí)際的視圖，并且重建緩存
   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):
 # 見(jiàn)下文，這個(gè)cache_key由 request的完整url 以及 key_prefix 唯一確定
 cache_key = _generate_cache_header_key(key_prefix, request)
 if cache is None:
  # cache 是一個(gè)緩存實(shí)例
  cache = caches[settings.CACHE_MIDDLEWARE_ALIAS]
 # Vary 是一個(gè)HTTP響應(yīng)頭字段。其內(nèi)容是一個(gè)或多個(gè)http頭部名稱
 # 比如 `Vary: User-Agent` 表示此響應(yīng)根據(jù)請(qǐng)求頭 `User-Agent` 的值有所不同
 # 只有當(dāng)下一個(gè)請(qǐng)求的 `User-Agent` 值與當(dāng)前請(qǐng)求相同時(shí)，才會(huì)使用當(dāng)前響應(yīng)的緩存
 if response.has_header('Vary'):
  headerlist = []
  for header in cc_delim_re.split(response['Vary']):
   # 將 Vary 中出現(xiàn)的 http頭部名稱 加到 headerlist 中去
   header = header.upper().replace('-', '_')
   headerlist.append('HTTP_' + header)
  headerlist.sort()
  # 當(dāng)前 cache_key 實(shí)際上是 cache_header_key，它存的是響應(yīng)頭中Vary字段的值
  cache.set(cache_key, headerlist, cache_timeout)
  # 這里返回的才是頁(yè)面內(nèi)容對(duì)應(yīng)的 cache_key，它由 
  # 出現(xiàn)在Vary字段中的request請(qǐng)求頭字段的值(有序拼在一起)、request的完整url、request的method、key_prefix 唯一確定
  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()返回的是完整的請(qǐng)求URL。如 http://127.0.0.1:8000/api/leaflet/filterList?a=1
 # 因此，請(qǐng)求同一個(gè)接口，但是接口參數(shù)不同，會(huì)生成兩個(gè)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是響應(yīng)頭中Vary字段的值
 for header in headerlist:
  # 出現(xiàn)在Vary字段中的request請(qǐng)求頭字段的值
  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)
​ 再看獲取緩存的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是之前緩存的 與當(dāng)前請(qǐng)求具有相同cache_header_key 的請(qǐng)求的響應(yīng)的響應(yīng)頭中Vary字段的值
 headerlist = cache.get(cache_key)
 # 即使響應(yīng)頭沒(méi)有Vary字段，還是會(huì)針對(duì)當(dāng)前 cache_header_key 存一個(gè)空數(shù)組
 # 因此如果headerlist為None，表示當(dāng)前請(qǐng)求沒(méi)有緩存
 if headerlist is not None:
  # 根據(jù) 出現(xiàn)在Vary字段中的request請(qǐng)求頭字段的值(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