JSON-LD: minimal introduction

JSON-LD (JavaScript Object Notation for Linked Data, see JSON-LD 1.1 specification) is a format for representing structured data as Linked Data (see Linked Data principles) using JSON.

In JSON-LD, data is expressed as a collection of objects that can be uniquely identified and linked to each other. This allows metadata to be represented as a graph of interconnected entities.

A minimal JSON-LD object

The same JSON-LD object can be written in different syntactic forms. The following example shows a minimal object in three equivalent representations:

  • expanded
  • compact-1
  • compact-2
{
  "@id": "_:example-person",
  "@type": "https://schema.org/Person",
  "https://schema.org/givenName": "Zaphod"
}

{
  "@context": {
    "@vocab": "https://schema.org/"
  },
  "@id": "_:example-person",
  "@type": "https://schema.org/Person",
  "givenName": "Zaphod"
}

{
  "@context": {
    "schema": "https://schema.org/"
  },
  "@id": "_:example-person",
  "@type": "https://schema.org/Person",
  "schema:givenName": "Zaphod"
}

These representations describe the same object. The expanded form uses full IRIs for all property names, while the compact forms use "@context" to shorten them. The underlying syntax is explained below.

JSON-LD syntax basics

The example above uses several JSON-LD keywords:

“@id” The keyword "@id" provides the identifier of an object. It can be a blank node identifier (e.g. "_:example-person") or a globally unique IRI. Within a metadata collection, each "@id" has to be unique.

“@type” The keyword "@type" specifies the type of an object. This is typically defined by a schema and expressed as an IRI (e.g. https://schema.org/Person).

“@context” The keyword "@context" defines how property names are interpreted by mapping them to IRIs (Internationalized Resource Identifiers), e.g. https://schema.org/givenName. Namespaces defined in "@context" are used to shorten these IRIs.

These representations differ in how namespaces are defined and used:

  • compact-1: "@vocab" defines a default namespace, allowing properties to be written directly

  • compact-2: a custom prefix (e.g. "schema") defines a namespace used in property names

  • expanded: no "@context" is defined, so full IRIs must be used

The following chapters show how these JSON-LD concepts are applied in openMINDS.