The Portland Common Data Model is a minimal, flexible linked data model designed to represent complex digital objects and collections in repository systems. Born from the collaborative needs of three major open-source digital repository communities — Fedora, Samvera (formerly Hydra), and Islandora — PCDM provides a shared vocabulary that enables interoperability across platforms while remaining simple enough to accommodate a wide range of content types.
Background
Digital repository platforms in the library and archives sector have historically used divergent internal data models, making it difficult to share content or migrate between systems. In 2014, developers and metadata specialists from the Fedora, Hydra, and Islandora communities convened at a meeting in Portland, Oregon, to address this interoperability gap. The result was the Portland Common Data Model, a deliberately minimal set of RDF classes and relationships that all three platforms could adopt as a shared foundation. The name reflects both the city where the initial work took place and the aspiration for a common approach to modeling digital collections.
Purpose & Scope
PCDM defines three core classes for organizing digital content:
| Class | Description |
|---|---|
| Collection | An aggregation of other Collections or Objects |
| Object | A digital resource that groups related Files |
| File | A single bitstream (e.g., a TIFF image, a PDF, an audio file) |
These classes are connected through two key relationships: hasMember (linking Collections to Objects or sub-Collections) and hasFile (linking Objects to their constituent Files). An additional hasRelatedObject property supports associative relationships between Objects.
This simplicity is intentional. PCDM is not meant to replace richer descriptive metadata standards like Dublin Core or MODS. Instead, it provides the structural scaffolding — the "bones" of a digital object — on top of which descriptive, administrative, and technical metadata can be layered using other vocabularies.
Technical Foundation
PCDM is built on two W3C standards. It extends the Object Reuse and Exchange (OAI-ORE) model, which defines how aggregations of web resources can be described and exchanged. It also aligns with the W3C Linked Data Platform (LDP) specification, which defines patterns for reading and writing linked data resources via HTTP. This alignment means that PCDM resources can be stored in and served from LDP-compliant containers, as implemented by the Fedora Repository platform.
The ontology is expressed in RDF and published in Turtle format at pcdm.org. The namespace URI is http://pcdm.org/models#. Extension ontologies have been defined for common use cases: PCDM Works adds classes for describing intellectual works and their relationships, while PCDM Use provides terms for indicating the intended use of a File (e.g., thumbnail, preservation master, access copy).
Governance & Maintenance
PCDM is governed by consensus among the participating repository communities. Development takes place on GitHub under the DuraSpace organization (now part of LYRASIS). Changes to the model are proposed through GitHub issues and discussed in community calls. The model has been intentionally kept stable and minimal, with extensions preferred over modifications to the core ontology.
Notable Implementations
PCDM is most prominently implemented in Fedora 4 and later versions, where it serves as the default data model for organizing repository content. The Samvera framework, built on top of Fedora, uses PCDM as its structural backbone for applications like Hyrax. Islandora 2.x (ISLE) also adopted PCDM as part of its migration to Fedora-based architecture. Academic and cultural heritage institutions worldwide use these platforms, making PCDM one of the most widely deployed structural models in the digital library community.
Related Standards
- OAI-ORE -- The Object Reuse and Exchange model that PCDM extends for resource aggregation
- Linked Data Platform (LDP) -- The W3C specification for HTTP-based linked data operations that Fedora implements
- Dublin Core -- Commonly used alongside PCDM for descriptive metadata on Objects and Collections