Dieter Maurer was as kind as he always is one the Zope mailing list and explained to me the difference between using the Cache-Control and the Expires header for inducing browsers to cache your web elements. He explains:

Cache control is the modern (HTTP 1.1) way, "Expires" the old (HTTP 1.0) one.

My problem is that I've know about each but never bothered to find out why there are two ways of doing the same thing. In most of my projects I've used the oldschool method of setting RFC822 formatted future dates on the Expires header, but from now on I'll use both. Dieter continues on my question "What is best to use and for what?":

Use them both:

HTTP 1.1 clients will honour "Cache-Control" (which is easier to use and much more flexible).

HTTP 1.0 clients will ignore "Cache-Control" but honour "Expires". With "Expires" you get thus at least a bit control for these old clients.

Now, let's speak code. Here's what I now do to set caching on my pages when I want to:

def doCache(self, hours=10):
    """ set cache headers on this request if not in debug mode """
    if not self.doDebug():
        response = self.REQUEST.RESPONSE
        now = DateTime()
        then = now+int(hours/24.0)
        response.setHeader('Cache-Control', 'public,max-age=%d' % int(3600*hours))

This I can then use in say a stylesheet like this:

<dtml-call "doCache(48)">



How do I add this to my site. I would like it so when the user goes back you get an Expired page appear. I have looked over the web for a solution but cannot seem to find one. Can I simply put the above method in a script and call it each time a page is loaded.

Colin Wallace

Could you kindly explain what the DateTime object is? Perhaps it's because I'm using Python 2.7, but the datetime class in the datetime module has no rfc822 method.

Peter Bengtsson

DateTime was a utility library used in Zope2.

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