Having trouble scaling your website? Is that new marketing campaign killing your performance? Are you using a framework or a CMS that causes too much overhead? Give Varnish a try.
Varnish is a reverse proxy that serves cached pages to your visitors instead of hitting the backend servers all the time. Cache is stored in RAM and there's a domain specific language that allows you to hook into your caching logic.
If you respect the rules and use the tools, Varnish will make your site bullet proof!