Hash director and backend health

[Thiago Figueiro]

Hi, Ken.

Ken Brownfield wrote:

> You could have the hash backends each be individual round/random 
> directors with your intended server having a very high weight 
> and your intended backup backend a weight of 1.  Awkward and 
> somewhat lengthy, but it works.  The only downside is that the 
> fallback will get a non-zero amount of traffic.

> director server1d random {
>        { .backend = server1;   .weight = 10000000; }
>        { .backend = fallback1; .weight = 1; }
> }

I'm not sure I got your idea correctly as "hash backend" is not something I'm aware of.  Are you suggesting I cut-up the distribution manually?

I guess what I'm really after is a CARP[1] implementation such as the one in F5's BigIP [2].  In their implementation the sick servers are removed from the pool and the hash algorithm does its thing, redistributing the URLs amongst the healthy servers.

The other way I understood your suggestion was to do something like this:

director R_director random {
        { .backend = centre_rail_ausyd_fap01; .weight = 1; }
(...)
        { .backend = centre_rail_ausyd_fap08; .weight = 1; }
}

director H_director hash {
        { .backend = centre_rail_ausyd_fap01; }
(...)
        { .backend = centre_rail_ausyd_fap08; }
}

director example random {
        { .backend = H_centre_rails; .weight = 999999; }
        { .backend = R_centre_rails; .weight = 1; }
}

That could work but varnish 2.1 doesn't take directors in .backend.

I'm trying to avoid using our BigIP to do the job because Varnish is currently responsible for figuring-out the correct backend based on the request.  If I use the BigIP for this I'll need to either get rid of Varnish altogether add another BigIP VIP to select the backend based on CARP (BigIP-PublicVIP -> Varnish -> BigIP-InternalVIP -> Backends).


[1] http://en.wikipedia.org/wiki/Cache_Array_Routing_Protocol
[2] http://support.f5.com/kb/en-us/solutions/public/11000/300/sol11362.html



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