backend timeouts/503s vs grace cache

[Andrei]

bump

On Sun, Nov 5, 2017 at 2:12 AM, Andrei <lagged at gmail.com> wrote:

> Hello everyone,
>
> One of the backends we have configured, runs through an SSH tunnel which
> occasionally gets restarted. When the tunnel is restarted, Varnish is
> returning a 503 since it can't reach the backend for pages which would
> normally be cached (we force cache on the front page of the related site).
> I believe our grace implementation might be incorrect, as we would expect a
> grace period cache return instead of 503.
>
> Our grace ttl is set to 21600 seconds based on a global variable:
>
> sub vcl_backend_response {
>   set beresp.grace = std.duration(variable.global_get("ttl_grace") + "s",
> 6h);
> }
>
> Our grace implementation in sub vcl_hit is:
>
>   sub vcl_hit {
>     # We have no fresh fish. Lets look at the stale ones.
>     if (std.healthy(req.backend_hint)) {
>       # Backend is healthy. Limit age to 10s.
>       if (obj.ttl + 10s > 0s) {
>         #set req.http.grace = "normal(limited)";
>         std.log("OKHITDELIVER: obj.ttl:" + obj.ttl + " obj.keep: " +
> obj.keep + " obj.grace: " + obj.grace);
>         return (deliver);
>       } else {
>         # No candidate for grace. Fetch a fresh object.
>         std.log("No candidate for grace. Fetch a fresh object. obj.ttl:" +
> obj.ttl + " obj.keep: " + obj.keep + " obj.grace: " + obj.grace);
>         return(miss);
>       }
>     } else {
>       # backend is sick - use full grace
>         if (obj.ttl + obj.grace > 0s) {
>         #set req.http.grace = "full";
>         std.log("SICK DELIVERY: obj.hits: " +   obj.hits + " obj.ttl:" +
> obj.ttl + " obj.keep: " + obj.keep + " obj.grace: " + obj.grace);
>         return (deliver);
>       } else {
>         # no graced object.
>         std.log("No graced object. obj.ttl:" + obj.ttl + " obj.keep: " +
> obj.keep + " obj.grace: " + obj.grace);
>         return (miss);
>       }
>     }
>
>     # fetch & deliver once we get the result
>     return (miss); # Dead code, keep as a safeguard
>   }
>
>
> Occasionally we see:
> -   VCL_Log        No candidate for grace. Fetch a fresh object.
> obj.ttl:-1369.659 obj.keep: 0.000 obj.grace: 21600.000
>
> For the most part, it's:
> -   VCL_Log        OKHITDELIVER: obj.ttl:26.872 obj.keep: 0.000 obj.grace:
> 21600.000
>
> Are we setting the grace ttl too low perhaps?
>
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