science

XML schema

XML schema 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 XML schema rather than just read about it. In short: An XML Schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself. These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governi…

Key takeaways

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

Reference excerpt

An XML Schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself. These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as uniqueness and referential integrity constraints. There are languages developed specifically to express XML Schemas. The document type definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas. Two more expressive XML Schema languages in widespread use are XML Schema (with a capital S) and RELAX NG. The mechanism for associating an XML document with a schema varies according to the schema language. The association may be achieved via markup within the XML document itself, or via some external means. The XML Schema Definition is commonly referred to as XSD.

Validation The process of checking to see if an XML document conforms to a schema is called validation, which is separate from XML's core concept of syntactic well-formedness. All XML documents must be well-formed, but it is not required that a document be valid unless the XML parser is "validating", in which case the document is also checked for conformance with its associated schema. DTD-validating parsers are most common, but some support XML Schema or RELAX NG as well. Validation of an instance document against a schema can be regarded as a conceptually separate operation from XML parsing. In practice, however, many schema validators are integrated with an XML parser.

Languages There are several different languages available for specifying an XML Schema. Each language has its strengths and weaknesses. The primary purpose of a schema language is to specify what the structure of an XML document can be. This means which elements can reside in which other elements, which attributes are and are not legal to have on a particular element, and so forth. A schema is analogous to a grammar for a language; a schema defines what the vocabulary for the language may be and what a valid "sentence" is. There are historic and current XML schema languages:

The main ones (see also the ISO 19757's endorsed languages) are described below. Though there are a number of schema languages available, the primary three languages are Document Type Definitions, W3C XML Schema, and RELAX NG. Each language has its own advantages and disadvantages.

Document Type Definitions

Tool support DTDs are perhaps the most widely supported schema language for XML. Because DTDs are one of the earliest schema languages for XML, defined before XML even had namespace support, they are widely supported. Internal DTDs are often supported in XML processors; external DTDs are less often supported, but only slightly. Most large XML parsers, ones that support multiple XML technologies, will provide support for DTDs as well.

W3C XML Schema

Advantages over DTDs Features available in XSD that are missing from DTDs include:

Names of elements and attributes are namespace-aware Constraints ("simple types") can be defined for the textual content of elements and attributes, for example to specify that they are numeric or contain dates. A wide repertoire of simple types are provided as standard, and additional user-defined types can be derived from these, for example by specifying ranges of values, regular expressions, or by enumerating the permitted values. Facilities for defining uniqueness constraints and referential integrity are more powerful: unlike the ID and IDREF constraints in DTDs, they can be scoped to any part of a document, can be of any data type, can apply to element as well as attribute content, and can be multi-part (for example the combination of first name and last name must be unique). Many requirements that are traditionally handled using parameter entities in DTDs have explicit support in XSD: examples include substitution groups, which allow a single name (such as "block" or "inline") to refer to a whole class of elements; complex types, which allow the same content model to be shared (or adapted by restriction or extension) by multiple elements; and model groups and attribute groups, which allow common parts of component models to be defined in one place and reused. XSD 1.1 adds the ability to define arbitrary assertions (using XPath expressions) as constraints on element content. XSD schemas are conventionally written as XML documents, so familiar editing and transformation tools can be used. As well as validation, XSD allows XML instances to be annotated with type information (the Post-Schema-Validation Infoset (PSVI)) which is designed to make manipulation of the XML instance easier in application programs. This may be by mapping the XSD-defined types to types in a programming language such as Java ("data binding") or by enriching the type system of XML processing languages such as XSLT and XQuery (known as "schema-awareness").

Commonality with RELAX NG RELAX NG and W3C XML Schema allow for similar mechanisms of specificity. Both allow for a degree of modularity in their languages, including, for example, splitting the schema into multiple files. And both of them are, or can be, defined in an XML language.

Advantages over RELAX NG

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with XML schema

Start with the simplest possible case. Write down what XML schema 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 XML schema 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 XML schema 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 XML schema

In research: XML schema 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 XML schema 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: XML schema is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data modeling languages, ISO standards, World Wide Web Consortium standards, so understanding it makes those chapters shorter.
In everyday life: Look for XML schema 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 XML schema in 20 minutes

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

Frequently asked questions

What is XML schema in simple terms?

Why does XML schema 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 XML schema?

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 XML schema.