In contrary to a CQ instance which can handle and resolve specific dependencies between handles, the CQ dispatcher cache works quite simple. It uses so-called “stat-files” to indicate wheter cached files are still valid or need to refetched from the configured CQ.
The basic principle is that the last modification of a stat-file is compared to the last modification of content file. If the stat-file has been modified after the content file, the content file is stale and needs to be refetched. This refetching includes an update of the last-modifcation date to the current date. Otherwise the cached file can be delivered as up-to-date content.
The cache mirrors the content structure of the content in CQ. Using the statement “\statfileslevel” in the dispatcher.any file you can specify the depth for which these stat-files are created. If the dispatcher receives an invalidation request, it calculates the directory in which the files are which should be invalidated. If the directory in which this file is doesn’t contain a stat-file the dispatcher goes up in the directory structure until a stat-file is found. Then this stat-file is touched setting the last modification time to now. (If there are also stat-files in directories below the one of the file which is invalidated, they are also invalidated.)
If a requested file is contained in the cache, the dispatcher looks for a stat-file. If there is no stat-file in the directory of the to-be-delivered file, the dispatcher checks recursively the higher directories for a stat-file until it finds one. Then the last-modification times are compared and acted accordingly (as described above).
For correctness: There isn’t any recursionin the code anymore, since with deep nested content structures it produced Stack-Overflows killing the delivering webserver process. If you encounter this problem update to latest available dispatcher, currently version 4.0.2.
Update (2009-01-26): I just fell over the presentation by Dominique Jaeggi he gave on Tech Summit 2007. He talked over performance tuning and covers the proper use of the dispatcher in depth and explains the proper configuration of the invalidation