[PATCH] Random director tries all backends before giving up

[Adrian Otto]

Jack,

This approach is probably not a good idea if (a) you have a large cluster, (b) a heavily loaded cluster, and/or (c) if your backends are sensitive to overload. You are likely to trigger a cascading failure. It might be smarter to have a configurable number of backends to try... perhaps 2 or 3. Imagine if you have 50 backends. There is no point in trying 50 times to find a healthy backend. Changes are that if 25% of your backends are down, trying more is just going to exacerbate the problem. 

Adrian

On Apr 11, 2010, at 4:35 PM, Jack Lindamood wrote:

> The following is a patch I’ve made to varnish that I hope improves the random director: which anyone’s welcome to use (even varnish trunk?).  My motivation was to reduce the number of vcl_error calls when a director is mostly good.  You can get the entire patch at this link.
>  
> http://github.com/cep21/Varnish/commit/6f5e98143ac2636504d9febf574b14c3c1a072fc
>  
> Here’s the commit message:
>  
> Random director tries all backends before giving up
>  
> Summary:
> The current random director gives up when it can't get a FD to the backend it wants retries times in a row.  Rather than give up and return NULL, which is guaranteed to cause a vcl_error, as a last ditch effort we try all other healthy backends until we get one that works.  This is mostly useful in the between time after a backend server dies and before the health check fails enough to mark a backend unhealthy.
>  
> Backwards Compatibility =  Not strictly backwards compatible.  In cases when the old code would of fallen through to vcl_error this will give a shot at getting a good result.
>  
> Performance = In the worse case, this will add extra calls for getting a FD, but only for situations that vcl_error
>  
> Test Plan: New varnish unittest.  It fails in the old code and works in this new code.
>  
>  
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