It's not at all hard! A typical Wikipedia page has the "chrome", which changes based on your logon, and then content, which generally does not. Cache invalidation is trivial, because there's only one atomic "blob" to track instead of the output of some complex query.

This is why it amuses me that people seem to think that Wikipedia "requires" tens of millions of dollars of infrastructure to host. It doesn't require it. It's just that some things are better/easier with more kit, such as maintenance.