Class CachingPolicy

Represent a single class of cachable objects:

  - class membership is defined by 'predicate', a TALES expression
    with access to the following top-level names:

    'object' -- the object itself

    'view' -- the name of the view method

    'keywords' -- keywords passed to the request

    'request' -- the REQUEST object itself

    'member' -- the authenticated member, or None if anonymous

    'modules' -- usual TALES access-with-import

    'nothing' -- None

    'time' -- A DateTime object for the current date and time

  - The "Last-modified" HTTP response header will be set using
    'mtime_func', which is another TALES expression evaluated
    against the same namespace.  If not specified explicitly,
    uses 'object/modified'.

  - The "Expires" HTTP response header and the "max-age" token of
    the "Cache-control" header will be set using 'max_age_secs',
    if passed;  it should be an integer value in seconds.

  - The "Vary" HTTP response headers will be set if a value is 
    provided. The Vary header is described in RFC 2616. In essence,
    it instructs caches that respect this header (such as Squid
    after version 2.4) to distinguish between requests not just by
    the request URL, but also by values found in the headers showing
    in the Vary tag. "Vary: Cookie" would force Squid to also take 
    Cookie headers into account when deciding what cached object to 
    choose and serve in response to a request.

  - The "ETag" HTTP response header will be set if a value is
    provided. The value is a TALES expression and the result 
    after evaluation will be used as the ETag header value.

  - Other tokens will be added to the "Cache-control" HTTP response
    header as follows:

     'no_cache=1' argument => "no-cache" token

     'no_store=1' argument => "no-store" token

     'must_revalidate=1' argument => "must-revalidate" token