Serve stale content if backend is healthy but "not too healthy"

[Dridi Boukelmoune]

On Tue, Sep 21, 2021 at 1:33 PM Luca Gervasi <luca.gervasi at gmail.com> wrote:
>
> Hello everyone,
>
> We have a backend that actually proxies different services (mangling
> the original response). Sometimes one of those backends are not
> available and the general response goes from 200 to a 50x.
> Is there a way to serve a stale (valid) content (if present) for a
> request that comes from a backend in a healthy state?
>
> I was thinking about something like this:
> sub backend_fetch {
>   if (beresp.status >= 500) {
>     return_a_stale;
>   }
> }
>
> From the state machine
> (https://varnish-cache.org/docs/6.0/reference/states.html) it seems
> that I'm not allowed to return(hash) nor switch to an unhealthy
> backend (that i keep configured) to reach what I want.
>
> Please forgive me if do exists a facility to reach my goal and feel
> free to direct me to the right document.
>
> Ah. Varnish 6.x.

Hi Luca,

Varnish Cache does not have this feature, you should be able to do
that with Varnish Enterprise instead. What you are looking for is
stale-if-error and you may find some implementations using VCL but I
can't vouch for any, not having experience with them.

https://docs.varnish-software.com/varnish-cache-plus/vmods/stale/#description

Cheers,
Dridi