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RO-Crate Metadata

Table of contents

  1. RO-Crate uses Linked Data principles
  2. Base metadata standard: Schema.org
    1. Differences from Schema.org
  3. Additional metadata standards
  4. Summary of Coverage
  5. Future coverage
  6. Recommended Identifiers

RO-Crate aims to capture and describe the Research Object using structured metadata.

The RO-Crate Metadata File Descriptor contains the metadata that describes the RO-Crate and its content, in particular:

  • Root Data Entity - the RO-Crate Dataset itself, a gathering of data
  • Data Entities - the data payload, in the form of files and folders
  • Contextual Entities - related things in the world (e.g. people, organizations, places), providing provenance for the data entities and the RO-Crate.

This machine-readable metadata can also be represented for human consumption in the RO-Crate Website, linking to data and Web resources.

RO-Crate uses Linked Data principles

RO-Crate makes use of the Linked Data principles for its description. In particular:

  1. (Meta)data should be made available as Open Data on the web.
  2. (Meta)data should be machine-readable in a structured format.
  3. (Meta)data should not require proprietary software packages.
  4. (Meta)data should use open standards from W3C, such as RDF and SPARQL.
  5. (Meta)data should link to other people’s data to provide context, using URIs as global identifiers

RO-Crate realizes these principles using a particular set of technologies and best practices:

  1. The RO-Crate Metadata File and RO-Crate Website can be directly published on the web together with the RO-Crate payload. In addition, a data package (e.g. BagIt Zip archive) that contain the RO-Crate can also be published on the web.
  2. The RO-Crate Metadata File is based on the structured data format JSON-LD.
  3. Multiple open source tools/libraries are available for JSON and for JSON-LD.
  4. The RO-Crate Website is HTML 5, and the RO-Crate Metadata File is JSON-LD, one of the W3C RDF 1.1 formats.
  5. The RO-Crate Metadata File reuse common vocabularies like Schema.org, and this specification recommend identifiers it should link to.

Base metadata standard: Schema.org

Schema.org is the base metadata standard for RO-Crate. Schema.org was chosen because it is widely used on the World Wide Web and supported by search engines, on the assumption that discovery is likely to be maximized if search engines index the content.

As far as we know there is no alternative, well-maintained linked-data schema for research data with the coverage needed for this project - i.e. a single standard for expressing all the examples presented in this specification.

RO-Crate relies heavily on Schema.org, using a constrained subset of JSON-LD, and this document gives opinionated recommendations on how to represent the metadata using existing linked data best practices.

Differences from Schema.org

Generally, the standard type and property names (terms) from Schema.org should be used. However, RO-Crate uses variant names for some elements, specifically:

  • File is mapped to http://schema.org/MediaObject which was chosen as a compromise as it has many of the properties that are needed to describe a generic file. Future versions of Schema.org or a research data extension may re-define File.
  • Journal is mapped to http://schema.org/Periodical.

JSON-LD examples given on the Schema.org website may not be in flattened form; any nested entities in RO-Crate JSON-LD SHOULD be described as separate contextual entities in the flat @graph list.

To simplify processing and avoid confusion with string values, the RO-Crate JSON-LD Context requires URIs and entity references to be given in the form "author": {"@id": "http://example.com/alice"}, even where Schema.org for some properties otherwise permit shorter forms like "author": "http://example.com/alice".

See the appendix RO-Crate JSON-LD for details.

Additional metadata standards

RO-Crate also uses the Portland Common Data Model (PCDM version https://pcdm.org/2016/04/18/models) to describe repositories or collections of digital objects and imports these terms:

The terms RepositoryObject and RepositoryCollection are renamed to avoid collision between other vocabularies and the PCDM terms Collection and Object. The term RepositoryFile is renamed to avoid clash with RO-Crate’s File mapping to http://schema.org/MediaObject.

RO-Crate use the Profiles Vocabulary to describe profiles using these terms:

From Dublin Core Terms RO-Crate use:

These terms are being proposed by Bioschemas profile ComputationalWorkflow 1.0-RELEASE and FormalParameter 1.0-RELEASE to be integrated into Schema.org:

In this specification the proposed Bioschemas terms use the temporary https://bioschemas.org/ namespace; future releases of RO-Crate may reflect mapping to the http://schema.org/ namespace.

Summary of Coverage

RO-Crate is simply a way to make metadata assertions about a set of files and folders that make up a Dataset. These assertions can be made at two levels:

  • Assertions at the RO-Crate level: for an RO-Crate to be useful, some metadata should be provided about the dataset as a whole (see minimum requirements for different use-cases below). In the RO-Crate Metadata File, we distinguish the Root Data Entity which represents the RO-Crate as a whole, from other Data Entities (files and folders contained in the RO-Crate) and Contextual Entities, e.g. a person, organisation, place related to an RO-Crate Data Entity
  • Assertions about files and folders contained in the RO-Crate: in addition to providing metadata about the RO-Crate as a whole, RO-Crate allows metadata assertions to be made about any other Data Entity

This document has guidelines for ways to represent common requirements for describing data in a research context, e.g.:

  • Contact information for a data set.
  • Descriptive information for a dataset and the files within it and their contexts such as an abstract, spatial and temporal coverage.
  • Associated publications.
  • Funding relationships.
  • Provenance information of various kinds; who (people and organizations) and what (instruments and computer programs) created or contributed to the data set and individual files within it.
  • Workflows that operate on the data using standard workflow descriptions including ‘single step workflows’; executable files or environments such as singularity containers or Jupyter notebooks.

However, as RO-Crate uses the Linked Data principles, adopters of RO-Crate are free to supplement RO-Crate using Schema.org metadata and/or assertions using other Linked Data vocabularies.

Future coverage

A future version of this specification aim to cater for variable-level assertions: In some cases, e.g. for tabular data, additional metadata may be provided about the structure and variables within a given file. See the use case Describe a tabular data file directly in RO-Crate metadata for work-in-progress.

RO-Crate JSON-LD SHOULD use the following IDs where possible:

In the absence of the above, RO-Crates SHOULD contain stable persistent URIs to identify all entities wherever possible.