XSD Full Form

What is the full form of XSD?

An Extensible Markup Language (XML) document’s element must be properly described according to the World Wide Web Consortium's (W3C) proposal known as XSD (XML Schema Definition). Programmers can use it to check that every piece of material in a document corresponds to the description of the element it is placed in. Like all XML schema languages, XSD can be used to specify a set of guidelines that an XML document needs to follow to be regarded as "valid" per that schema. However, unlike most other schema languages, XSD was also intended to produce a collection of data complying with data types because of determining a document's validity. An Infoset like this after validation can be helpful.

History of XSD

One of the XML schema languages is XML Schema, which was published as a W3C recommendation in May 2001. It was the first distinct XML schema language to be given W3C recommendation status. Some user communities referred to this language as WXS, an initialism for W3C XML Schema, while others referred to it as XSD, an initialism for XML Schema Definition, due to the confusion between XML Schema as a specific W3C specification, and the use of the same term to describe schema languages generally. The W3C decided to use XSD as the preferred designation in Version 1.1, and this article uses that name.

DTDs and other early XML schema projects like DDML, SOX, XML-Data, and XDR are acknowledged as having a significant influence in the XSD specification's appendix of references. Although it is a compromise, it incorporates elements from all these plans. After XML Schema was released, only XDR and SOX of those languages remained in use and support. Up until the release of MSXML 6.0 (which abandoned XDR in favor of XML Schema) in December 2006, several Microsoft products supported XDR. Up until its bankruptcy in late 2004, Commerce One, Inc. supported its SOX schema language.

Namespace awareness and datatypes, i.e., the capability to define element and attribute content as holding values such as integers and dates rather than arbitrary text, are the most obvious characteristics supplied by XSD that are not available in XML's native Document Type Definitions (DTDs).

The XSD 1.0 specification was first released in 2001, and a second edition was released in 2004 to address numerous problems. In April 2012, XSD 1.1 was designated as a W3C Recommendation.

Characteristics of XSD

1. The structure of an XML document is described by XSD.

2. Namespaces are supported, and they can be expanded to include new functionality.

3. Different kinds of data are supported by XSD.

4. The W3C consortium supports XSD for XML documents.

5. The fundamental elements of a document's legal structure are defined by XSD.

6. They comprise the document's attributes and elements, the data types, default and fixed values, and the number and order of each element.

7. Every day, several standardized XML formats are used, and XSD is a potent substitute for the DTD.

Features of XSD

1. Users may design and recognize data patterns more easily thanks to XSD.

2. In data communication, they add additional security.

3. There is no need for a parser to process XSD.

4. The integrity of a file is checked and updated.

5. For future enhancements, XSDs can be suitably extended.

Schemas and Schema Documents

Technically speaking, a schema is a set of abstract metadata declarations for elements, attributes, and sophisticated and simple type definitions. These components are often produced by processing a set of schema documents that include their source language definitions. A schema document, however, is frequently referred to as a schema in everyday speech.

The identified schema components are all members of a target namespace, which is a characteristic of the entire schema document. Schema documents are arranged according to namespaces. Other schema documents for the same namespace may be included in a schema document, and it may import schema documents for another namespace.

The schema to be used for validation can either be supplied as a parameter to the validation engine or it can be accessed directly from the instance document using two specific attributes, xsi: schema Location and xsi: no Namespace Schema Location. (For the latter approach to work, the client invoking validation must have enough confidence in the document to know that it is being validated against the right schema. The namespace's traditional prefix is "xsi."

The filename extension ".xsd" is typically used for XML Schema Documents. According to RFC 3023, "application/XML" or "text/XML" should be used because XSDs do not yet have a registered Internet Media Type.

Schema Components

The main components of a schema are:

Element declarations

Which specify the attributes of elements. The target namespace and element name are two examples. The type of the element is a crucial characteristic because it limits the characteristics and children the element can have. The type of an element may in XSD 1.1 depend on the values of its attributes. A substitution group is a collection of elements; if element E is in the substitution group of element H, then E may substitute for H anywhere H is allowed by the schema. There are two types of integrity requirements that might apply to elements: uniqueness constraints, which mandate that certain values be unique inside the subtree that is rooted in the element, and referential constraints, which mandate that values match the identifier of another element. The ability to use global or local element declarations.

Attribute declarations

Which enumerates the attributes' properties. The target namespace and attribute name are again included in the properties. The values that an attribute may have been limited by the type of the attribute. Additionally, a fixed value or a default value may be included in an attribute declaration, making them the sole possible values for that attribute.

Simple and Complex types: These are explained in the section below.

Model and Attribute group

In essence, these are named collections of components and characteristics that can be utilized in a variety of types of definitions.

Indicating whether an attribute is required or optional when used in a complex type, an attribute use depicts the relationship between an attribute declaration and a complex type.

The relationship between a complex type and an element declaration is represented similarly by an element particle, which also specifies the minimum and the maximum number of times an element may appear in the content. Model group particles, which function like non-terminals in a grammar to define the choice and repetition units within the sequence of allowed elements, can also be included in content models in addition to elementary particles. Furthermore, the use of wildcard particles, which allow a variety of various elements, is permitted (perhaps any element provided it is in a certain namespace).

Annotations, assertions, notations, and the schema component, which provides details on the schema, are some of the other, more specialized components.

Types

The textual values that can be used in an element or attribute are limited by simple types, often known as data types. This is one of the most notable ways that XML Schema and DTDs are different from one another. A property might, for instance, be limited to storing only a valid date or a decimal integer.

