backend timeouts/503s vs grace cache

[Andrei]

Hi Guillaume,

Thanks for getting back to me

On Wed, Nov 15, 2017 at 8:11 AM, Guillaume Quintard <
guillaume at varnish-software.com> wrote:

> Hi,
>
> Why bother with the complex vcl_hit? Since you are saying that the cache
> is regularly primed, I don't really see the added value.
>
I was mainly going by an example a while back, and not all sites/urls are
primed in the same manner. It just stuck in the conf ever since


>
> (note, after a quick glance at it, I think it could just be a race
> condition where the backend appears up in vcl_hit and is down by the time
> you ask it the content)
>
How would you suggest "restarting" the request to try and force a grace
cache object to be returned if present in that case?



>
> --
> Guillaume Quintard
>
> On Wed, Nov 15, 2017 at 6:02 AM, Andrei <lagged at gmail.com> wrote:
>
>> 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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>>
>
>
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