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Link Resolvers

A link resolver is the institution's gatekeeper for context-sensitive linking — it parses an incoming OpenURL, consults a knowledge base, and routes the user.

What a link resolver is

A link resolver is a library service that receives an OpenURL, works out which copy of the cited item the library can provide, and sends the user there. It is operated by a library, a consortium, or a vendor on the library's behalf. The decision is made against a knowledge base, a per-institution record of titles, subscriptions, coverage dates and full-text targets, plus local rules such as preferred platforms.

When "checking the link resolver" comes up in e-resources work, it usually means testing that this chain works for a given title: that the database emits a correct OpenURL, that the knowledge base has the right holdings, and that the target link lands on the article.

What it does with an OpenURL

  1. Parses the query string, separating transport and administrative keys from Entity descriptors (accepting both 0.1 and 1.0 syntax).
  2. Identifies the Referent by identifier (DOI, PMID, ISBN) when one is present, by metadata otherwise.
  3. Looks up the Referent in the knowledge base and collects candidate services: full text on publisher or aggregator platforms, open-access copies, print holdings in the catalogue, interlibrary loan or document delivery, citation export.
  4. Filters and orders them by coverage, licence terms and local preference.
  5. Either sends the user straight to the preferred full text (often called a "direct link" or "one-click") or shows a menu.

Knowledge bases

A resolver is only as accurate as its knowledge base. Vendors maintain large shared knowledge bases that each library customises to its own holdings, for example the Alma Community Zone and SFX KnowledgeBase (Ex Libris), the knowledge base behind 360 Link and Intota, EBSCO's Holdings & Linking Management, and the WorldCat knowledge base (OCLC). Title lists from content providers flow into them, ideally in the NISO KBART format; see OpenURL & KBART.

Products in use

Listed without endorsement or comparison:

  • SFX (Ex Libris, part of Clarivate). The first commercial link resolver, developed from the SFX research at Ghent University.
  • Alma link resolver (Ex Libris). Built into the Alma library services platform and used with Primo discovery.
  • 360 Link (originally Serials Solutions, later ProQuest, now Ex Libris/Clarivate).
  • EBSCO Full Text Finder (EBSCO; its earlier resolver product was LinkSource).
  • WorldCat Discovery / WorldShare link resolver (OCLC), using the WorldCat knowledge base.
  • Umlaut (open source, Ruby on Rails), developed at Johns Hopkins University. It is no longer actively developed; its code repository has had no changes since 2016.

All of these accept OpenURLs in the SAP1 form, KEV ContextObjects sent Inline over HTTP(S) GET, and in practice also the older 0.1 syntax.

Where the resolver's address comes from

Every resolver has a base URL, such as https://xx.alma.exlibrisgroup.com/view/uresolver/INST_CODE/openurl for Alma or https://XXXXXX.search.serialssolutions.com/ for 360 Link. Libraries register it with Google Scholar's Library Links programme and with databases, which then add "Find it at…" buttons for users on campus or logged in. See How to find your library's OpenURL resolver for step-by-step instructions.

Common questions

Is a link resolver the same as a proxy server such as EZproxy?
No. The resolver decides where to send the user; the proxy handles authentication so that the publisher recognises the user as a member of the institution. They are often chained: the resolver's target links are prefixed with the proxy URL.
Is a link resolver the same as a discovery layer?
No. A discovery layer (Primo, EBSCO Discovery Service, Summon, WorldCat Discovery) is a search interface over a large central index. It uses the link resolver to turn each result into an access link.
Is a link resolver the same as a DOI resolver?
No. doi.org resolves a DOI to the one URL registered by the publisher, whoever the user is. An OpenURL link resolver takes the user's institution into account. See OpenURL & DOI.
Is a link resolver the same as a "URL resolver" or persistent-URL service?
Persistent-identifier resolvers (Handle, PURL, ARK, DOI) redirect an identifier to a current location. An OpenURL link resolver works from a citation and returns context-dependent services.

Sources