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An OPAC, or Online Public Access Catalog, is the public-facing search interface of a library catalog. Patrons search by title, author, subject, ISBN, or keyword and see availability, location, due date, and reservation status — without asking a librarian. It replaced the wooden card catalog in the 1980s and now ships as the front-end of every library management system.
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An OPAC reads from the library's bibliographic database, where each item carries a MARC 21 or UNIMARC record describing title, author, ISBN, publisher, edition, subject headings, classification number (Dewey, LCC, UDC), and physical location. The OPAC indexes those fields in a search engine — typically Solr, Elasticsearch, or a built-in inverted index — and renders results as a web page. When a patron clicks a result, the OPAC checks the circulation table for current availability: on-shelf, checked out (with due date), reserved, lost, in repair, or in transit between branches. Authenticated patrons can place holds, renew loans, view their borrowing history, save reading lists, and pay fines. Federated search modules let one OPAC query peer libraries via Z39.50 or SRU, so a student at one campus can find a book at another and request inter-library loan.
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School and university libraries deploy an OPAC to give students and faculty 24/7 catalog access from any device, cut the front-desk lookup queue, and surface borrowing data the librarian needs for collection development. Without an OPAC, patrons either ask staff or browse stacks by call number — both slow, both incomplete. With an OPAC, the average lookup drops from minutes to seconds, reservations move online, and analytics reveal which subjects circulate, which titles sit unread, and which acquisitions to prioritize. For accreditation (NAAC, NBA, AACSB, regional accreditors), the OPAC supplies usage statistics that inspectors expect. Many institutions also expose the OPAC through the school portal or LMS so a course reading list resolves directly to library holdings — no copy-paste, no broken links.
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- Search across title, author, subject, ISBN, call number, and full-text where licensed
- Real-time availability — on-shelf, checked out (with due date), reserved, lost, in repair
- Patron self-service — place holds, renew, view history, save reading lists, pay fines
- Faceted refinement by format, language, year, location, subject heading
- Z39.50 / SRU federated search across peer libraries and inter-library loan request
- Mobile-responsive interface with optional barcode-scan-to-search on phones
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What does OPAC stand for?
OPAC stands for Online Public Access Catalog. The "online" distinguishes it from the paper card catalog it replaced; "public access" signals that any patron can search it without staff assistance; "catalog" means the full bibliographic record set, not just current circulation.
What is the difference between an OPAC and a discovery layer?
An OPAC searches one library's own catalog. A discovery layer (Primo, EBSCO Discovery Service, Summon, WorldCat Discovery) searches the catalog plus subscription databases, journal articles, e-books, and institutional repositories in one unified result set. Most academic libraries now deploy a discovery layer in front of the OPAC; school and small college libraries usually run the OPAC alone, sometimes federated via Z39.50 to peer libraries.
Does OPAC support MARC records?
Yes. MARC 21 (US/anglophone) and UNIMARC (parts of Europe and Asia) are the standard bibliographic formats the OPAC reads. The cataloger creates the MARC record once — title in field 245, author in 100, ISBN in 020, subject headings in 6XX, call number in 050 (LCC) or 082 (Dewey) — and the OPAC indexes every relevant subfield. MARC import via Z39.50 (from Library of Congress, OCLC, British Library) lets a cataloger pull a record in seconds rather than typing it.
Is there an open-source OPAC?
Yes. Koha and Evergreen are the two leading open-source library management systems, both ship with full OPAC interfaces, and both are used by thousands of libraries worldwide. OpenEduCat's openeducat_library module provides catalog and circulation features integrated with the broader education ERP — student records, fees, reservations — and integrates with Koha or Evergreen via Z39.50 for institutions that want a dedicated library system alongside their education ERP.
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