1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
|
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
This file is generated from xml source: DO NOT EDIT
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-->
<title>mod_cache - Apache HTTP Server</title>
<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
<link href="../images/favicon.ico" rel="shortcut icon" /></head>
<body>
<div id="page-header">
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.4</p>
<img alt="" src="../images/feather.gif" /></div>
<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
<div id="path">
<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.4</a> > <a href="./">Modules</a></div>
<div id="page-content">
<div id="preamble"><h1>Apache Module mod_cache</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_cache.html" title="English"> en </a> |
<a href="../fr/mod/mod_cache.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
<a href="../ja/mod/mod_cache.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="../ko/mod/mod_cache.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>RFC 2616 compliant HTTP caching filter.</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>cache_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_cache.c</td></tr></table>
<h3>Summary</h3>
<div class="warning">This module should be used with care, as when the
<code class="directive"><a href="#cachequickhandler">CacheQuickHandler</a></code> directive is
in its default value of <strong>on</strong>, the <code class="directive"><a href="../mod/mod_access_compat.html#allow">Allow</a></code> and <code class="directive"><a href="../mod/mod_access_compat.html#deny">Deny</a></code> directives will be circumvented.
You should not enable quick handler caching for any content to which you
wish to limit access by client host name, address or environment
variable.</div>
<p><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> implements an <a href="http://www.ietf.org/rfc/rfc2616.txt">RFC 2616</a> compliant
<strong>HTTP content caching filter</strong>, with support for the caching
of content negotiated responses containing the Vary header.</p>
<p>RFC 2616 compliant caching provides a mechanism to verify whether
stale or expired content is still fresh, and can represent a significant
performance boost when the origin server supports <strong>conditional
requests</strong> by honouring the
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26">If-None-Match</a>
HTTP request header. Content is only regenerated from scratch when the content
has changed, and not when the cached entry expires.</p>
<p>As a filter, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> can be placed in front of
content originating from any handler, including <strong>flat
files</strong> (served from a slow disk cached on a fast disk), the output
of a <strong>CGI script</strong> or <strong>dynamic content
generator</strong>, or content <strong>proxied from another
server</strong>.</p>
<p>In the default configuration, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> inserts the
caching filter as far forward as possible within the filter stack,
utilising the <strong>quick handler</strong> to bypass all per request
processing when returning content to the client. In this mode of
operation, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> may be thought of as a caching
proxy server bolted to the front of the webserver, while running within
the webserver itself.</p>
<p>When the quick handler is switched off using the
<code class="directive"><a href="#cachequickhandler">CacheQuickHandler</a></code> directive,
it becomes possible to insert the <strong>CACHE</strong> filter at a
point in the filter stack chosen by the administrator. This provides the
opportunity to cache content before that content is personalised by the
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code> filter, or optionally compressed by the
<code class="module"><a href="../mod/mod_deflate.html">mod_deflate</a></code> filter.</p>
<p>Under normal operation, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> will respond to
and can be controlled by the
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9">Cache-Control</a>
and
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32">Pragma</a>
headers sent from a client in a request, or from a
server within a response. Under exceptional circumstances,
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> can be configured to override these headers
and force site specific behaviour, however such behaviour will be limited
to this cache only, and will not affect the operation of other caches
that may exist between the client and server, and as a result is not
recommended unless strictly necessary.</p>
<p>RFC 2616 allows for the cache to return stale data while the existing
stale entry is refreshed from the origin server, and this is supported
by <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> when the
<code class="directive"><a href="#cachelock">CacheLock</a></code> directive is suitably
configured. Such responses will contain a
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46">Warning</a>
HTTP header with a 110 response code. RFC 2616 also allows a cache to return
stale data when the attempt made to refresh the stale data returns an
error 500 or above, and this behaviour is supported by default by
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>. Such responses will contain a
<a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.46">Warning</a>
HTTP header with a 111 response code.</p>
<p><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> requires the services of one or more
storage management modules. One storage management module is included in
the base Apache distribution:</p>
<dl>
<dt><code class="module"><a href="../mod/mod_cache_disk.html">mod_cache_disk</a></code></dt>
<dd>Implements a disk based storage manager. Headers and bodies are
stored separately on disk, in a directory structure derived from the
md5 hash of the cached URL. Multiple content negotiated responses can
be stored concurrently, however the caching of partial content is not
supported by this module. The <code class="program"><a href="../programs/htcacheclean.html">htcacheclean</a></code> tool is
provided to list cached URLs, remove cached URLs, or to maintain the size
of the disk cache within size and inode limits.</dd>
