avoid rendering documents with an external cache

Created by . Last modified on 2003/08/05.

There are several headers that will let external caches avoid retrieving a page from your server. To use headers that set expiration dates, like Expires and Cache-Control, you need to be able to know the earliest date that a page will change.

A cache can send an If-Modified-Since header with a date along with a request to indicate the date of it's cached page. The server can return a 304 status message instead of the content to signify that the page has not been modified. This fragment, modified from ImageFile.py, sends that response if the date is earlier than a given date:

 def checkNotMod(self, modDate, REQUEST, RESPONSE):
    """
    If REQUEST contains an If-Modifed-Since header, and modDate is not
    after that date, set the appropriate status on RESPONSE and return
    true, else return None.
    """
    # ripped from ImageFile.py
    header=REQUEST.get_header('If-Modified-Since', None)
    if header is not None:
        header=string.split(header, ';')[0]
        # Some proxies seem to send invalid date strings for this
        # header. If the date string is not valid, we ignore it
        # rather than raise an error to be generally consistent
        # with common servers such as Apache (which can usually
        # understand the screwy date string as a lucky side effect
        # of the way they parse it).
        try:    mod_since = DateTime(header)
        except: mod_since = None
        if mod_since is not None and modDate <= mod_since:
            RESPONSE.setStatus(304)
            return 1
        return None

Another way to help a cache is to send an ETag header for every unique piece of content. The WebDAV code in the Zope source tree currently has some classes to help with this.