Too many 503 backend errors

Robert Shilston rtshilston at
Thu Dec 23 10:32:37 CET 2010


My VCL extract coped with a backend failure - if all backends fail on their monitoring probe (which, for example, might monitor any database replication lag etc), then Varnish will fail over to sending requests to any backend, regardless of its perceived health.  This allows for the possibility of service continuing in a worst case scenario, whilst still behaving well when there's just a partial degradation of backends.


On 23 Dec 2010, at 08:05, Traian Bratucu wrote:

> Anand, 
> You may want to look at grace mode, it could resolve your issue. When the backend is too busy you can serve stale content for a minute or something.
> Have a look:
> Traian
> ________________________________________
> From: varnish-misc-bounces at [varnish-misc-bounces at] On Behalf Of Anand Shah [imanandshah at]
> Sent: Thursday, December 23, 2010 6:26 AM
> To: Robert Shilston
> Cc: varnish-misc at
> Subject: Re: Too many 503 backend errors
> Thanks robert,
> But i have already tried all this stuff's. The problem with probing is that if your back-end is unhealthy for a second and if varnish declares it unhealthy due to some reason than it shows backend unhealthy for next 5 mins or so whereas the interval is only 5s. So if i m running on one back-end all the requests for this backend fails.
> The one i m using is a very domain with 3000 req/s.
> so cannot afford a second of unhealthy network or unreachability.
> Regards,
>     Anand
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