This is a short kata in which you are going to build a very basic cache.
The cache is initially empty and retrieves its content (from an unspecified location) the first time content is requested. Subsequent requests will be returned directly from the cache, until a certain expiration time.
After the expiration time, a request for the cached content results in an update of the cached content and the updated content is returned.
The challenge of this kata is to write the cache in a TDD as if you meant it-style, which also means that you have to come up with a mechanism to deal with the expiry mechanism (and system time).
Martin Fowler has this to say about the implementation of a cache and state verification:
But before I do, let me throw in an edge case. Occasionally you do run into things that are really hard to use state verification on, even if they aren't awkward collaborations. A great example of this is a cache. The whole point of a cache is that you can't tell from its state whether the cache hit or missed - this is a case where behavior verification would be the wise choice for even a hard-core classical TDDer. I'm sure there are other exceptions in both directions.