nginx/backend keepalive question

[Maxim Dounin]

Hello!

On Wed, Apr 30, 2014 at 02:08:57PM -0700, Yu Zhao wrote:

> Greetings!
> 
> I couldn't find my answer of following questions in the manual or somewhere
> else. Your help is much appreciated!

See src/http/modules/ngx_http_upstream_keepalive_module.c.

> 1) does nginx manage frontend/nginx and nginx/backend connections
> separately? In another word, will a nginx/backend connection be bond to a
> certain frontend/nginx connection for the lifetime of the frontend/nginx
> connection that is also kept-alive? My guess is it's not, nginx uses M:N
> model which M is the number of frontend/nginx connections that may or may
> not be kept-alive and N is the number of nginx/backend kept-alive
> connections, and M connections share N connections.

Cache of connections to upstream servers is completely unrelated 
to client connections.

> 2) If my guess is correct, how does nginx make sure the request/response
> pair is not broken/mismatched? Nginx wil NOT send new request to a backend
> until the response of the last one is received, right?

Yes.

Moreover, even documentation specifically talks about "idle 
keepalive connections to upstream servers that are preserved in 
the cache", see http://nginx.org/r/keepalive.

> 3) If I'm still on the right track: because nginx has to wait the response
> from a backend connection, it cannot reuse it for next request. That's why
> nginx still needs to open new connections to backend even there are
> existing connections. But It will only do so when all the existing
> connection are non-idle (i.e. responses from them haven't been fully
> received).

Yes, see above.

-- 
Maxim Dounin
http://nginx.org/