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An online public access catalog, sometimes abbreviated as OPAC, is a way for public users of a library or other system to access information about the facility's holdings. Usually, library patrons can access the public access catalog online within the library, but the same catalog can often be viewed on any computer with Internet access because the catalog is not physically confined to the library's computers. There are several different strategies that can be used to give the user ways to find documents in the online public access catalog, and there are many different designs for online catalogs. Generally, public users are given access to a different set of information than employees or library workers, and the online public access catalog cooperates with the system used by employees.
From a library patron's perspective, an online public access catalog is a way to interact with a library. Patrons often use the same catalog within the library as they use at home. In many cases, patrons may also interact with a library account through the system. Even though the catalog itself is usually thought to be only a list of the library's holdings, it is better thought of as the primary way in which library users access library information.
Beyond finding books, the same site is typically used to find information about the user's library account, place holds, or even request purchases. Some systems allow users to add metadata to the system, such as reviews or keywords. Depending on the library, an online public access catalog may provide direct access to materials such as audio books that can be downloaded from the Internet. It is unlikely that an online library will replace the physical location in which books are held, but as more resources become available online, more of the user's experience of a library will be located in the online catalog.
As the possible ways in which users can interact with a database evolve, the design of online public access catalogs evolves as well. For instance, while users were once confined to searches using the metadata supplied by librarians, it is now possible to search or browse based on the relations between books. Moreover, an online public access catalog usually does not exist in isolation, and users can utilize resources outside the library site to locate books, returning to the catalog when ready to interact directly with the library. There is no way to predict what features will be offered by future online catalogs, but it is clear that the technology that exists now will continue to improve and change.