HttpUpstreamModule: Need more detailed Information

[Axel]

Hello all,

i'm new to nginx and first of all i have to say it's a great piece of 
software.

I need some more detailed Information about nginx behaviour regarding 
HttpUpstreamModule and I hope you can give me some hints and links where 
i can learn more about it.
I set up nginx/1.2.2 as reverse proxy in front of a bunch of apache 
server(*) which are located in different housing locations.

Now I have some questions and I can't find any docs or wiki pages with 
detailed answers.

- how does nginx detect if one ore more upstream servers has diappeared?
   - what kind of mechanism does nginx use? icmp or something else?
   - how often does nginx request the status of upstream servers?

- how can I monitor the status of upstream server seen by nginx (I 
monitor the status of running apache prcesses on the upstream server 
separately)


It's a is really simple setup atm and I have not activated any other module:

upstream frontend {
   server    frontend1:80 weight=100 max_fails=2 fail_timeout=10s;
   server    frontend2:80 weight=100 max_fails=2 fail_timeout=10s;
   server    frontend3:80 weight=100 max_fails=2 fail_timeout=10s;
   server    frontend4:80 weight=100 max_fails=2 fail_timeout=10s;
   server    frontend5:80 weight=100 max_fails=2 fail_timeout=10s;
}

location / {
   proxy_next_upstream     http_502 http_503 error;
   proxy_set_header        Host $host;
   proxy_set_header        X-Real-IP $remote_addr;
   proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
   proxy_pass              http://frontend/;
   proxy_redirect          default;
}

I googled around and found this thread 
http://forum.nginx.org/read.php?29,108477 but got no answer.

Any help is appreciated

Regards, Axel