Turning a folder into an RO-Crate

Estimated time: 7 minutes

Overview

Questions

• How can I start a new RO-Crate?

Objectives

• Creating a skeleton RO-Crate Metadata File
• Use the JSON-LD pre-amble to enable Linked Data

Turning a folder into an RO-Crate

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In the simplest form, to describe some data on disk, an RO-Crate Metadata File is placed in a folder alongside a set of files or folders.

First create a new folder crate1/ and add a single file data.csv to represent our dataset:

"Date","Minimum temperature (°C)","Maximum temperature (°C)","Rainfall (mm)"
2022-02-01,16.0,28.4,0.6
2022-02-02,16.3,17.2,12.4

Next, to turn this folder into an RO-Crate, we need to add the RO-Crate Metadata File, which has a fixed filename. Create the file ro-crate-metadata.json using Visual Studio Code or your favourite editor, then add the following JSON:

JSON

{
  "@context": "https://w3id.org/ro/crate/1.1/context",
  "@graph": [

  ]
}

Your folder should now look like this:

Folder listing of crate1, including data.csv and ro-crate-metadata.json

The presence of the reserved ro-crate-metadata.json filename means that crate1 (and its children) can now be considered to be an RO-Crate. We call the top-level folder of the crate for the RO-Crate Root and can now refer to its content with relative file paths.

We also need to make some declaration within the JSON file to turn it into a valid RO-Crate Metadata Document, explained in the next session.

JSON-LD preamble

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The preamble of @context and @graph are JSON-LD structures that help provide global identifiers to the JSON keys and types used in the rest of the RO-Crate document. These will largely map to definitions in the schema.org vocabulary, which can be used by RO-Crate extensions to provide additional metadata beyond the RO-Crate specifications. It is this feature of JSON-LD that helps make RO-Crate extensible for many different purposes – this is explored further in the specification’s appendix on JSON-LD. In short, only JSON keys (properties) and types defined this way can be used within the RO-Crate Metadata Document.

However, in the general case it should be sufficient to follow the RO-Crate JSON examples directly without deeper JSON-LD understanding. The RO-Crate Metadata Document contains a flat list of entities as JSON objects in the @graph array. These entities are cross-referenced using @id identifiers, rather than being deeply nested. This is one major difference from JSON structures you may have experienced before. The @type keyword associates an object with a predefined type from the JSON-LD context. Almost any property can alternatively be used with an [] array to provide multiple values.

The rest of this tutorial, and indeed most of the RO-Crate specification, specify which entities can be added to the @graph array.

Key Points

• Adding a RO-Crate Metadata file to a folder turns it into an RO-Crate
• The RO-Crate Root is the top-level folder of the crate
• RO-Crate uses schema.org as base vocabulary
• The JSON-LD context enables optional Linked Data processing
• Descriptions are listed flatly as entities in the @graph array