</dl>
<p>Further details, discussion, and examples, are provided in the
<a href="../caching.html">Caching Guide</a>.</p>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#cachedefaultexpire">CacheDefaultExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachedetailheader">CacheDetailHeader</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachedisable">CacheDisable</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheenable">CacheEnable</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheheader">CacheHeader</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorecachecontrol">CacheIgnoreCacheControl</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignoreheaders">CacheIgnoreHeaders</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorenolastmod">CacheIgnoreNoLastMod</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorequerystring">CacheIgnoreQueryString</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachekeybaseurl">CacheKeyBaseURL</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelastmodifiedfactor">CacheLastModifiedFactor</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelock">CacheLock</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelockmaxage">CacheLockMaxAge</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelockpath">CacheLockPath</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachemaxexpire">CacheMaxExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheminexpire">CacheMinExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachequickhandler">CacheQuickHandler</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestaleonerror">CacheStaleOnError</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestoreexpired">CacheStoreExpired</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestorenostore">CacheStoreNoStore</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestoreprivate">CacheStorePrivate</a></li>
</ul>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#related">Related Modules and Directives</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#sampleconf">Sample Configuration</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#thunderingherd">Avoiding the Thundering Herd</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#finecontrol">Fine Control with the CACHE Filter</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#status">Cache Status and Logging</a></li>
</ul><h3>See also</h3>
<ul class="seealso">
<li><a href="../caching.html">Caching Guide</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="related" id="related">Related Modules and Directives</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_cache_disk.html">mod_cache_disk</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/mod_cache_disk.html#cacheroot">CacheRoot</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cachedirlevels">CacheDirLevels</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cachedirlength">CacheDirLength</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cacheminfilesize">CacheMinFileSize</a></code></li><li><code class="directive"><a href="../mod/mod_cache_disk.html#cachemaxfilesize">CacheMaxFileSize</a></code></li></ul></td></tr></table>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="sampleconf" id="sampleconf">Sample Configuration</a></h2>
<div class="example"><h3>Sample httpd.conf</h3><p><code>
#<br />
# Sample Cache Configuration<br />
#<br />
LoadModule cache_module modules/mod_cache.so<br />
<br />
<IfModule mod_cache.c><br />
<span class="indent">
LoadModule cache_disk_module modules/mod_cache_disk.so<br />
<IfModule mod_cache_disk.c><br />
<span class="indent">
CacheRoot c:/cacheroot<br />
CacheEnable disk /<br />
CacheDirLevels 5<br />
CacheDirLength 3<br />
</span>
</IfModule> <br />
<br />
# When acting as a proxy, don't cache the list of security updates<br />
CacheDisable http://security.update.server/update-list/<br />
</span>
</IfModule>
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="thunderingherd" id="thunderingherd">Avoiding the Thundering Herd</a></h2>
<p>When a cached entry becomes stale, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> will submit
a conditional request to the backend, which is expected to confirm whether the
cached entry is still fresh, and send an updated entity if not.</p>
<p>A small but finite amount of time exists between the time the cached entity
becomes stale, and the time the stale entity is fully refreshed. On a busy
server, a significant number of requests might arrive during this time, and
cause a <strong>thundering herd</strong> of requests to strike the backend
suddenly and unpredicably.</p>
<p>To keep the thundering herd at bay, the <code class="directive">CacheLock</code>
directive can be used to define a directory in which locks are created for
URLs <strong>in flight</strong>. The lock is used as a <strong>hint</strong>
by other requests to either suppress an attempt to cache (someone else has
gone to fetch the entity), or to indicate that a stale entry is being refreshed
(stale content will be returned in the mean time).
</p>
<h3>Initial caching of an entry</h3>
<p>When an entity is cached for the first time, a lock will be created for the
entity until the response has been fully cached. During the lifetime of the
lock, the cache will suppress the second and subsequent attempt to cache the
same entity. While this doesn't hold back the thundering herd, it does stop
the cache attempting to cache the same entity multiple times simultaneously.
</p>
<h3>Refreshment of a stale entry</h3>
<p>When an entity reaches its freshness lifetime and becomes stale, a lock
will be created for the entity until the response has either been confirmed as
still fresh, or replaced by the backend. During the lifetime of the lock, the
second and subsequent incoming request will cause stale data to be returned,
and the thundering herd is kept at bay.</p>
<h3>Locks and Cache-Control: no-cache</h3>
<p>Locks are used as a <strong>hint only</strong> to enable the cache to be
more gentle on backend servers, however the lock can be overridden if necessary.
If the client sends a request with a Cache-Control header forcing a reload, any
lock that may be present will be ignored, and the client's request will be
honored immediately and the cached entry refreshed.</p>
<p>As a further safety mechanism, locks have a configurable maximum age.
