Too many 503 backend errors

[Michael Alger]

On Wed, Dec 22, 2010 at 12:42:24PM +0530, Anand Shah wrote:
> I am running varnish on 64 bit Centos. Getting too many 503 errors
> recently and i tried debugging it with some help from Forums on
> mailinglist.

> VARNISHLOGS:
> 
>   180 VCL_call     c miss fetch
>   180 FetchError   c no backend connection
>   180 VCL_call     c error deliver

In my experience, "no backend connection" indicates that whatever
backend was selected to handle this request was unavailable due to the
health check/probe returning an error.

What was the backend health at the time? You can use the 'debug.health'
command in the CLI to view health information, or

  varnishlog -i Backend_health

to see the results of the health checks as they occur. If a backend is
marked as 'sick' then Varnish won't send any requests to it, since it's
unlikely to help the situation.