Misbehaving servers

A key feature of Varnish is its ability to shield you from misbehaving web- and application servers.

Grace mode

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 back end.

If you are serving thousands of hits per second this queue can get huge. Nobody likes to wait so there is a possibility to serve stale content to waiting users. In order to do this we must instruct Varnish to keep the objects in cache beyond their TTL. So, to keep all objects for 30 minutes beyond their TTL use the following VCL::

sub vcl_fetch {
  set beresp.grace = 30m;
}

Varnish still won’t serve the stale objects. In order to enable Varnish to actually serve the stale object we must enable this on the request. Lets us say that we accept serving 15s old object.:

sub vcl_recv {
  set req.grace = 15s;
}

You might wonder why we should keep the objects in the cache for 30 minutes if we are unable to serve them? Well, if you have enabled Health checks you can check if the backend is healthy and serve the content for as longer.:

if (! req.backend.healthy) {
   set req.grace = 5m;
} else {
   set req.grace = 15s;
}

Saint mode

Sometimes servers get flaky. They start throwing out random errors. You can instruct Varnish to try to handle this in a more-than-graceful way - enter Saint mode. Saint mode enables you to discard a certain page from one backend server and either try another server or serve stale content from cache. Lets have a look at how this can be enabled in VCL::

sub vcl_fetch {
  if (beresp.status == 500) {
    set beresp.saintmode = 10s;
    restart;
  }
  set beresp.grace = 5m;
}

When we set beresp.saintmode to 10 seconds Varnish will not ask that server for URL for 10 seconds. A blacklist, more or less. Also a restart is performed so if you have other backends capable of serving that content Varnish will try those. When you are out of backends Varnish will serve the content from its stale cache.

This can really be a life saver.

God mode

Not implemented yet. :-)