backend timeouts/503s vs grace cache

[Andrei]

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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