XML Technologies and Standards

Overview

Throughout the years, Extensible Markup Language, or XML, has proven to be a resilient and versatile component. Its value lies in its ability to represent structured data, making it applicable to various technologies and underpinning numerous web standards.

Moving forward, we'll dive into XML's influence across a range of applications, along with the critical standards that build on its capabilities.

XML's Role

XML's primary strength is its simplicity, generality, and usability over the internet. In a way, its role is not to perform computation tasks, but rather to structure data in a universally comprehensible way, making it equally accessible to machines and humans. The versatile nature of XML extends its reach across multiple domains.

Take for example the healthcare sector. It uses XML to facilitate the exchange of patient records and lab reports between systems. Similarly, financial institutions employ XML to ensure consistent transactions and reporting. In addition, the publishing industry leverages XML for its ability to separate data from presentation, simplifying data reuse across different platforms.

While XML is used in many more contexts than represented here, this gives a glimpse into its far-reaching implications.

A Selection of XML Standards and Tools

These are some of the significant standards and tools in XML's ecosystem, but many others also contribute to its flexibility and wide usage.

Simple API for XML (SAX) and Document Object Model (DOM): SAX and DOM are interfaces that enable interaction with XML data programmatically. While SAX provides a read-only, event-based method ideal for processing large XML documents due to its low memory usage, DOM is more interactive. DOM provides read-write access, allowing modifications to the XML document, like adding, modifying, or deleting nodes.
XML Namespaces: In the early days of XML usage, conflicts between element names became evident. To solve this, W3C introduced XML Namespaces, which associates a URI with element names to guarantee uniqueness across different XML documents. This standardization by W3C was instrumental in XML's wide adoption.
XSL (Extensible Stylesheet Language) and XSLT (XSL Transformations): These are languages used to style, format, and transform XML documents. They make it possible to present the same XML data in multiple visual formats. XSLT, for instance, can repurpose XML data into HTML for viewing in web browsers, making the data more adaptable to different presentation needs.
XLink and XPointer: These specifications enable the creation of hyperlinks and connections within XML documents. XLink defines how to create hyperlinks in XML, while XPointer allows for pointing to specific parts within XML documents, akin to the hyperlinking mechanism within HTML.
XML Schema (XSD) and DTD (Document Type Definition): XML Schema and DTD are essential for defining the structure, elements, and data types allowed in XML documents. XML Schema is a more comprehensive and sophisticated method for validation, offering a wider array of data types compared to DTD.

XML Query Languages

Data stored in XML is not just statically sitting; it is often required to be queried and manipulated. XML query languages, like XQuery and XPath, allow for this interaction. XQuery is a tool for extracting and manipulating data from XML documents and collections. Suppose a retail company maintains an XML-based product catalog. They could use XQuery to retrieve all products in a particular category priced below a certain amount. XPath, often used within XSLT and XQuery, allows for selecting nodes in an XML document, adding to the versatility of XML data handling.

XML and Metadata

XML has a significant role in encapsulating metadata — data that provides additional information about other data. Libraries, digital collections, and online repositories utilize XML to represent metadata, thereby increasing data's portability and reusability. With XML metadata, these entities can improve the searchability and interoperability of their collections, expanding the value of XML beyond mere data organization.

Web Services and XML

In the early days of Internet communication, there was a need for standardized protocols to allow different systems, potentially written in different programming languages and running on different platforms, to interact effectively over the Internet. Among the solutions developed, three significant ones – SOAP, WSDL, and UDDI – relied heavily on XML.

SOAP (Simple Object Access Protocol) is a messaging protocol that uses XML to encode the information in a web service request and response messages before sending them over a network. SOAP messages are independent of any operating system or protocol and can be used in conjunction with a variety of HTTP or SMTP, for example.

WSDL (Web Services Description Language) is a language based on XML, designed to detail the methods and services offered by a specific web service. Its purpose is to provide a machine-readable description of how the service can be interacted with.

UDDI (Universal Description, Discovery, and Integration) is a platform-independent, XML-based registry for businesses worldwide to list themselves on the internet. UDDI allows businesses to list their web service applications in a universally recognized registry, making the web service easier to find and use.

These XML-based protocols have been critical in enabling complex applications to communicate over the Internet. However, they are often considered heavyweight and can introduce significant overhead, leading to the rise of alternative approaches like REST.

