Contents

vcl 4.0;

backend server1 {
      .host = "server1.example.com";
      .probe = {
              .url = "/";
              .timeout = 1s;
              .interval = 5s;
              .window = 5;
              .threshold = 3;
      }
}

sub vcl_hit {
    # Called when a cache lookup is successful.

    if (obj.ttl >= 0s) {
        # A pure unadultered hit, deliver it
        return (deliver);
    }

    # https://www.varnish-cache.org/docs/trunk/users-guide/vcl-grace.html
    # When several clients are requesting the same page Varnish will send one request to the backend and place the others on hold while fetching one copy from the backend. In some products this is called request coalescing and Varnish does this automatically.
    # If you are serving thousands of hits per second the queue of waiting requests can get huge. There are two potential problems - one is a thundering herd problem - suddenly releasing a thousand threads to serve content might send the load sky high. Secondly - nobody likes to wait. To deal with this we can instruct Varnish to keep the objects in cache beyond their TTL and to serve the waiting requests somewhat stale content.

# if (!std.healthy(req.backend_hint) && (obj.ttl + obj.grace > 0s)) {
#     return (deliver);
# } else {
#     return (fetch);
# }

    # 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)";
            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) {
            #set req.http.grace = "full";
            return (deliver);
        } else {
            # no graced object.
            return (fetch);
        }
    }

    # fetch & deliver once we get the result
    return (fetch); # Dead code, keep as a safeguard
}

Version data entries

1 entries across 1 versions & 1 rubygems

Version	Path
rouge-3.16.0	lib/rouge/demos/varnish