https://issues.apache.org/bugzilla/show_bug.cgi?id=50317
Summary: Possible error in mod_cache's "Avoiding the Thundering
Herd"
Product: Apache httpd-2
Version: 2.2.17
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P2
Component: mod_cache
AssignedTo: [email protected]
ReportedBy: [email protected]
Created an attachment (id=26333)
--> (https://issues.apache.org/bugzilla/attachment.cgi?id=26333)
Proposed fix
Hello.
I configured apache (2.2.17) as written in
http://httpd.apache.org/docs/current/mod/mod_cache.html#thunderingherd
Nevertheless, more than one request hit the backend. The Backend sends neither
a ETag nor Last-Modified header but it sends an Expires header. I took a look
into mod_cache and made the attached patch.
The problem was in cache_storage.c:
irv = cache->provider->remove_url(h, r->pool);
Because of this line, there is no stale content left to serve, while getting a
newer version from the backend. There is no use in deleting this entry anyway,
because at this point, there is still interest in this url and the cache entry
will be regenerated very soon.
While investigating this, I found:
rv = ap_cache_try_lock(conf, r, NULL);
in mod_cache.c. This seems useless, because the lockfile is already created
in cache_util.c ap_cache_check_freshness(). Because of the NULL, it uses
the same lockfile for different URLs. I am unsure about the intention of
this second lock.
With kind regards,
Julius Gehr
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