Once this age has been reached, the lock is removed, and a new request is
given the opportunity to create a new lock. This maximum age can be set using
the <code class="directive">CacheLockMaxAge</code> directive, and defaults to 5
seconds.
</p>
<h3>Example configuration</h3>
<div class="example"><h3>Enabling the cache lock</h3><p><code>
#<br />
# Enable the cache lock<br />
#<br />
<IfModule mod_cache.c><br />
<span class="indent">
CacheLock on<br />
CacheLockPath /tmp/mod_cache-lock<br />
CacheLockMaxAge 5<br />
</span>
</IfModule>
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="finecontrol" id="finecontrol">Fine Control with the CACHE Filter</a></h2>
<p>Under the default mode of cache operation, the cache runs as a quick handler,
short circuiting the majority of server processing and offering the highest
cache performance available.</p>
<p>In this mode, the cache <strong>bolts onto</strong> the front of the server,
acting as if a free standing RFC 2616 caching proxy had been placed in front of
the server.</p>
<p>While this mode offers the best performance, the administrator may find that
under certain circumstances they may want to perform further processing on the
request after the request is cached, such as to inject personalisation into the
cached page, or to apply authorisation restrictions to the content. Under these
circumstances, an administrator is often forced to place independent reverse
proxy servers either behind or in front of the caching server to achieve this.</p>
<p>To solve this problem the <code class="directive"><a href="#cachequickhandler ">CacheQuickHandler
</a></code> directive can be set to <strong>off</strong>, and the server will
process all phases normally handled by a non-cached request, including the
<strong>authentication and authorisation</strong> phases.</p>
<p>In addition, the administrator may optionally specify the <strong>precise point
within the filter chain</strong> where caching is to take place by adding the
<strong>CACHE</strong> filter to the output filter chain.</p>
<p>For example, to cache content before applying compression to the response,
place the <strong>CACHE</strong> filter before the <strong>DEFLATE</strong>
filter as in the example below:</p>
<div class="example"><p><code>
# Cache content before optional compression<br />
CacheQuickHandler off<br />
AddOutputFilterByType CACHE;DEFLATE text/plain<br /><br />
</code></p></div>
<p>Another option is to have content cached before personalisation is applied
by <code class="module"><a href="../mod/mod_include.html">mod_include</a></code> (or another content processing filter). In this
example templates containing tags understood by
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code> are cached before being parsed:</p>
<div class="example"><p><code>
# Cache content before mod_include and mod_deflate<br />
CacheQuickHandler off<br />
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html<br /><br />
</code></p></div>
<p>You may place the <strong>CACHE</strong> filter anywhere you wish within the
filter chain. In this example, content is cached after being parsed by
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code>, but before being processed by
<code class="module"><a href="../mod/mod_deflate.html">mod_deflate</a></code>:</p>
<div class="example"><p><code>
# Cache content between mod_include and mod_deflate<br />
CacheQuickHandler off<br />
AddOutputFilterByType INCLUDES;CACHE;DEFLATE text/html<br /><br />
</code></p></div>
<div class="warning"><h3>Warning:</h3>If the location of the
<strong>CACHE</strong> filter in the filter chain is changed for any reason,
you may need to <strong>flush your cache</strong> to ensure that your data
served remains consistent. <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> is not in a position
to enforce this for you.</div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="status" id="status">Cache Status and Logging</a></h2>
<p>Once <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> has made a decision as to whether or not
an entity is to be served from cache, the detailed reason for the decision
is written to the subprocess environment within the request under the
<strong>cache-status</strong> key. This reason can be logged by the
<code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code> directive as
follows:</p>
<div class="example"><p><code>
LogFormat "%{cache-status}e ..."
