NGSI-LD

Key takeaways

NGSI-LD belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NGSI-LD to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NGSI-LD from memory before moving on to harder problems.

Reference excerpt

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.

Design

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 (a.k.a. "attributed 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 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. A 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. A 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). A 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). A 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. The flexible information model allows the specification of any kind of entity. In order to allow interoperability between NGSI-LD users, standardized entities are collaboratively defined at Smart Data Models Program and made available at its repository with an open-source license.

Architecture The NGSI-LD specification consists of an information model and an API. The API provides functionalities to support the architectural roles described in the following.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with NGSI-LD

Start with the simplest possible case. Write down what NGSI-LD claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NGSI-LD before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NGSI-LD ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NGSI-LD

In research: NGSI-LD appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NGSI-LD in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NGSI-LD is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer standards, Information science, Knowledge representation, so understanding it makes those chapters shorter.
In everyday life: Look for NGSI-LD outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study NGSI-LD in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NGSI-LD means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NGSI-LD out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NGSI-LD in simple terms?

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.

Why does NGSI-LD matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NGSI-LD?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NGSI-LD.