HTML, XML, PDF, and PostScript document formats are currently the most popular, but format popularity has changed quickly in the past, and a variety of important specialized formats must be supported too. So significant time must be anticipated to retrieve documents from digital libraries. A big library is unusable unless users have a good way to find things in it. So flexible query interfaces are the most popular. Both recall and precision are important, but some applications may need to emphasize one more than the other.
Query interfaces require good indexing of the library.
This requires an online thesaurus giving synonym and superconcept information for each sense of each possible word. Librarians have developed useful thesauri for manual indexing, and these can be implemented in software, but they traditionally classify entire books only and permit some imprecision in terminology to fit every book into a category. Digital libraries are well suited to distributed processing because of their often relatively independent contents. Most documents will be used by only one user at a time, so coordination between distributed processes will not be needed. Hence cached pages will rarely used by more than one user, and caching can be minimal.