</code></p></div>
<p>Based on the caching decision made, the reason is also written to the
subprocess environment under one the following three keys, as appropriate:</p>
<dl>
<dt>cache-hit</dt><dd>The response was served from cache.</dd>
<dt>cache-revalidate</dt><dd>The response was stale and was successfully
revalidated, then served from cache.</dd>
<dt>cache-miss</dt><dd>The response was served from the upstream server.</dd>
</dl>
<p>This makes it possible to support conditional logging of cached requests
as per the following example:</p>
<div class="example"><p><code>
CustomLog cached-requests.log common env=cache-hit<br />
CustomLog uncached-requests.log common env=cache-miss<br />
CustomLog revalidated-requests.log common env=cache-revalidate<br />
</code></p></div>
<p>For module authors, a hook called <var>cache_status</var> is available,
allowing modules to respond to the caching outcomes above in customised
ways.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDefaultExpire" id="CacheDefaultExpire">CacheDefaultExpire</a> <a name="cachedefaultexpire" id="cachedefaultexpire">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The default duration to cache a document when no expiry date is specified.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheDefaultExpire <var>seconds</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheDefaultExpire 3600 (one hour)</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheDefaultExpire</code> directive specifies a default time,
in seconds, to cache a document if neither an expiry date nor last-modified date are provided
with the document. The value specified with the <code class="directive">CacheMaxExpire</code>
directive does <em>not</em> override this setting.</p>
<div class="example"><p><code>
CacheDefaultExpire 86400
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDetailHeader" id="CacheDetailHeader">CacheDetailHeader</a> <a name="cachedetailheader" id="cachedetailheader">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Add an X-Cache-Detail header to the response.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheDetailHeader <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheDetailHeader off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cachedetailheader">CacheDetailHeader</a></code> directive
is switched on, an <strong>X-Cache-Detail</strong> header will be added to the response
containing the detailed reason for a particular caching decision.</p>
<p>It can be useful during development of cached RESTful services to have additional
information about the caching decision written to the response headers, so as to
confirm whether <code>Cache-Control</code> and other headers have been correctly
used by the service and client.</p>
<p>If the normal handler is used, this directive may appear within a
<code class="directive"><a href="../mod/core.html#<directory>"><Directory></a></code> or
<code class="directive"><a href="../mod/core.html#<location>"><Location></a></code> directive. If the quick handler
is used, this directive must appear within a server or virtual host context, otherwise
the setting will be ignored.</p>
<div class="example"><p><code>
# Enable the X-Cache-Detail header<br />
CacheDetailHeader on<br />
</code></p></div>
<div class="example"><p><code>
X-Cache-Detail: "conditional cache hit: entity refreshed" from localhost<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDisable" id="CacheDisable">CacheDisable</a> <a name="cachedisable" id="cachedisable">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Disable caching of specified URLs</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheDisable <var>url-string</var> | <var>on</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheDisable</code> directive instructs
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> to <em>not</em> cache urls at or below
<var>url-string</var>.</p>
<div class="example"><h3>Example</h3><p><code>
CacheDisable /local_files
</code></p></div>
<p>If used in a <code class="directive"><Location></code> directive,
the path needs to be specified below the Location, or if the word "on"
is used, caching for the whole location will be disabled.</p>
<div class="example"><h3>Example</h3><p><code>
<Location /foo><br />
<span class="indent">
CacheDisable on<br />
</span>
</Location><br />
</code></p></div>
<p>The <code>no-cache</code> environment variable can be set to
disable caching on a finer grained set of resources in versions
2.2.12 and later.</p>
<h3>See also</h3>
<ul>
<li><a href="../env.html">Environment Variables in Apache</a></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheEnable" id="CacheEnable">CacheEnable</a> <a name="cacheenable" id="cacheenable">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Enable caching of specified URLs using a specified storage
manager</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheEnable <var>cache_type</var> [<var>url-string</var>]</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheEnable</code> directive instructs
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> to cache urls at or below
<var>url-string</var>. The cache storage manager is specified with the
<var>cache_type</var> argument. The <code class="directive">CacheEnable</code>
directive can alternatively be placed inside either
<code class="directive"><Location></code> or
<code class="directive"><LocationMatch></code> sections to indicate
the content is cacheable.
