https://issues.apache.org/bugzilla/show_bug.cgi?id=44696
Summary: mod_cache sometimes serves old content despite having
fetched new content
Product: Apache httpd-2
Version: 2.2.8
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P2
Component: mod_cache
AssignedTo: [email protected]
ReportedBy: [EMAIL PROTECTED]
There appears to be a race condition which can occur when many clients request
the same object at once. If the object is in the cache, but expired, and a
newer version is available at the backend, the first client can get served the
old content with the timestamp of the new content.
Example: using Apache 2.2.8, with mod_cache and mod_disk_cache, suppose there's
a 3300000-byte file at /rand which has been downloaded and cached, but has now
expired. Suppose at Thu, 27 Mar 2008 16:44:40 GMT the file is updated so that
it's now 2100000 bytes.
If a hundred requests are made for the file almost simultaneously:
for x in $(seq -w 00 99);
do (echo -e 'GET /rand HTTP/1.0\n\n' | nc localhost 8000 >response.$x &);
done
then the first few responses are:
response.00
--------------------------------------------
HTTP/1.1 200 OK
Date: Thu, 27 Mar 2008 16:44:45 GMT
Server: Apache/2.2.8 (Unix)
Last-Modified: Thu, 27 Mar 2008 16:44:40 GMT
ETag: "72c013-200b20-4496de6d7ae00"
Accept-Ranges: bytes
Content-Length: 2100000
Connection: close
Content-Type: text/plain
?tuX`i%DJ!X.....
response.01
--------------------------------------------
HTTP/1.1 200 OK
Date: Thu, 27 Mar 2008 16:44:45 GMT
Server: Apache/2.2.8 (Unix)
ETag: "72c013-200b20-4496de6d7ae00"
Accept-Ranges: bytes
Content-Length: 3300000
Last-Modified: Thu, 27 Mar 2008 16:44:40 GMT
Connection: close
Content-Type: text/plain
_qj*mB(3m(f.....
response.02
--------------------------------------------
HTTP/1.1 200 OK
Date: Thu, 27 Mar 2008 16:44:45 GMT
Server: Apache/2.2.8 (Unix)
ETag: "72c013-200b20-4496de6d7ae00"
Accept-Ranges: bytes
Content-Length: 2100000
Last-Modified: Thu, 27 Mar 2008 16:44:40 GMT
Connection: close
Content-Type: text/plain
?tuX`i%DJ!X.....
On this occasion the second request gets handled first (it appears first in the
access log) and is served the old 3300000-byte content from the cache, but with
the timestamp of the new content.
The other 99 requests are served the correct new 2100000-byte content.
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