This module can be used to glue a cache to a PSGI applications
or middleware. A *cache* is an object that provides at least two methods to
get and set data, based on a key. Existing cache modules on CPAN include
CHI, Cache and Cache::Cache.
Plack::Middleware::Cached is put in front of a PSGI application as
middleware. Given a request in form of a PSGI environment E, it either
returns the matching response R from its cache, or it passed the request to
the wrapped application, and stores the application's response in the
cache:

________ _____
Request ===E===>| |---E--->| |
| Cached | | App |
Response <==R====|________|<--R----|_____|

In most cases, only a part of the environment E is relevant to the request.
This relevant part is called the caching *key*. By default, the key is set
to the value of REQUEST_URI from the environment E.