Caching documents with active properties

Caching in the Placeless Documents system poses new challenges because users can attach active properties to documents. Active properties can modify the document's content as seen by a user. Thus, the caching mechanisms must take into account that a document's content not only depends on when the document was last modified, but also on the set of personal and universal properties attached to the document and the information on which these properties depend. Interestingly, active properties can be used to help caches manage their contents by notifying them of events that affect cache consistency, by providing caches with document-specific verifiers to further check on a document's consistency, and by returning information that can aid in decisions of which documents to cache.