Permanent Identifiers for Semantic Artifacts

# Persistent Identifiers (PIDs)

Persistent identifiers (PIDs) are long-lasting references to any digital object on the Web, providing access to the object it refers to for years to come. It remains permanent to identify digital objects, regardless of changes to their location on the Web. More details can be found here (opens new window).

PIDA PURLs are functionally URLs. Thus, PIDA acts as a resolver service: instead of pointing directly to the location of a resource on the web, a PIDA PURL points to the PIDA service which re-directs requests according to the to the registered location (the corresponding URL).

# Objective

Occasionally, a resource's location or name changes. However, the original location had already been shared with users. Then this original information is not useful anymore to locate the resource, and the client gets a "404" error. We call this phenomenon link rot (opens new window). By using an intermediate resolver, this can be prevented from happening in the future: by updating the record stored in the resolver service, a registrar can repair a broken link with minimal effort. The resource becomes available again!

# Format

PIDA PIDs are persistent URLs and therefore look like URLs. They consist of 3 parts:

  • Protocol: The protocol used to access the PURL resolver (https://). The protocol may differ from that used to access the resource associated with the PID.
  • Resolver address: The IP address or domain name of the PURL resolver (purls.helmholtz-metadaten.de/). This part of the PURL is resolved by the Domain Name Server (DNS).
  • Artifact namespace: This part of the PURL is a user-defined namespace. For semantic artifacts typically 3-5 letter abbreviations of the artifact name are used (e.g.: bfo for "basic formal ontology"). Artifact classes, annotation properties or object properties are typically identified based on an extension of the namespace that may be based on the label of a class or an associated numeric extension.

# Updating PIDA PURLs

Should you need to update your PIDA PURL, the .htaccess file registered in the PIDA system needs to be adapted. To do this, please follow the steps described in Manage PURLs (opens new window).

# History of PURLs

The PURL concept was developed by Stuart Weibel and Erik Jul at OCLC (opens new window) in 1995. A PURL service was implemented using the Apache HTTP Server. OCLC released versions 1 and 2 of the Apache-based source tree, initially in 1999 under the OCLC Research Public License 1.0 License and later under the OCLC Research Public License 2.0 License (opens new window).