varnish redundancy

[Kristian Lyngstol]

On Mon, Sep 14, 2009 at 05:49:38PM +0100, Laurence Rowe wrote:
> 2009/9/3 Poul-Henning Kamp <phk at phk.freebsd.dk>:
> 
> > For it to be really smart you want to use directors for the
> > "other_varnish" and probes to ascertain health.
> >
> > We do not have a "priority_director" (we probably should have)
> > but you can get much the same effect with the random director
> > and very uneven weights:
> >
> >        director other_backend {
> >                { .backend = b_other_varnish ; weight=100000; }
> >                { .backend = b_real_backend ; weight=1; }
> >        }
> >
> > Should the probes mark the other_varnish unhealthy, all trafic
> > will go to the real backend.
> 
> Is there an advantage in using a director here instead of the following?
> 
> sub vcl_recv {
>     set req.backend = haproxy01;
>     if (!req.backend.healthy) {
>         set req.backend = haproxy02;
>     }

Both approaches have their benefits.

One benefit of using a random director is that it can have multiple
fallbacks, but then again, you can achieve that by having two directors: a
pool of primary directors (or a single backend), and do 

if (!req.backend.healthy) { 
	set req.backend = fallbackdirector;
}

Using a single director with the weight-approach does have a benefit of
being nicer to read and maintain, but will give the fallback backends some
marginal amount of traffic even when the primary backends are healthy.

-- 
Kristian Lyngstøl
Redpill Linpro AS
Tlf: +47 21544179
Mob: +47 99014497
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