backend timeouts/503s vs grace cache

[Guillaume Quintard]

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.

(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)

-- 
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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