REST (Representational State Transfer) is an architectural style for distributed hypermedia systems, like the World Wide Web. RESTful APIs typically use HTTP and can work with any data format, including XML and JSON. JSON is often preferred over XML in RESTful services for its simplicity and better compatibility with JavaScript, one of the predominant languages in web development.

Understanding the foundational role of XML in these aspects offers a historical perspective on the evolution of Internet communication and highlights why certain technologies have been adopted or abandoned as needs and capabilities have evolved.

RSS - Really Simple Syndication

RSS (Really Simple Syndication) is a technology that uses XML for content syndication, allowing users to subscribe to "feeds" from a blog, news site, or other content providers. The idea is to push the same content out in multiple locations, making it accessible for users without them having to manually check each site.

An RSS feed document typically includes a list of items, each representing an individual piece of content. Each item can contain a title, description, link, publication date, and other metadata.

Observe an illustrative example of what an RSS feed document might look like:

<rss version="2.0">
<channel>
  <title>My Blog</title>
  <link>https://myblog.com</link>
  <description>This is my blog.</description>
  <item>
    <title>Blog Entry 1</title>
    <link>https://myblog.com/entry1</link>
    <description>This is my first blog entry.</description>
  </item>
  <!-- More items here -->
</channel>
</rss>

In this XML document, <rss> is the root element that encapsulates the entire feed. The <channel> element contains metadata about the feed itself (e.g., title, link, and description). Each <item> element represents an individual piece of content, complete with its title, link, and description. While this example is simplified, real-world RSS feeds may contain more complex structures and additional elements.

Learn more about the syntax in our XML Syntax - A Detailed Overview article

Despite the rise of social media platforms, RSS feeds remain a vital tool for content syndication, particularly for news websites, blogs, and podcasts. They offer a standardized format for applications, such as RSS reader apps, to consume and display content from various sources.

AJAX - XML in Asynchronous Web Applications

AJAX, standing for Asynchronous JavaScript and XML, encompasses a group of techniques, concepts, and technologies. It enables web applications to send and retrieve data from a server in the background (asynchronously) without interfering with the display and behavior of the existing page.

While XML was initially the primary format for data exchange in AJAX, JSON has become more popular due to its compatibility with JavaScript. The XMLHttpRequest object, often used in AJAX, provides a method to exchange data with a server behind the scenes.

Consider this simplified example:

var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    var responseXML = this.responseXML;
    // Process the XML data
  }
};
xhttp.open("GET", "data.xml", true);
xhttp.send();

In the above example, an XMLHttpRequest object requests data from a web server. When the server's response is ready (readyState == 4 and status == 200), the XML data is accessed through the responseXML property.

Over time, new methods and libraries have emerged, such as the Fetch API and Axios, simplifying the process of making asynchronous requests. The advent of ES6 Promises and async/await in JavaScript has made asynchronous programming more straightforward and less error-prone.

Though the use of XML in AJAX has declined with the rise of JSON, understanding AJAX and its roots in XML can be beneficial for understanding the evolution of web technologies. These modern approaches have built upon the foundation laid by XMLHttpRequest and AJAX's original techniques.

Legacy Systems, Niche Uses, and XML's Lingering Relevance

In technology, things move fast. However, not every system or protocol keeps up with the newest trends. While XML may not be as commonly used for new developments, its prevalence in legacy systems and specialized applications remains undeniable.

Over the years, XML has found its way into many industries and has been employed by a myriad of technologies. From Android development, which utilizes XML for layout designs, to the publishing world, where XML is leveraged for formats like DITA and DocBook, XML's impact can still be felt. Similarly, XML has been key in the creation of Scalable Vector Graphics (SVG), a format that has become a mainstay in data visualization libraries such as d3.js.

Enterprise systems, especially those following the WS-* (Web Services) architecture and using SOAP and WSDL, have extensively employed XML for communication. These systems are commonly used by large corporations, and while new projects may opt for more modern protocols, these legacy systems are likely to continue using XML for many years to come.

Additionally, XML's usage in server-side programming shouldn't be underestimated. Despite the rise of JSON, many server-side languages, such as Java, Python, and PHP, provide robust support for XML processing. XML can be utilized server-side and then transformed into JSON for client-side usage. This process leverages XML's robust validation mechanisms through XML schemas, allowing the validation of data before conversion to JSON.

We've covered a lot of ground, but our exploration doesn't exhaust the full extent of XML's capabilities or its applications. To partly aid you in that, you can refer to the articles found below, covering the introductory aspects of XML that provide more depth into the nuts and bolts of working with XML.