PURLs and DOIs

This page defines DRUIDs, PURLs, and DOIs, and explains the differences between them, including who makes them, when they are used, and what they do. It also explains why DOIs are an important complement to SDR PURLs.


A DRUID is a Digital Resource Unique IDentifier. This is a string of 11 letters and numbers, e.g. cm430hn3276, that uniquely identifies each item in the Stanford Digital Repository (SDR). We create and assign these DRUIDs here at Stanford. It's also the string you'll see at the end of one of our PURLs, e.g. https://purl.stanford.edu/cm430hn3276

A PURL, or Persistent URL, is the stable location on the web of a particular item of digital content. Here at Stanford Libraries, we assign PURLs to all the objects in the Stanford Digital Repository. If you want to access any SDR content, you need only type the PURL into your browser.


A DOI, or Digital Object Identifier, is also a string -- but different from our DRUIDs -- that uniquely identifies a digital object. A DOI points to the actual location of content on the web. In other words, it's a redirect to an actual web location. In order to create a DOI you must provide a URL where the content can be found. For content in the SDR, DOIs point to PURL pages.

DOIs were introduced to the world in 2000 and were quickly adopted by the scholarly communications community to uniquely identify publications, particularly in scholarly journals. DOIs are issued by one of ten DOI registration agencies around the world. DOIs can now be found attached to many types of digital objects and help enable citation and reuse of research data, software, conference publications, preprints, computational notebooks, data management plans, and more. DOIs are typically expressed as URLs.

DOIs streamline and standardize the tracking of citations. If you have a DOI, you can very quickly access the object that it identifies, and it's very easy to track every citation to a particular DOI. When publications, datasets, and software are assigned DOIs, and when those objects reference people and funders and organizations by the persistent identifers available for those entities (ORCIDs, FundRef IDs, and RORs), we can quickly connect all of these entitites to each other to identify even more information about a topic of interest or to understand the scholarly publication or research landscape from a particular perspective. In other words, DOIs make it easier to connect publications with their authors in a networked information system.

Does content with a PURL need a DOI?

SDR PURLs are strong identifiers, but they are not strong enough. PURLs are not used or tracked consistently in expanding areas where services are growing to manage scholarly publications and communication. Most scholarly or research information systems track DOIs, not PURLs. We make our content more discoverable by many more information systems when we assign DOIs to our SDR PURLs.