<var>cache_type</var> <code>disk</code> instructs
<code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> to use the disk based storage manager
implemented by <code class="module"><a href="../mod/mod_cache_disk.html">mod_cache_disk</a></code>.</p>
<p>In the event that the URL space overlaps between different
<code class="directive">CacheEnable</code> directives (as in the example below),
each possible storage manager will be run until the first one that
actually processes the request. The order in which the storage managers are
run is determined by the order of the <code class="directive">CacheEnable</code>
directives in the configuration file. <code class="directive">CacheEnable</code>
directives within <code class="directive"><Location></code> or
<code class="directive"><LocationMatch></code> sections are processed
before globally defined <code class="directive">CacheEnable</code> directives.</p>
<p>When acting as a forward proxy server, <var>url-string</var> can
also be used to specify remote sites and proxy protocols which
caching should be enabled for.</p>
<div class="example"><p><code>
# Cache content<br />
<Location /foo><br />
<span class="indent">
CacheEnable disk<br />
</span>
</Location><br /><br />
# Cache regex<br />
<LocationMatch foo$><br />
<span class="indent">
CacheEnable disk<br />
</span>
</LocationMatch><br /><br />
# Cache proxied url's<br />
CacheEnable disk /<br /><br />
# Cache FTP-proxied url's<br />
CacheEnable disk ftp://<br /><br />
# Cache content from www.example.org<br />
CacheEnable disk http://www.example.org/<br />
</code></p></div>
<p>A hostname starting with a <strong>"*"</strong> matches all hostnames with
that suffix. A hostname starting with <strong>"."</strong> matches all
hostnames containing the domain components that follow.</p>
<div class="example"><p><code>
# Match www.example.org, and fooexample.org<br />
CacheEnable disk http://*example.org/<br />
# Match www.example.org, but not fooexample.org<br />
CacheEnable disk http://.example.org/<br />
</code></p></div>
<p> The <code>no-cache</code> environment variable can be set to
disable caching on a finer grained set of resources in versions
2.2.12 and later.</p>
<h3>See also</h3>
<ul>
<li><a href="../env.html">Environment Variables in Apache</a></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheHeader" id="CacheHeader">CacheHeader</a> <a name="cacheheader" id="cacheheader">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Add an X-Cache header to the response.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheHeader <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheHeader off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cacheheader">CacheHeader</a></code> directive
is switched on, an <strong>X-Cache</strong> header will be added to the response
with the cache status of this response. If the normal handler is used, this
directive may appear within a <code class="directive"><a href="../mod/core.html#<directory>"><Directory></a></code>
or <code class="directive"><a href="../mod/core.html#<location>"><Location></a></code> directive. If the quick
handler is used, this directive must appear within a server or virtual host
context, otherwise the setting will be ignored.</p>
<dl>
<dt><strong>HIT</strong></dt><dd>The entity was fresh, and was served from
cache.</dd>
<dt><strong>REVALIDATE</strong></dt><dd>The entity was stale, was successfully
revalidated and was served from cache.</dd>
<dt><strong>MISS</strong></dt><dd>The entity was fetched from the upstream
server and was not served from cache.</dd>
</dl>
<div class="example"><p><code>
# Enable the X-Cache header<br />
CacheHeader on<br />
</code></p></div>
<div class="example"><p><code>
X-Cache: HIT from localhost<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreCacheControl" id="CacheIgnoreCacheControl">CacheIgnoreCacheControl</a> <a name="cacheignorecachecontrol" id="cacheignorecachecontrol">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore request to not serve cached content to client</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreCacheControl On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreCacheControl Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, requests containing a Cache-Control: no-cache or
Pragma: no-cache header value will not be served from the cache. The
<code class="directive">CacheIgnoreCacheControl</code> directive allows this
behavior to be overridden. <code class="directive">CacheIgnoreCacheControl On</code>
tells the server to attempt to serve the resource from the cache even
if the request contains no-cache header values. Resources requiring
authorization will <em>never</em> be cached.</p>
<div class="example"><p><code>
CacheIgnoreCacheControl On
</code></p></div>
<div class="warning"><h3>Warning:</h3>
This directive will allow serving from the cache even if the client has
requested that the document not be served from the cache. This might
result in stale content being served.
</div>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#cachestoreprivate">CacheStorePrivate</a></code></li>
<li><code class="directive"><a href="#cachestorenostore">CacheStoreNoStore</a></code></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreHeaders" id="CacheIgnoreHeaders">CacheIgnoreHeaders</a> <a name="cacheignoreheaders" id="cacheignoreheaders">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Do not store the given HTTP header(s) in the cache.
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreHeaders <var>header-string</var> [<var>header-string</var>] ...</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreHeaders None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>According to RFC 2616, hop-by-hop HTTP headers are not stored in
the cache. The following HTTP headers are hop-by-hop headers and thus
do not get stored in the cache in <em>any</em> case regardless of the
setting of <code class="directive">CacheIgnoreHeaders</code>:</p>
<ul>
<li><code>Connection</code></li>
<li><code>Keep-Alive</code></li>
<li><code>Proxy-Authenticate</code></li>
<li><code>Proxy-Authorization</code></li>
<li><code>TE</code></li>
<li><code>Trailers</code></li>
<li><code>Transfer-Encoding</code></li>
<li><code>Upgrade</code></li>
</ul>
<p><code class="directive">CacheIgnoreHeaders</code> specifies additional HTTP
headers that should not to be stored in the cache. For example, it makes
sense in some cases to prevent cookies from being stored in the cache.</p>
<p><code class="directive">CacheIgnoreHeaders</code> takes a space separated list
of HTTP headers that should not be stored in the cache. If only hop-by-hop
headers not should be stored in the cache (the RFC 2616 compliant
behaviour), <code class="directive">CacheIgnoreHeaders</code> can be set to
<code>None</code>.</p>
<div class="example"><h3>Example 1</h3><p><code>
CacheIgnoreHeaders Set-Cookie
</code></p></div>
<div class="example"><h3>Example 2</h3><p><code>
CacheIgnoreHeaders None
</code></p></div>
<div class="warning"><h3>Warning:</h3>
If headers like <code>Expires</code> which are needed for proper cache
management are not stored due to a
<code class="directive">CacheIgnoreHeaders</code> setting, the behaviour of
mod_cache is undefined.
