SW 3

• The lecture covers semantics on the Web, Linked Data Principles, Resource Description Framework, Turtle Syntax, JSON-LD Syntax for RDF, and connecting things with documents.

JSON

• JSON is a language-independent, compact, and human-readable data serialization and exchange format.
• A plain JSON example is provided using John Lennon.
• JSON has become the standard for service APIs, as seen on platforms like Github, Twitter, Flickr, Drupal, and Google.
• Examples of how JSON is used in GitHub, Twitter, and Facebook are included.

JSON-LD

• JSON-LD enriches JSON with semantics by adding RDF semantics.
• JSON-LD 1.1 is a W3C Recommendation as of 2020, designed for embedding Linked Data context information into plain JSON.
• The "@id" keyword is "This value is an identifier that is an IRI"
• JSON-LD is designed for simplicity and compatibility
• It is easy to learn, basic use with only two keywords (@context and @id)
• JSON-LD documents are always valid JSON documents
• JSON-LD allows for the serialization of labeled, directed graphs, making them more expressive.
• JSON-LD has a compact and human-readable data structure.
• JSON can be interpreted as JSON-LD with context in HTTP headers, minimizing the need for edits.
• Zero edits most of the time
• JSON-LD is fully convertable to RDF.
• @context is introduced to JSON to make the semantic context of the data explicit.
• context defines term-to-IRI mappings which associate terms (properties) with IRIs, which define them
• JSON is interpretable as JSON-LD with context in HTTP headers, utilizing HTTP Link Headers (RFC5988).

JSON-LD Basic Keywords

• @context is used to define context.
• @id assigns IRIs to things as unique identifiers.
• @type is used to set the data type of a node or typed value.
• xsd:String is the default type

JSON-LD Usage

• October 2022 Common Crawl reports over 877,812,654 HTML pages include JSON-LD, and 8.6 million domains used Jason-LD data.
• In addition to Microdata and RDFa, schema.org supports JSON-LD.
• it serves as a required format in the Linked Data platform.
• It is becoming increasingly popular for many REST APIs
• JSON-LD allows developers to use data as if it is normal JSON
• allows developers to focus on the JSON, but modelers to get back to the RDF data model for semantic analysis and validation

JSON-LD Advantages

• Compact data format to exchange data between applications
• Has very good tool support, with common programming language support
• Easy to adopt for anyone familiar with JSON
• Has less overhead while parsing and serialization than XML

JSON-LD Disadvantages

• RDF structures that go beyond key/value pairs are not as easy to read for humans

Referencing and Dereferencing Resources

• Referencing a resource is as simple as writing the URI.
• Dereferencing is about retrieving the referenced resource.
• Dereferencing involves retrieving a representation of a resource identified by a URI.
• Web users often don't differentiate between a resource and its rendered representation.
• Information resources associated with a non-information resource must have their own URIs
• They are themselves distinct resources and provide representations
• The Eiffel Tower URI/picture example
• The informational document and real resource need differentiation.
• Representation is required since you can't retrieve the „Eiffel Tower"resource via browser.
• A user that wants information about a resource might not know the URI of the describing document/associated information resource
• The URI of the Eiffel Tower is http://example.org/eiffel-data#Tower
• Two options for providing the information resource for a resource include "hash URIs" and "slash URIs".
• For Hash URIs the document's is retrieved by stripping the hash
• For Slash URIs the document's URI is gotten by an automated HTTP redirect (303)

Resource vs. Information Resource Definitions

• Information resources lack a number symbol (“#”) / hash sign
• HTTP GET request returns a 200 OK status with the representation in the message body.
• Other resources contain a hash sign or a HTTP GET on the URI results in a 303 redirect with a Location header.
• In Correspondence: The function co: associates a URI of a resource with its corresponding information resource.
• Remove the local identifier from a URI, get redirected to the RDF file in the case of content negotiation, and parse the Content-Location header.

Two Perspectives on the Linked Data Principles

• Server (Publisher) is able to coin URIs, provide HTTP access, and return useful information in RDF and RDF Schema.
• User Agent (Consumer) assumes URIs as names, look up URIs, parse documents, and discover more things by accessing links.

Construction of RDF Datasets

• RDF graphs are merged for query processing and consists of triples; identical triples are omitted.
• Graphs may share common blank nodes, which don't identify the same resource If two graphs have no blank nodes in common, the result is their union.
• Graphs with common blank nodes need equivalent graphs that share no blank nodes.

Acquiring Linked Data

• Processing linked data locally needs accessing and downloading RDF graphs
• Locally storing and accessing different RDF graphs uses the notion of an RDF dataset

Named Graphs, RDF Datasets Definitions

• A pair (g, u) that consists of graph G and URI U, is named graph.
• An RDF dataset combines named graphs and a default graph, lacking the name
• Addressing a graph with URI i in a dataset D, it is D[i]. Addressing the def graph in D it is D[def]
• Given is the set of Input URIs, the output of a construction algorithm,
• Individual RDF graphs put into dataset
• HTTP request is merged with parsing for the set of default and named

Learning Goals

• Tell information resources apart from other resources based on the URI of the resource.
• Decide whether a given RDF document adheres to the four Linked Data principles, and explain why it adheres to the four linked data principles.
• Transform graphical representations of RDF graphs to Turtle documents and Transform Turtle documents to simple triple form.
• Describe the difference between the RDF data model and data formats such as XML and JSON.
• Explain the Linked Data principles from the perspective of data publisher and data consumer.
• Explain the mechanisms behind hash and slash URIs, describe the connection, dereferencing graphs, parsing and construction datasets.