science

XLink

XLink is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand XLink rather than just read about it. In short: XML Linking Language, or XLink, is an XML markup language and W3C specification that provides methods for creating internal and external links within XML documents, and associating metadata with those links. The XLink specification XLink 1.1 is a W3C recommendation and the successor of XLink 1.0, which was also a W3C recommendation.

Key takeaways

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

Reference excerpt

The XLink specification XLink 1.1 is a W3C recommendation and the successor of XLink 1.0, which was also a W3C recommendation.

Linking with XLink XLink defines a set of attributes that may be added to elements of other XML namespaces. XLink provides two kinds of hyperlinking for use in XML documents. Simple links connect only two resources, similar to HTML links. Extended links can link an arbitrary number of resources.

Simple links A simple link creates a unidirectional hyperlink from one element to another via a URI. Example:

Extended links Extended links allow multiple resources, either remote or local, to be connected by multiple arcs. An arc is information about the origin, destination and behavior of a link between two resources. The origin and destination resources are defined by labels. By using one or more arcs, an extended link can achieve specific sets of connections between multiple resources. For example, if all resources in an extended link were given the label A, then an arc within that link declaring from="A", to="A" would form connections between all resources. Extended links do not need to be contained in the same document as the elements they link to. This makes it possible to associate metadata or other supplementary information with resources without editing those resources. XLink also supports richer information about link types and the roles of each resource in an arc.

Support for XLink

Within other specifications

SVG Hypertext links in Scalable Vector Graphics can currently be defined as simple XLinks. The working draft of SVG 1.2 proposes using extended XLinks as well. In the SVG 2 specification, XLink was deprecated in favor of non-namespaced equivalent attributes.

RDDL The Resource Directory Description Language, an extension to XHTML Basic that is used to describe XML Namespaces, uses simple XLinks.

XBRL The eXtensible Business Reporting Language has used simple and extended XLinks since the XBRL 2.0 specification was published in 2001. Most large XBRL taxonomies contain extensive linkbases. As of 2009, XBRL is probably the most extensive use of XLink in production systems.

METS The Metadata Encoding and Transmission Standard, supported and maintained by the Library of Congress for describing file aggregations, uses simple XLinks in pointing to file locations as well as linkbases which describe relationships among external files (though these restrict to and from attributes to type IDREF instead of NMTOKEN).

GML Geography Markup Language uses simple XLinks to implement referencing. In particular, GML uses xlink:href to support a graph model for geospatial information. GML's graph model is essentially the same as RDF, on which early versions of GML were based. The GML specification constrains the semantics of XLinks to be essentially the same as rdf:resource (from the RDF/XML syntax) i.e. the referent can logically be placed in-line and the data is still valid.

Implementations

Mozilla Firefox Mozilla Firefox has supported simple XLinks since version 1.5, but only for SVG and MathML documents. It is unsupported in other XML documents. Only the xlink:href, xlink:show, xlink:target and xlink:title attributes are supported.

Prince Prince supports simple XLinks.

References

External links W3C Recommendation W3C Recommendation (version 1.0) XLink: Who Cares? XML Linking Implementations (Last Updated 2000) Demo of XLinks for Mozilla browsers

Worked examples

Example 1 — a first encounter with XLink

Start with the simplest possible case. Write down what XLink claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In 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 XLink 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 XLink 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 XLink

In research: XLink appears in 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 XLink 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: XLink is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hypertext, Markup languages, World Wide Web Consortium standards, so understanding it makes those chapters shorter.
In everyday life: Look for XLink 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.

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How to study XLink in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what XLink 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 XLink out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is XLink in simple terms?

XML Linking Language, or XLink, is an XML markup language and W3C specification that provides methods for creating internal and external links within XML documents, and associating metadata with those links. The XLink specification XLink 1.1 is a W3C recommendation and the successor of XLink 1.0, w…

Why does XLink matter?

Because it connects several 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 XLink?

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 XLink.