HTTP caching solutions such as Varnish are great for improving web application performance. Filling up the cache is easy, but how do you get rid of cache entries when they become stale? There are three ways: expiration, validation and invalidation. The first two are nice and simple, but have their limitations. The other option, invalidation, is frowned upon for its complexity. Still, it can be very useful. Case in point: an e-learning web platform we built that serves a large number of Dutch pupils and their teachers. I will tell you why and how we added an invalidation scheme to this platform. I will also show you our lightweight Symfony bundle that makes cache invalidation a little less complex.