In order for Jetspeed to support remote content (XML) subscription and publication, it is important that this information is buffered locally. The DiskCache mechanism handles fetching content and assuring that it is always updated. All Jetspeed code should use this mechanism to ensure performance and reliability.

The disk cache will download remote URLs and store them as files on the local filesystem. Local URLs will be served from the webserver. Static URLs that are served from the webservers will have better performance than URLs served from dynamic content engines (PHP, ASP, JSP, Servlet, etc).

Jetspeed employees numerous stategies for ensuring that content is continually updated:

• DiskCacheDaemon: Continually goes over every entry within the cache at a set interval (the frequency is controlled using daemon.diskcachedaemon.interval property - you can also start this process manually using the DaemonAdminPortlet). If an entry is noticed to be out of date (either by HTTP header info or by a MAX interval) it is fetched and placed and the updated version placed within the cache. All Portlets and code that utilize this URL will be updated. External URL entry that returns an "Expires:" header will use the returned value. The URL entry that specifies "0" or does not have an "Expires:" header will be expired based on cache.default.expiration property (every 15 minutes by default).
• Request Trigger: If Jetspeed notices that a URL was requested but is not within the cache. It will throw an IOException which states this this URL is currently not available. The DiskCache will then asynchronously update itself so that the next requests will have this URL available.
• Asynchonous Bulk Update: The OCS support in Jetspeed will bulk update the cache with remote content. It is a fair assumption that if OCS uses this content that it will be needed within a Portlet at some time in the future.
• OCS interval monitoring: If an OCS entry specifies that an interval should be updated at a specified interval, Jetspeed will honor this and fetch the updated content.

There are also some performance features that ensure that Jetspeed NEVER wastes CPU time during content updates:

• One update per interval: Jetspeed will NEVER update content more than once per interval. If it didn't it might be possible for more than one resource to trigger a content fetch which would waste both CPU and bandwidth.
• Bad URL monitoring: Jetspeed keeps track of URLs that it can't fetch. If it is ever restarted it will reload this so that time isn't wasted on failed content.