Introduction

NGSI-LD is an evolution of the NGSI v2 information model, which has been modified to improve support for linked data (entity relationships), property graphs and semantics (exploiting the capabilities offered by JSON-LD). This work has been conducted under the ETSI ISG CIM initiative and the updated specification has been branded as NGSI-LD. The main constructs of NGSI-LD are: Entity, Property and Relationship. NGSI-LD Entities (instances) can be the subject of Properties or Relationships. In terms of the traditional NGSI v2 data model, Properties can be seen as the combination of an attribute and its value. Relationships allow to establish associations between instances using linked data.

NGSI v2 Data Model

The NGSI v2 data model is quite simple. It can be summarized as shown below:

The core element of NGSI v2 is the data entity, typically a real object with a changing state (such as a Store, a Shelf and so on). Entities have attributes (such as name and location) and these in turn hold metadata such as accuracy - i.e. the accuracy of a location reading.

Every entity must have a type which defines the sort of thing the entity describes, but giving an NGSI v2 entity the type=Store is relatively meaningless as no-one is obliged to shape their own Store entities in the same fashion. Similarly adding an attribute called name doesn't suddenly make it hold the same data as someone else's name attribute.

Relationships can be defined using NGSI v2, but only so far as giving the attribute an appropriate attribute name defined by convention ( e.g. starting with ref, such as refManagedBy) and assigning the attribute type=Relationship which again is purely a naming convention with no real semantic weight.

NGSI-LD is an information model and API for publishing, querying and subscribing to context information. It is meant to facilitate the open exchange and sharing of structured information between different stakeholders. It is used across application domains such as Smart Cities,Smart Industry, Smart Agriculture, and more generally for the Internet of Things, Cyber-Physical Systems, Systems of systems and Digital Twins.

NGSI-LD has been standardized by ETSI (European Telecommunications Standardization Institute) through the Context Information Management Industry Specification Group, following a request from the European Commission. Its takeup and further development are spelled out in the EU's "Rolling plan for ICT standardization". NGSI-LD builds upon a decades-old corpus of research in context management frameworks and context modelling. The acronym NGSI stands for "Next Generation Service Interfaces", a suite of specifications originally issued by the OMA which included Context Interfaces. These were taken up and evolved as NGSIv2 by the European Future Internet Public-Private-Partnership (PPP), which spawned the FIWARE open source community.

The NGSI-LD information model represents Context Information as entities that have properties and relationships to other entities. It is derived from property graphs, with semantics formally defined on the basis of RDF and the semantic web framework. It can be serialized using JSON-LD. Every entity and relationship is given a unique IRI reference as identifier, making the corresponding data exportable as Linked data datasets. The -LD suffix denotes this affiliation to the Linked Data universe.

Information Model The NGSI-LD information model can be considered as the first formal specification by a de jure standards organization of the property graph model, which has emerged since the early 2000s as an informal common denominator model for graph databases.

The core concepts are:

A property graph is a directed multigraph, made up of nodes (vertices) connected by directed links, where nodes and arcs both may have multiple optional attached properties (i.e. attributes) Properties (similar to attributes in object models) have the form of arbitrary key-value pairs. Keys are character strings and values are arbitrary data types. By contrast to RDF graphs, properties are not arcs of the graph. Relationships are arcs (directed edges) of the graph, which always have an identifier, a start node and an end node The NGSI-LD meta-model formally defines these foundational concepts (Entities, Relationships, Properties) on the basis of RDF/RDFS/OWL, and partially on the basis of JSON-LD.

An NGSI-LD Entity is the informational representative of something (a referent) that is supposed to exist in the real world, outside of the computational platform using NGSI-LD. This referent need not be something strictly physical (it could be a legal or administrative entity), nor self-contained (it may be a distributed system-level construct). Any instance of such an entity is supposed to be uniquely identified by an IRI, and characterized by reference to one or more NGSI-LD Entity Type(s). In property-graph language, it is a node. An NGSI-LD Property is an instance that associates a characteristic, an NGSI-LD Value, to either an NGSI-LD Entity, an NGSI-LD Relationship or another NGSI-LD Property. Properties of properties are explicitly allowed and are encouraged e.g. to express the accuracy of a particular measured value. An NGSI-LD Relationship is a directed link between a subject (starting point), that may be an NGSI-LD Entity, an NGSI-LD Property, or another NGSI-LD Relationship, and an object (end-point), that is an NGSI-LD Entity. A NGSI-LD Relationship from a Property to an Entity can for example be used to express that the Property was measured by that Entity (Provenance of the measurement). An NGSI-LD value is a JSON value (i.e. a string, a number, true or false, an object, an array), or a JSON-LD typed value (i.e. a string as the lexical form of the value together with a type, defined by an XSD base type or more generally an IRI), or a JSON-LD structured value (i.e. a set, a list, or a language-tagged string). An NGSI-LD type is an OWL class that is a subclass of either the NGSI-LD Entity, NGSI-LD Relationship, NGSI-LD Property or NGSI-LD Value classes defined in the NGSI-LD meta-model. NGSI-LD pre-defines a small number of types, but is otherwise open to any types defined by users. Complementing this metamodel, the NGSI-LD information model specification also provides a cross-domain ontology that defines key constructs related to spatial, temporal or system-composition characteristics of entities.

Example of NGSI-LD data entity (supermarket)