author | Pawel Solyga <Pawel.Solyga@gmail.com> |
Sun, 06 Sep 2009 23:31:53 +0200 | |
changeset 2864 | 2e0b0af889be |
parent 109 | 620f9b141567 |
permissions | -rwxr-xr-x |
109
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""" |
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This module contains helper functions for controlling caching. It does so by |
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managing the "Vary" header of responses. It includes functions to patch the |
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header of response objects directly and decorators that change functions to do |
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that header-patching themselves. |
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For information on the Vary header, see: |
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|
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http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.44 |
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Essentially, the "Vary" HTTP header defines which headers a cache should take |
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into account when building its cache key. Requests with the same path but |
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different header content for headers named in "Vary" need to get different |
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cache keys to prevent delivery of wrong content. |
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A example: i18n middleware would need to distinguish caches by the |
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"Accept-language" header. |
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""" |
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import datetime, md5, re |
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from django.conf import settings |
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from django.core.cache import cache |
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cc_delim_re = re.compile(r'\s*,\s*') |
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def patch_cache_control(response, **kwargs): |
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""" |
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This function patches the Cache-Control header by adding all |
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keyword arguments to it. The transformation is as follows: |
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|
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* All keyword parameter names are turned to lowercase, and underscores |
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are converted to hyphens. |
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* If the value of a parameter is True (exactly True, not just a |
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true value), only the parameter name is added to the header. |
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* All other parameters are added with their value, after applying |
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str() to it. |
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""" |
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def dictitem(s): |
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t = s.split('=',1) |
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if len(t) > 1: |
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return (t[0].lower().replace('-', '_'), t[1]) |
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else: |
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return (t[0].lower().replace('-', '_'), True) |
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def dictvalue(t): |
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if t[1] == True: |
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return t[0] |
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else: |
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return t[0] + '=' + str(t[1]) |
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if response.has_header('Cache-Control'): |
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cc = cc_delim_re.split(response['Cache-Control']) |
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cc = dict([dictitem(el) for el in cc]) |
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else: |
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cc = {} |
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for (k,v) in kwargs.items(): |
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cc[k.replace('_', '-')] = v |
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cc = ', '.join([dictvalue(el) for el in cc.items()]) |
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response['Cache-Control'] = cc |
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vary_delim_re = re.compile(r',\s*') |
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|
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def patch_response_headers(response, cache_timeout=None): |
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""" |
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Adds some useful headers to the given HttpResponse object: |
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ETag, Last-Modified, Expires and Cache-Control |
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|
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Each header is only added if it isn't already set. |
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|
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cache_timeout is in seconds. The CACHE_MIDDLEWARE_SECONDS setting is used |
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by default. |
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""" |
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if cache_timeout is None: |
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cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS |
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now = datetime.datetime.utcnow() |
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if not response.has_header('ETag'): |
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response['ETag'] = md5.new(response.content).hexdigest() |
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if not response.has_header('Last-Modified'): |
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response['Last-Modified'] = now.strftime('%a, %d %b %Y %H:%M:%S GMT') |
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if not response.has_header('Expires'): |
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expires = now + datetime.timedelta(0, cache_timeout) |
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response['Expires'] = expires.strftime('%a, %d %b %Y %H:%M:%S GMT') |
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if cache_timeout < 0: |
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cache_timeout = 0 # Can't have max-age negative |
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patch_cache_control(response, max_age=cache_timeout) |
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|
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def add_never_cache_headers(response): |
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""" |
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Add headers to a response to indicate that |
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a page should never be cached. |
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""" |
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patch_response_headers(response, cache_timeout=-1) |
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def patch_vary_headers(response, newheaders): |
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""" |
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Adds (or updates) the "Vary" header in the given HttpResponse object. |
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newheaders is a list of header names that should be in "Vary". Existing |
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headers in "Vary" aren't removed. |
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""" |
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# Note that we need to keep the original order intact, because cache |
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# implementations may rely on the order of the Vary contents in, say, |
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# computing an MD5 hash. |
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vary = [] |
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if response.has_header('Vary'): |
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105 |
vary = vary_delim_re.split(response['Vary']) |
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oldheaders = dict([(el.lower(), 1) for el in vary]) |
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for newheader in newheaders: |
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108 |
if not newheader.lower() in oldheaders: |
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109 |
vary.append(newheader) |
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response['Vary'] = ', '.join(vary) |
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111 |
|
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112 |
def _generate_cache_key(request, headerlist, key_prefix): |
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113 |
"Returns a cache key from the headers given in the header list." |
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114 |
ctx = md5.new() |
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115 |
for header in headerlist: |
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116 |
value = request.META.get(header, None) |
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117 |
if value is not None: |
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118 |
ctx.update(value) |
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119 |
return 'views.decorators.cache.cache_page.%s.%s.%s' % (key_prefix, request.path, ctx.hexdigest()) |
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120 |
|
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121 |
def get_cache_key(request, key_prefix=None): |
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122 |
""" |
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123 |
Returns a cache key based on the request path. It can be used in the |
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124 |
request phase because it pulls the list of headers to take into account |
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125 |
from the global path registry and uses those to build a cache key to check |
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126 |
against. |
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127 |
|
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128 |
If there is no headerlist stored, the page needs to be rebuilt, so this |
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129 |
function returns None. |
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130 |
""" |
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131 |
if key_prefix is None: |
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132 |
key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX |
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cache_key = 'views.decorators.cache.cache_header.%s.%s' % (key_prefix, request.path) |
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134 |
headerlist = cache.get(cache_key, None) |
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135 |
if headerlist is not None: |
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136 |
return _generate_cache_key(request, headerlist, key_prefix) |
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137 |
else: |
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138 |
return None |
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139 |
|
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140 |
def learn_cache_key(request, response, cache_timeout=None, key_prefix=None): |
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141 |
""" |
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142 |
Learns what headers to take into account for some request path from the |
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143 |
response object. It stores those headers in a global path registry so that |
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144 |
later access to that path will know what headers to take into account |
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145 |
without building the response object itself. The headers are named in the |
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146 |
Vary header of the response, but we want to prevent response generation. |
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147 |
|
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148 |
The list of headers to use for cache key generation is stored in the same |
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149 |
cache as the pages themselves. If the cache ages some data out of the |
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150 |
cache, this just means that we have to build the response once to get at |
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151 |
the Vary header and so at the list of headers to use for the cache key. |
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152 |
""" |
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153 |
if key_prefix is None: |
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154 |
key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX |
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155 |
if cache_timeout is None: |
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156 |
cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS |
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157 |
cache_key = 'views.decorators.cache.cache_header.%s.%s' % (key_prefix, request.path) |
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158 |
if response.has_header('Vary'): |
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159 |
headerlist = ['HTTP_'+header.upper().replace('-', '_') for header in vary_delim_re.split(response['Vary'])] |
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160 |
cache.set(cache_key, headerlist, cache_timeout) |
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161 |
return _generate_cache_key(request, headerlist, key_prefix) |
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162 |
else: |
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163 |
# if there is no Vary header, we still need a cache key |
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164 |
# for the request.path |
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165 |
cache.set(cache_key, [], cache_timeout) |
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166 |
return _generate_cache_key(request, [], key_prefix) |