</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreNoLastMod" id="CacheIgnoreNoLastMod">CacheIgnoreNoLastMod</a> <a name="cacheignorenolastmod" id="cacheignorenolastmod">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore the fact that a response has no Last Modified
header.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreNoLastMod On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreNoLastMod Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, documents without a last-modified date are not cached.
Under some circumstances the last-modified date is removed (during
<code class="module"><a href="../mod/mod_include.html">mod_include</a></code> processing for example) or not provided
at all. The <code class="directive">CacheIgnoreNoLastMod</code> directive
provides a way to specify that documents without last-modified dates
should be considered for caching, even without a last-modified date.
If neither a last-modified date nor an expiry date are provided with
the document then the value specified by the
<code class="directive">CacheDefaultExpire</code> directive will be used to
generate an expiration date.</p>
<div class="example"><p><code>
CacheIgnoreNoLastMod On
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreQueryString" id="CacheIgnoreQueryString">CacheIgnoreQueryString</a> <a name="cacheignorequerystring" id="cacheignorequerystring">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore query string when caching</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreQueryString On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreQueryString Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, requests with query string parameters are cached separately
for each unique query string. This is according to RFC 2616/13.9 done only
if an expiration time is specified. The
<code class="directive">CacheIgnoreQueryString</code> directive tells the cache to
cache requests even if no expiration time is specified, and to reply with
a cached reply even if the query string differs. From a caching point of
view the request is treated as if having no query string when this
directive is enabled.</p>
<div class="example"><p><code>
CacheIgnoreQueryString On
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheIgnoreURLSessionIdentifiers" id="CacheIgnoreURLSessionIdentifiers">CacheIgnoreURLSessionIdentifiers</a> <a name="cacheignoreurlsessionidentifiers" id="cacheignoreurlsessionidentifiers">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Ignore defined session identifiers encoded in the URL when caching
</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheIgnoreURLSessionIdentifiers <var>identifier</var> [<var>identifier</var>] ...</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheIgnoreURLSessionIdentifiers None</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Sometimes applications encode the session identifier into the URL like in the following
Examples:
</p>
<ul>
<li><code>/someapplication/image.gif;jsessionid=123456789</code></li>
<li><code>/someapplication/image.gif?PHPSESSIONID=12345678</code></li>
</ul>
<p>This causes cachable resources to be stored separately for each session, which
is often not desired. <code class="directive">CacheIgnoreURLSessionIdentifiers</code> lets
define a list of identifiers that are removed from the key that is used to identify
an entity in the cache, such that cachable resources are not stored separately for
each session.
</p>
<p><code>CacheIgnoreURLSessionIdentifiers None</code> clears the list of ignored
identifiers. Otherwise, each identifier is added to the list.</p>
<div class="example"><h3>Example 1</h3><p><code>
CacheIgnoreURLSessionIdentifiers jsessionid
</code></p></div>
<div class="example"><h3>Example 2</h3><p><code>
CacheIgnoreURLSessionIdentifiers None
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheKeyBaseURL" id="CacheKeyBaseURL">CacheKeyBaseURL</a> <a name="cachekeybaseurl" id="cachekeybaseurl">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Override the base URL of reverse proxied cache keys.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheKeyBaseURL <var>URL</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheKeyBaseURL http://example.com</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cachekeybaseurl">CacheKeyBaseURL</a></code> directive
is specified, the URL provided will be used as the base URL to calculate
the URL of the cache keys in the reverse proxy configuration. When not specified,
the scheme, hostname and port of the current virtual host is used to construct
the cache key. When a cluster of machines is present, and all cached entries
should be cached beneath the same cache key, a new base URL can be specified
with this directive.</p>
<div class="example"><p><code>
# Override the base URL of the cache key.<br />
CacheKeyBaseURL http://www.example.com/<br />
</code></p></div>
<div class="warning">Take care when setting this directive. If two separate virtual
hosts are accidentally given the same base URL, entries from one virtual host
will be served to the other.</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLastModifiedFactor" id="CacheLastModifiedFactor">CacheLastModifiedFactor</a> <a name="cachelastmodifiedfactor" id="cachelastmodifiedfactor">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The factor used to compute an expiry date based on the
LastModified date.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLastModifiedFactor <var>float</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLastModifiedFactor 0.1</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>In the event that a document does not provide an expiry date but does
provide a last-modified date, an expiry date can be calculated based on
the time since the document was last modified. The
<code class="directive">CacheLastModifiedFactor</code> directive specifies a
<var>factor</var> to be used in the generation of this expiry date
according to the following formula:
<code>expiry-period = time-since-last-modified-date * <var>factor</var>
expiry-date = current-date + expiry-period</code>
For example, if the document was last modified 10 hours ago, and
<var>factor</var> is 0.1 then the expiry-period will be set to
10*0.1 = 1 hour. If the current time was 3:00pm then the computed
expiry-date would be 3:00pm + 1hour = 4:00pm.
