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varnishvarnish-vclvarnish-4

Increase varnish grace time while backend is sick

We use Varnish Cache as a Frontend to a lot of our customers and we server stale content by grace while any backend is sick.

We do now have a failed backend and we want to increase the grace period (while it's sick), is that a possible scenario? I tried digging in docs and found nothing.

Varnish 4

Solution

  • Serving outdated content in Varnish Cache 4.x when a backend in sick is a common use cache. You simply need to implement your own vcl_hitsubroutine. The idea is caching contents using a high grace value (e.g. 24 hours), but limit grace to a small time window (e.g. 10 seconds) when your backend is healthy:

    sub vcl_hit {
    if (obj.ttl >= 0s) {
        # Normal hit.
        return (deliver);
    }

    # 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) {
            return (deliver);
        } else {
            # No candidate for grace. Fetch a fresh object.
            return(fetch);
        }
    } else {
        # Backend is sick. Use full grace.
        if (obj.ttl + obj.grace > 0s) {
            return (deliver);
        } else {
            # No graced object.
            return (fetch);
        }
    }
}

    For further information please check: