webpage check and load balancer weirdness

[Marc Powell]

Neil <mailto:neil-on-nagios at restricted.dyndns.org> wrote:
> Hey guys,
> 
> Just wondering about the alert I got this morning. I have 8
> webservers behind 1 load balancer. One webserver was providing HTTP
> internal error 500.  
> I got the alert for that and is correct. But I also have another
> check that checks the webpages on the load balancer. I was wondering
> why nagios is also seeing the HTTP 500 error but Internet Explorer
> gets the proper web pages from the remaining 7 webservers. Any ideas?

Depending on the configuration and capabilities of your LB, it may be
using a hash to determine what web server you get sent to. Alteon AD3's
do this if your metric is hash. Session persistence is another method.
Your first request get's sent to the least utilized web server and all
subsequent requests from you get sent to the same device. If your
monitoring machine was directed to the bad box on a request then using
either method would redirect you to the same box on subsequent requests.
You should consult your load balancer's manual and configuration for
specifics.

--
Marc


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