If the expiry-period would be longer than that set by
<code class="directive">CacheMaxExpire</code>, then the latter takes
precedence.</p>
<div class="example"><p><code>
CacheLastModifiedFactor 0.5
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLock" id="CacheLock">CacheLock</a> <a name="cachelock" id="cachelock">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Enable the thundering herd lock.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLock <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLock off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.2.15 and later</td></tr>
</table>
<p>The <code class="directive">CacheLock</code> directive enables the thundering herd lock
for the given URL space.</p>
<p>In a minimal configuration the following directive is all that is needed to
enable the thundering herd lock in the default system temp directory.</p>
<div class="example"><p><code>
# Enable cache lock<br />
CacheLock on<br /><br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLockMaxAge" id="CacheLockMaxAge">CacheLockMaxAge</a> <a name="cachelockmaxage" id="cachelockmaxage">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Set the maximum possible age of a cache lock.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLockMaxAge <var>integer</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLockMaxAge 5</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLockMaxAge</code> directive specifies the maximum
age of any cache lock.</p>
<p>A lock older than this value in seconds will be ignored, and the next
incoming request will be given the opportunity to re-establish the lock.
This mechanism prevents a slow client taking an excessively long time to refresh
an entity.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLockPath" id="CacheLockPath">CacheLockPath</a> <a name="cachelockpath" id="cachelockpath">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Set the lock path directory.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLockPath <var>directory</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLockPath /tmp/mod_cache-lock</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLockPath</code> directive allows you to specify the
directory in which the locks are created. By default, the system's temporary
folder is used. Locks consist of empty files that only exist for stale URLs
in flight, so is significantly less resource intensive than the traditional
disk cache.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheMaxExpire" id="CacheMaxExpire">CacheMaxExpire</a> <a name="cachemaxexpire" id="cachemaxexpire">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The maximum time in seconds to cache a document</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheMaxExpire <var>seconds</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheMaxExpire 86400 (one day)</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheMaxExpire</code> directive specifies the maximum number of
seconds for which cachable HTTP documents will be retained without checking the origin
server. Thus, documents will be out of date at most this number of seconds. This maximum
value is enforced even if an expiry date was supplied with the document.</p>
<div class="example"><p><code>
CacheMaxExpire 604800
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheMinExpire" id="CacheMinExpire">CacheMinExpire</a> <a name="cacheminexpire" id="cacheminexpire">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>The minimum time in seconds to cache a document</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheMinExpire <var>seconds</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheMinExpire 0</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheMinExpire</code> directive specifies the minimum number of
seconds for which cachable HTTP documents will be retained without checking the origin
server. This is only used if no valid expire time was supplied with the document.</p>
<div class="example"><p><code>
CacheMinExpire 3600
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheQuickHandler" id="CacheQuickHandler">CacheQuickHandler</a> <a name="cachequickhandler" id="cachequickhandler">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Run the cache from the quick handler.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheQuickHandler <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheQuickHandler on</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Apache HTTP Server 2.3.3 and later</td></tr>
</table>
<p>The <code class="directive"><a href="#cachequickhandler">CacheQuickHandler</a></code> directive
controls the phase in which the cache is handled.</p>
<p>In the default enabled configuration, the cache operates within the quick
handler phase. This phase short circuits the majority of server processing,
and represents the most performant mode of operation for a typical server.
