Hello!

On Wed, Jun 30, 2010 at 07:35:06PM +0200, scyth wrote:

> On Wed, 30 Jun 2010 20:53:58 +0400
> Maxim Dounin <mdounin@mdounin.ru> wrote:
>
> > Hello!
> >
> > On Wed, Jun 30, 2010 at 06:28:51PM +0200, Zeljko Tomic wrote:
> >
> > > I wrote a small, but sometimes very useful module for
> > > limiting the number of active connections, in a "soft" way.
> > >
> > > Description and usage:
> > > -----------------------
> > > In situation when using nginx for serving static files and
> > > primarily as a proxy/proxy_cache with some slower backend
> > > (or even php fastcgi), limits of connections and requests
> > > that nginx can handle are much higher than they are on
> > > backend server(s). When you know the limits of your
> > > backend, it is bad practice to apply those limits on nginx's
> > > worker_connections, because it's obvious that when limits
> > > are met, new requests won't come through and this leaves you
> > > with bad visitor experience.
> >
> > It would be a good idea to clarify differences from limit_conn
> > module. Right now it looks like use cases you consider are
> > better handled by limit_conn.
>
> The difference is that limit_conn (from limit_zone module)
> limits the number of simultaneous connections PER IP.

No. It limits number of simultaneous connections per variable
value. You are free to use any variable, including one identical
for all requests.

See here for details:

http://wiki.nginx.org/NginxHttpLimitZoneModule#limit_zone

> This is
> useful in many cases, but soflimit_module, as stated above,
> compares its limit with total number of active connections,
> which is different. For example, site can have 2000 different
> clients with 1 connection each, but backend can handle only 300
> without performance issues. Actual limits of course vary from
> site to site.

As long as you are trying to limit *backend* load - you probably
don't want to use number of active connections (you are talking
about ngx_stat_active, right? sorry, I haven't had time to read
your code yet) as it includes keepalive connections, connections
just sending requests to us and connections for non-backend
requests.

limit_conn limits number of currently running requests in certain
location (from preaccess phase till request pool cleanup) and this
is much closer to real number of backend connections than
ngx_stat_active (though it's still not perfect as it includes
reading request body and sending big buffered responses).

....not even talking about multiple backends with quite a different
limits running behind the same nginx frontend...

Maxim Dounin

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