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From Rainer Jung <rainer.j...@kippdata.de>
Subject Re: mod_proxy. mod_proxy_balancer and failovers
Date Tue, 02 Jun 2009 14:08:36 GMT
On 02.06.2009 15:24, Kevac Marko wrote:
> On Tue, Jun 2, 2009 at 4:15 PM, Nick Kew <nick@webthing.com> wrote:
>> How is the balancer supposed to know that one valid HTTP response
>> is an error while another is correct?  A 404 or other error response
>> doesn't mean there's a problem with the server!
> 
> These HTTP responses can be configured in httpd.conf. Something like:
> 
> PROXY_BALANCER_RETRY 404
> PROXY_BALANCER_RETRY 405
> PROXY_BALANCER_RETRY 406
> 
> Yes, you are right, 404 does not mean that there is an error, but it
> could. I'd rather try asking few more workers (attempt limit) before
> returning error, than return right now error.
> 
> Feature that I am talking about gives freedom. It should be very
> useful when creating high availability systems. Right now mod_proxy
> declares policy that only theeese errors are true errors and only if
> getting these we should retry. I think it is not right.

You have chosen an especially problematic example. If a "not found"
triggers error status for the worker, then you will have continuous
denial of service.

Nearly every productive web server I know of, has every now and then
(most of them more often) a "not found" in the logs.  Examples are
people don't have a favourites icon, have broken links, broken proxies
in between, the usual attacker URLs e.g. for mysqladmin etc. Any random
URL I send to your web server will trigger a "not found".

You said "make it configurable". I doubt, that you will find many cases,
that are really worth making them configurable, i.e. they happen in real
world with a broken backend, but still can be used as a stable indicator
for a broken backend.

Regards,

Rainer

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