The cache <strong>bolts onto</strong> the front of the server, and the
majority of server processing is avoided.</p>
<p>When disabled, the cache operates as a normal handler, and is subject to
the full set of phases when handling a server request. While this mode is
slower than the default, it allows the cache to be used in cases where full
processing is required, such as when content is subject to authorisation.</p>
<div class="example"><p><code>
# Run cache as a normal handler<br />
CacheQuickHandler off<br /><br />
</code></p></div>
<p>It is also possible, when the quick handler is disabled, for the
administrator to choose the precise location within the filter chain where
caching is to be performed, by adding the <strong>CACHE</strong> filter to
the chain.</p>
<div class="example"><p><code>
# Cache content before mod_include and mod_deflate<br />
CacheQuickHandler off<br />
AddOutputFilterByType CACHE;INCLUDES;DEFLATE text/html<br /><br />
</code></p></div>
<p>If the CACHE filter is specified more than once, the last instance will
apply.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStaleOnError" id="CacheStaleOnError">CacheStaleOnError</a> <a name="cachestaleonerror" id="cachestaleonerror">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Serve stale content in place of 5xx responses.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStaleOnError <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStaleOnError on</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.3.9 and later</td></tr>
</table>
<p>When the <code class="directive"><a href="#cachestaleonerror">CacheStaleOnError</a></code> directive
is switched on, and when stale data is available in the cache, the cache will
respond to 5xx responses from the backend by returning the stale data instead of
the 5xx response. While the Cache-Control headers sent by clients will be respected,
and the raw 5xx responses returned to the client on request, the 5xx response so
returned to the client will not invalidate the content in the cache.</p>
<div class="example"><p><code>
# Serve stale data on error.<br />
CacheStaleOnError on<br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStoreExpired" id="CacheStoreExpired">CacheStoreExpired</a> <a name="cachestoreexpired" id="cachestoreexpired">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Attempt to cache responses that the server reports as expired</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStoreExpired On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStoreExpired Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Since httpd 2.2.4, responses which have already expired are not
stored in the cache. The <code class="directive">CacheStoreExpired</code>
directive allows this behavior to be overridden.
<code class="directive">CacheStoreExpired</code> On
tells the server to attempt to cache the resource if it is stale.
Subsequent requests would trigger an If-Modified-Since request of
the origin server, and the response may be fulfilled from cache
if the backend resource has not changed.</p>
<div class="example"><p><code>
CacheStoreExpired On
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStoreNoStore" id="CacheStoreNoStore">CacheStoreNoStore</a> <a name="cachestorenostore" id="cachestorenostore">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Attempt to cache requests or responses that have been marked as no-store.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStoreNoStore On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStoreNoStore Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, requests or responses with Cache-Control: no-store header
values will not be stored in the cache. The
<code class="directive">CacheStoreNoStore</code> directive allows this
behavior to be overridden. <code class="directive">CacheStoreNoStore</code> On
tells the server to attempt to cache the resource even if it contains
no-store header values. Resources requiring authorization will
<em>never</em> be cached.</p>
<div class="example"><p><code>
CacheStoreNoStore On
</code></p></div>
<div class="warning"><h3>Warning:</h3>
As described in RFC 2616, the no-store directive is intended to
"prevent the inadvertent release or retention of sensitive information
(for example, on backup tapes)." Enabling this option could store
sensitive information in the cache. You are hereby warned.
</div>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#cacheignorecachecontrol">CacheIgnoreCacheControl</a></code></li>
<li><code class="directive"><a href="#cachestoreprivate">CacheStorePrivate</a></code></li>
</ul>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheStorePrivate" id="CacheStorePrivate">CacheStorePrivate</a> <a name="cachestoreprivate" id="cachestoreprivate">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Attempt to cache responses that the server has marked as private</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheStorePrivate On|Off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheStorePrivate Off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>Ordinarily, responses with Cache-Control: private header values will not
be stored in the cache. The <code class="directive">CacheStorePrivate</code>
directive allows this behavior to be overridden.
<code class="directive">CacheStorePrivate</code> On
tells the server to attempt to cache the resource even if it contains
private header values. Resources requiring authorization will
<em>never</em> be cached.</p>
<div class="example"><p><code>
CacheStorePrivate On
</code></p></div>
<div class="warning"><h3>Warning:</h3>
This directive will allow caching even if the upstream server has
requested that the resource not be cached. This directive is only
ideal for a 'private' cache.
</div>
<h3>See also</h3>
<ul>
<li><code class="directive"><a href="#cacheignorecachecontrol">CacheIgnoreCacheControl</a></code></li>
<li><code class="directive"><a href="#cachestorenostore">CacheStoreNoStore</a></code></li>
</ul>
</div>
</div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/mod/mod_cache.html" title="English"> en </a> |
<a href="../fr/mod/mod_cache.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
<a href="../ja/mod/mod_cache.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="../ko/mod/mod_cache.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
</div><div id="footer">
<p class="apache">Copyright 2012 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
</body></html>
|
