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From Joshua Slive <>
Subject Re: using proxy/cache for apache mirrors
Date Wed, 07 Dec 2005 01:16:07 GMT
[ This really should be on infrastructure; oh well.]

Perhaps I should have mentioned off the top that I envision setting 30+ 
day expiry times on all .gz/.zip/.msi/.jar/etc files under dist/.  These 
files should never change without being renamed.

Colm MacCarthaigh wrote:
> 	* It's vastly more complicated than neccessary and adds a burden
> 	  to what admins have to manage. Why should they have to worry
> 	  about managing a cache? They're busy enough trying to give us
> 	  free resources in the first place.

Either you manage the cache or you manage rsync.  I don't really see why 
one is easier than the other.

> 	* It adds massive dependencies to what a mirror server needs to run.
> 	  Adding modules, especially proxy, is not resource-free. These
> 	  things eat memory, research time and security work.

We're not going to get rid of rsync mirroring, so mirrors that don't 
want to use this don't need to.

> 	* It defeats a huge part of the point of having a mirroring
>           system in the first place. Mirroring isn't just a way of
> 	  decreasing bandwidth usage on the primary, it's also a means
> 	  of building content resilience. When goes down, users 
>           want their mirror to work. And worst of all, in the case of
> 	  infrequently used mirrors, this is exactly when they'll
> 	  suddenly get a lot of queries - all of which will end up in IOWAIT
> 	  land, with a boat-load of back-end TCP connections, and no 
> 	  content served. That really sucks, for both them and their
> 	  users.

Certainly for mirrors that see themselves as providing large-scale 
backups, this would not be a good technique.  From the point 
of view, people have no way of even finding our recommended mirrors if 
we are down, so it doesn't really help.  And for frequently requested 
files, the long expiry time will allow the mirrors to continue to serve 

> 	* mod_cache + mod_proxy is trivially vulnerable to all of the latest
> 	  DNS cache-poisoning trickery, with no easy fix. At the very
> 	  least we should recommend that admins hard-code
> 	  in their /etc/hosts file, and that INFRA get some PI-space and
> 	  guarantee availability at a particular IP address for
> 	  eternity. Or deploy DNSSEC, and insist that mirrors verify the
> 	  records.

I don't really see how the situation with mod_proxy is any worse than 
the existing situation in that regard.  It could even be better given 
that cache expiry times will far exceed rsync frequencies.

> 	* We havn't fixed all of the thundering herd problems :/

Again, with long expiry times, I don't see this as a problem.

> 	* It's HTTP only. A lot of users use rsync and FTP to fetch
> 	  content from a local mirror.

I generally discourage ftp mirrors.  But yes, they would continue to 
need to do rsync.

> 	* Next time gets compromised, the exposure
> 	  will be two to four times as great compared to the rsync
> 	  mirrors. CacheMaxExpire can fix this problem though.

Again, long expiry times seem to make this problem less severe than with 

Just to explain the reasoning behind this a little: our dist/ directory 
is rapidly approaching 10GB.  Although I don't have any statistics to 
back this up, I strongly suspect that a very small portion of that 
accounts for a very large portion of the downloads.  The rest gets 
rsynced to our hundreds of mirrors for no good reason (other than 
backups; but we don't need hundreds of backups).  In addition, our 
projects are always clammering for faster releases -- they don't want to 
delay their announcements to wait for mirrors to sync.  I know you have 
"push" ideas for how to solve that, but the proxy technique works as well.

(There are other ways to address these issues, of course.  We could stop 
recruiting mirrors and limit ourselves to a dozen or so more reliable 
mirrors.  But that would be a major change in thinking.)


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