Patron-driven acquisition (PDA), also referred to as demand-driven acquisition (DDA), is a model of library collection development in which a library only purchases materials when it is clear that a patron wants them. In an ideal transaction, libraries provide the patron with access to search engines, academic databases and/or library catalogs from which the patron can request items. When certain thresholds are reached for an item (e.g., number of pages read or number of requests), the library purchases the item and delivers instant access to patrons. The library may acquire the resource permanently, or acquire a license to use the resource only at certain times or in certain ways. Since content purchased is in digital format, “PDA emphasizes collecting for and at the moment of need” instead of collecting with a long-term focus.