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From Rex Young <rex.yo...@oracle.com>
Subject [users@httpd] mod_cache: will a Thundering Herd be caused by reading cached files?
Date Mon, 14 Mar 2011 18:42:22 GMT
Hi guys,

I just read about mod_cache and Thundering Herd at 
http://httpd.apache.org/docs/2.2/mod/mod_cache.html

It describes a case of Thundering Herd which may happen when a cached 
file is being updated.  I was wondering why it does not mention another 
possible Thundering Herd caused by reading a cached file.  In latter 
case, when Apache is serving a request by reading a cached file, it may 
last for a big amount of time.  Because the client that requests the 
cached file may be very slow, or the file may be very large in size.

While serving such a request, other requests may arrived and found that 
the cached file is stale and needs to be updated.  But at the same time, 
the Apache has to wait until the read operation to finish.  Because on 
some OSes e.g. Windows you can not delete/replace a file when it is open.

If you know details how Apache handles the situation, could you please 
share it with me? Thank you.

Regards,
Rex


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