19 primitive data types are offered by XSD (anyURI, base64Binary, boolean, date, dateTime, decimal, double, duration, float, hexBinary, gDay, gMonth, gMonthDay, gYear, gYearMonth, NOTATION, QName, string, and time). Through three techniques, it enables the construction of new data types from these primitives:

• limitation (limiting the range of allowed values)

• list (permitting a series of values)

• union (enabling a choice of values from different types).

There are 25 derived types defined in the standard itself, and users can specify more derived types in their schemas.

The means for limiting data types include regular expressions, the ability to set minimum and maximum values, restrictions on the length of strings, and restrictions on the number of digits in decimal numbers. Again, included in XSD 1.1 are assertions and the use of an XPath 2.0 expression to specify any constraint.

The allowable content of an element, including its element and text children and its properties, are described by complex types. A set of attribute uses plus a content model make up a complicated type of definition. The several types of content models are:

• content that is exclusively made up of elements and cannot contain text (other than whitespace, or text enclosed by a child element).

• Simple content allows for the appearance of text but forbids the appearance of child elements.

• Empty content does not allow for the appearance of either text or child elements.

• Mixed content allows for both appearances.

By restriction (forbidding some components, properties, or values that the underlying type permits) or by extension, a complex type can be created from another complex type (allowing additional attributes and elements to appear). A complicated type may be limited in XSD 1.1 by assertions, which are XPath 2.0 expressions against the content that must evaluate to be true.

Post-Schema-Validation Infoset

It is feasible to represent the structure and content of an XML document in terms of the data model that was implied during validation after XML Schema-based validation. Included in the XML Schema data model are:

• The vocabulary (element and attribute names)

• The content model (relationships and structure)

• The data types

The Post-Schema-Validation Infoset is the name of this data set (PSVI). The PSVI supports treating a valid XML document as an object using object-oriented programming (OOP) paradigms by defining the document's "type".

Secondary uses for XML Schemas

The main goal of creating an XML schema is to properly describe an XML document, but it also serves a variety of other purposes outside of simple validation.

Code Generation

The schema can be used to build so-called XML Data Binding code. The programming environment may now treat the contents of XML documents as objects thanks to this code.

Generation of XML file structure documentation

A human-readable documentation of an XML file structure can be created using the schema; this is especially helpful when authors have used the annotation elements. There is no official standard for creating documentation, but there are a lot of tools out there that can create understandable, high-quality HTML and printed materials, like the Xs3p stylesheet.

Criticism

XML Schema has received a tremendous degree of harsh criticism, possibly more so than any other W3C Recommendation, despite being extensively adopted and accomplishing most of what it set out to do. Rick Jelliffe, David Webber, Anders Moller, and Michael Schwartzbach, as well as James Clark, Anders Moller, and Michael Schwartzbach, provide excellent descriptions of the criticisms.

General Problem:

• It is difficult for non-experts to use because of its complexity (the specification is hundreds of pages long and written in a highly technical language), but many non-experts use schemas to specify data types. Reading the W3C Recommendation is very challenging. The XML Schema Primer from W3C is a lot simpler to understand for most consumers.

• No explicit mathematical specification exists for XSD. (This makes it difficult to establish that a schema alteration is backward compatible, for example.)

• The language has a lot of unexpected things, such as how the restriction of elements differs from the restriction of attributes

Practical Limitations of Expressibility:

• Support for unordered content is quite limited in XSD.

• No specific root element can be mandated by XSD (so extra information is required to validate even the simplest documents).

• Character data cannot in any manner be restricted when representing mixed content (not even a set of valid characters can be specified).

• Elements of element context cannot be used to depend on the content or attribute declarations (this was also listed as a central problem of DTD).

• Even though that was an initial design need, it is not entirely self-describing (as a trivial example, see the prior point).

• Elements' default values can only be character data, and defaults cannot be provided apart from declarations, making it difficult to create families of schemas that only differ in their default values (not containing markup).

Technical problems:

• Although it formally complies with the namespace, it doesn't seem to uphold the namespace spirit (e.g. "unqualified locals").

• There was no way to specify in XSD 1.0 that the existence or value of one property depends on the presence or value of another attribute (so-called co-occurrence constraints). XSD 1.1 contains a patch for this.

• The available XSD datatypes are a very random selection.

• Maintaining separation between the two activities of validation and augmentation (adding type information and default values) is important.

Version 1.1

In April 2012, XSD 1.1 received W3C Recommendation status, making it a recognized W3C specification.

In XSD 1.1, the following noteworthy new features are included:

• the use of XPath 2.0 expressions to define assertions against the content of documents (an idea borrowed from Schematron).

• the capacity to choose the type against which an element will be checked based on the values of the element's attributes ("conditional type assignment").

• easing the restrictions that say explicit content model elements can't match wildcards that the model also permits.

• the capacity to define wildcards that apply to all types in the schema and adhere to the same extensibility policy for both elements and attributes.

• Before the Proposed Recommendation draft, XSD 1.1 also suggested the addition of a new numeric data type, precision Decimal. This raised questions and was consequently removed from the standard.

Benefit of XSD

• On any type of data, XSD can impose restrictions.

• It can feasibly be extended by generating new elements from the ones that already exist. However, DTD cannot be extended.

• Standard values are supported by XSD. The default values of the involved items can then be specified.

• You can limit an element's content and it supports all data types.

• Because XSD is defined in XML, no intermediary parsing is necessary.

• Referencing external XSD schemas is also supported by XSD. A single XML schema can also contain or import additional XML schemas.