What Is XML or Extensible Markup Language?

Extensible Markup Language, a subset of the standard universal markup language, referred to as XML. It is a markup language used to mark electronic files to make them structured. ^[1]

Scalable

XML has two pioneers: SGML and HTML, both of which are very successful markup languages, but both have some inherent flaws. XML was born to solve their shortcomings. ^[4]

XML has the following characteristics: ^[4]

The XML file format is a plain text format, similar to HTML in many respects. XML consists of XML elements. Each XML element includes a start tag (<title>), an end tag (</ title>), and between two tags. , For example, you can mark XML elements as price, order number, or name. A tag is a description of the document storage format and logical structure. Formally, tags may include comments, references, character data segments, start tags, end tags, empty elements, document type declarations (DTDs), and preambles. ^[5]

(1) Extensibility: HTML does not allow users to define their own identifiers or attributes. In XML, users can define new identifiers and attribute names according to their needs in order to better semantically modify data. ^[6]

Although the XML standard itself is simple, there are many types of standards related to XML. There are more than twenty related standards formulated by the W3C, and more than ten important e-commerce standards formulated using XML. On the one hand, it shows that XML is indeed a very practical structured general markup language, and has been widely used; on the other hand, it has brought some difficulties to understand these standards. In addition to the wide variety of standards, the standard They also refer to each other, especially the application standards. Their formulation is not only using the XML standard itself, but also many other standards. The system of the XML standard is very similar to the system of the SGML standard. XML-related standards can also be divided into three levels: meta-language standards, basic standards, and application standards. ^[6]

( Meta-Language) Extensible Markup Language Meta-Language

Describes the meta-language used to describe the standard. In the XML standard system, it is the XML standard, which is the core of the entire system. Other XML-related standards are developed or served by it. ^[6]

( Foundation Standards) Extensible Markup Language Foundation Standards

This level of standards is a standard developed for the further practical application of XML, which specifies some common features, methods or rules when using XML to develop standards. For example, XML Schema describes a more strict method for defining XML documents so that XML documents can be processed more automatically; XMLNamespace is used to ensure the consistency of names in XML DTDs so that names in different DTDs can be merged into one document when needed. XSL is a language that describes the style and transformation of XML documents; XLink is used to describe hyperlinks in XML documents; XPointer describes the method of positioning inside the XML document structure; DOM defines a platform and language-independent interface for programs And scripts to dynamically access and modify document content, structure, and style. ^[6]

( Application Standards) Extensible Markup Language Application Standards

XML has begun to be widely accepted, and a large number of application standards, especially for the Internet, have been formulated using XML. Some people even think that the XML standard is the ASCII standard in the Internet era. In this Internet age, almost all industry sectors are related to the Internet. And when they have a relationship with the Internet, they must have their industry standards, and these standards are often formulated using XML. ^[6]