Hash director and backend health

[KT Sin]

the hash director will fail if all the backends are failing. if you
define a random director with the same set of backends, and if the hash
director fails, the random director fails too?

kt

On Mon, Aug 01, 2011 at 05:44:01AM +0000, Thiago Figueiro wrote:
> Hi, all.
> 
> I'm experimenting with the hash director but the current documentation[1] is brief and I need some help. (background: our application servers do some local fragment caching and this is more efficient if the URLs go to servers which already have the necessary fragments).
> 
> As desired, the hash director is selecting the same backend for a given URL.  However, when the backend goes sick, I expect varnish to pick a healthy one; instead I get a 503 due to no backend available.
> 
> This is the backend definition:
> 
> director example hash {
> 	.retries = 5;
>         { .backend = server1; .weight = 1; }
> (...)
>         { .backend = server8; .weight = 1; }
> }
> 
> I assumed the "retries" parameter in the director definition would try different backends; it might be that it retries the same backend.
> 
> It could be relevant that my vcl_fetch has:
> 
> sub vcl_fetch {
> (...)
>         # If status not one of the below try again (up to max_restarts)
>         if (
>         beresp.status != 200 && beresp.status != 403 && beresp.status != 404 &&
>         beresp.status != 301 && beresp.status != 302 && beresp.status != 307 &&
>         beresp.status != 410) {
>                 if (req.http.X-Forwarded-Host) {
>                          set req.http.Host = req.http.X-Forwarded-Host;
>                 }
> 
>                 restart;
>         }
> 
> 
> To overcome this I thought about defining a random director with the same set of servers and, should the hash director fail, I'd update req.backend to use the random director.  I don't like this too much because it would double the number of health checks, which are not exactly cheap, and I would need to complicate the VCL a bit more.
> 
> Am I missing something?  Is there a better way of handling this situation?
> 
> 
> Thanks.
> 
> 
> [1] https://www.varnish-cache.org/docs/2.1/reference/vcl.html#the-hash-director
> ? 
> 
> Thiago Figueiro
> Technical Operations Manager
> 
> Westfield Digital Business
> T +61 (0)2 9358 7123
> M +61 (0)405 700 814
> W www.westfield.com.au
> E tfigueiro at au.westfield.com
> 
> 
> 
> 
> 
> 
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