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Entities Overview
An OpenURL ContextObject has room for six entities. One is required; the other five are optional. Each describes one role in the request.
The OpenURL framework models a request as a set of Entities: the things and parties involved in a reference and in the request for services about it. A ContextObject must contain exactly one Referent; the other five Entities add context when the source has it.
The six Entities
| Entity | KEV prefix | XML element | Number (KEV / XML) | Role |
|---|---|---|---|---|
| Referent | rft | referent | 1 / 1 | The resource being referenced |
| ReferringEntity | rfe | referring-entity | 0–1 / 0–1 | The resource that contains the reference |
| Requester | req | requester | 0–1 / 0–1 | The person or agent requesting services |
| ServiceType | svc | service-type | 0–1 / 0+ | The kind of service requested |
| Resolver | res | resolver | 0–1 / 0+ | The target of the request |
| Referrer | rfr | referrer | 0–1 / 0–1 | The system that generated the ContextObject |
The standard's own SAP1 appendix gives scholarly examples: the Referent is "a referenced journal article", the ReferringEntity "a referencing article", the Requester "the user clicking an OpenURL", the ServiceType "fulltext, ILL, etc.", the Resolver "a library's OpenURL linking server", and the Referrer the database that produced the link.
Describing an Entity
Every Entity can be described by any combination of four Descriptors: Identifier (_id), By-Value Metadata (_val_fmt + .key), By-Reference Metadata (_ref_fmt + _ref) and Private Data (_dat). The KEV prefix ties each key to its Entity:
rft_val_fmt=info:ofi/fmt:kev:mtx:journal
&rft.atitle=Some+Article+Title
&rft.jtitle=Some+Journal
&rft_id=info:doi/10.1103/PhysRev.47.777
&rfe_id=info:doi/10.1006/mthe.2000.0239
&rfr_id=info:sid/example.com:sourceWhat you typically see in practice
Most OpenURLs carry only two Entities:
- Referent, always, with
rft_val_fmtandrft.*metadata, anrft_id, or both. - Referrer, as
rfr_id=info:sid/…, naming the database or service that made the link.
Publishers' reference lists sometimes add a ReferringEntity (the DOI of the citing article). Requester, ServiceType and Resolver are uncommon in ordinary links and appear more in machine-to-machine uses.