We spent a lot of time on providing good caching, that is, saving intermediate results and serving saved copies instead of recalculating from scratch for the same requests. This works perfectly only in a perfect world (for example, a pure functional world where functions and, by extension, GET/HEAD HTTP requests do not have side effects). In the real world, two equal requests may sometimes lead to different responses. There are two basic reasons for it: the earlier-mentioned side effects, which change the state despite the perceived idempotence of GET/HEAD, or flawed equality relationship between requests. A good example of this is ignoring wall time when the response depends on it.

Such problems usually manifest themselves as complaints about seeing stale versions of some pages on your website or seeing pages that belong to other users. Although you can tolerate the first type to some extent (for example, as a compromise for performance), the second type is a...