Upstream Keepalive Questions

[Maxim Dounin]

Hello!

On Tue, Apr 08, 2014 at 05:41:17PM -0400, abstein2 wrote:

> Maxim,
> 
> Thanks so much for clarifying. Just to make sure I'm understanding
> correctly, if I had something like this pseudo-code
> 
> upstream upstream1 { }
> upstream upstream2 { }
> upstream upstream3 { }
> upstream upstream4 { }
> upstream upstream5 { }
> 
> server { server_name server1.com; proxy_pass http://upstream1; }
> server { server_name server2.com; proxy_pass http://upstream2; }
> server { server_name server3.com; proxy_pass http://upstream3; }
> server { server_name server4.com; proxy_pass http://upstream4; }
> server { server_name server5.com; proxy_pass http://upstream5; }
> 
> There would only be performance degradation if the setup was:
> 
> server {
> server_name server6.com;
> set $PROXY_TO 'upstream5';
> proxy_pass http://$PROXY_TO;
> }
> 
> Is that correct?

Yes.  The "degradation" is due to upstream{} blocks are in stored 
an array, and looking up an upstream uses linear array traversal.  
That is, cost of looking an upstream sever in the "proxy_pass 
http://$proxy_to" is O(n), where n - number of upstream{} blocks 
in the configuration.

I don't think the difference will be measurable even with 
thousands of upstream{} blocks though.

> And, if there was degradation, would it be limited to hosts
> that server block was trying to serve or would it impact overall
> performance?

See above, it is only related to upstream{} block lookup in 
"proxy_pass http://$proxy_to" and doesn't affect requests where 
this proxy_pass isn't used.

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