> I have a sitemap at the bottom of my website that contains recent > additions to the web page. When records are added to the database, > although most of the pages reflect the change, the sitemap doesn't.
Welcome to the swamp called Cache Invalidation. Doing it precisely is one of the genuinely Hard Problems in CS, so most people end up doing something that is Good Enough. (BTW, you normally do *not* want to enable caching on your development system! It can drive you crazy trying to find a bug that you *thought* you fixed but it's still there on the &#*$ page!) If you have deep knowledge of which cached pages (or fragments thereof) are dependent on a particular object's state, you can clear just that cache value. See http://cactuswax.net/archive/django-cache-invalidation/ for how one person did this for his blog. The plus is that you are only rebuilding page (-fragments) that are affected by the change. Unfortunately, it is common that you either don't have the knowledge for precision invalidation or maintaining the dependency tables to do it is a royal pain in the a**, so you use a more brute force method. The most brute force method of them all is to simply clear the entire cache every time there is any change to the DB -- it's guaranteed to work, but it could get expensive on a very dynamic site. Sometimes you can do something a little less ham-handed, such as a clear-all-cached-products (or similar class-specific) function. For your sitemap, you could build it up from cached template fragments, and clear those fragments which may be affected by the change. The more precise your invalidation, the more complicated the management becomes. In addition to deciding *what* cached object needs to be invalidated, you also have to decide when and how the cache-clearing is invoked. One way is to use a custom save() method, as shown in the blog example. This works OK when it involves only one class, but when the dependencies start to build, you can end up with some very nasty circular references amongst your various files. Another approach is to use Django's signals mechanism. For your sitemap, you could create a simple function that clears the cached sitemap. Then you register listeners for the post_save signal on all models with objects in the sitemap. When one of them fires, it causes the cached sitemap (and maybe other stuff) to be cleared. The signals docs are somewhat weak (compared to other Django docs), so here are some places to start reading: Primary docs: http://code.djangoproject.com/wiki/Signals Presentation by Jeremy Dunck (starts at page 45) http://toys.jacobian.org/presentations/2007/oscon/tutorial/ Chris Pratt's page http://www.chrisdpratt.com/2008/02/16/signals-in-django-stuff-thats-not-documented-well/ . HTH, Peter --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
