XML: Revolutionizing Web Data Exchange

Every time two different computer systems need to share information, they face a basic problem: one system stores data in its own way, and the other expects something different. A library’s catalogue software, a publisher’s database, and a mobile app may all hold the same book record, yet none of them speaks the same internal language. XML (Extensible Markup Language) was created to solve exactly this. It provides a plain-text, structured way to describe data so that almost any system can read it, regardless of the hardware, operating system, or programming language involved. This is why XML became one of the quiet workhorses of the internet, sitting behind web services, databases, and the digital catalogues used in libraries every day.

What XML is and how it works 🔗

XML is a markup language used to encode documents and data in a format that is both human-readable and machine-readable. It was developed by the World Wide Web Consortium (W3C) and released as a recommendation in 1998. The official specification describes XML as a class of data objects called XML documents, along with rules for how programs should process them. In simpler terms, XML lets you label pieces of information using tags that you define yourself.

Here is a small example describing a book record:

<book><title>Five Laws of Library Science</title><author>S. R. Ranganathan</author><year>1931</year></book>

Notice that the tags such as <title> and <author> are not fixed by any standard. You create them to match your data. This is what the word “extensible” means. Unlike HTML, where tags like <p> and <h1> are predefined and mainly control how things look, XML tags describe what the data is. Because of this, a program reading the file above immediately knows which text is a title and which is an author.

Separating data from presentation 🔗

One of the most important ideas behind XML is the clean separation of data from presentation. An XML file holds only the structured information. How that information looks on a screen or printed page is handled separately, often through stylesheets. According to the W3C’s description of XML activity, the formatting of data using XML is kept quite separate from the programs that process it. This matters because the same XML record can be displayed as a web page, fed into a database, or printed as a report without changing the underlying data at all.

Well-formed and valid documents 🔗

XML follows strict rules. Every opening tag must have a closing tag, tags must be properly nested, and there is a single root element that contains everything else. A document that follows these basic syntax rules is called well-formed. A stricter level is being valid, which means the document also matches a defined structure. That structure can be described using a Document Type Definition (DTD) or, more commonly today, an XML Schema. As the W3C notes in its comparison of the two languages, a DTD is optional in XML and is needed only when you want to validate a document against a fixed set of rules. This flexibility is one reason XML spread so quickly.

Comparing XML and SGML 🔗

To understand why XML was designed the way it is, it helps to look at its parent. XML did not appear from nowhere. It is a simplified subset of an older and far more complex standard called SGML (Standard Generalized Markup Language), which was published as an international standard, ISO 8879, back in 1986. The W3C specification states plainly that XML is an application profile, or restricted form, of SGML. In fact, by design, every valid XML document is also a conforming SGML document.

Why SGML needed simplifying 🔗

SGML is powerful and was widely used in large-scale publishing, technical documentation, and government records. But that power came with a cost. SGML contains many optional features that are rarely needed, and its full rules are complicated to implement. Building software that could correctly process every SGML feature was expensive and time-consuming, which made it a poor fit for the open, fast-moving environment of the web. A browser maker in the 1990s could not realistically support the entire SGML standard.

How XML keeps the strengths and drops the weight 🔗

The team behind XML, formed under the W3C in 1996, set out to keep what was useful about SGML while removing the parts that made it hard to use on the web. The stated goal in the specification was to enable generic SGML to be served, received, and processed on the web in the way that was already possible with HTML. XML retains SGML’s core strengths: extensibility, a clear structure, and the ability to validate documents. At the same time it strips away most optional features so that XML processors can be small, fast, and easy to write. The Library of Congress describes XML as a simple, very flexible text format derived from SGML, originally built to meet the challenges of large-scale electronic publishing.

A useful way to summarise the relationship is this: SGML is the large, full-featured toolbox, while XML is the lightweight, web-ready selection of the most useful tools. Content stored as XML is automatically valid SGML, but not all SGML content qualifies as valid XML. This careful trimming is exactly what made XML practical for the internet.

Applications of XML 🔗

The real proof of XML’s value lies in how widely it is used. Its platform-neutral and self-describing nature makes it suitable for many tasks where structured data must move between systems that were never designed to work together.

Web services and data exchange 🔗

One of the most common uses of XML is in web services, where applications communicate over the internet. The classic example is SOAP (Simple Object Access Protocol), an XML-based protocol for exchanging structured messages between systems. A SOAP message is itself an XML document made up of an envelope, an optional header, and a body that carries the actual content. Alongside SOAP, WSDL (Web Services Description Language) is an XML-based language used to describe what a web service does and how to call it. Databases also rely on this approach. Oracle’s documentation explains that applications can query a database and receive the results back as XML data, with web services acting as a clean interface to the underlying records.

Another familiar example is the RSS feed. When a news website or blog publishes an RSS feed, it is producing an XML file that lists recent items with their titles, links, and summaries. A reader application downloads that XML and displays the updates, often storing them in a database first. This lets people follow many sources from one place without visiting each website manually.

Databases and configuration files 🔗

Beyond communication, XML is widely used to store and configure software. Many applications keep their settings in XML configuration files because the format is easy to read, easy to edit, and easy for the program to parse. XML is also used for data serialization, which means converting data in a program into a storable or transmittable form and then rebuilding it later. Because XML is plain text, these files can be opened, inspected, and version-controlled without special tools, which is a real advantage during development and debugging.

Document sharing and library metadata 🔗

For students of library and information science, the most relevant use of XML is in metadata and document exchange. Libraries deal constantly with bibliographic records that must be shared between institutions, publishers, and vendors. XML provides a structured, standard way to do this. The Library of Congress developed MARCXML, an XML schema that can express a complete MARC record without losing any data, allowing full bibliographic records to travel between systems safely.

Where MARCXML preserves every detail, other XML standards offer different balances of simplicity and richness. MODS (Metadata Object Description Schema) is an XML schema with MARC-like meaning that uses readable, language-based tags instead of numeric codes, making it easier to learn while still being richer than the very simple Dublin Core. The Library of Congress even maintains crosswalks that convert records between MARC, MODS, and Dublin Core. These standards are also combined within METS (Metadata Encoding and Transmission Standard), which the American Library Association describes as a format used both for structuring digital objects and for exchanging them between institutions, including within long-term digital archives.

This is XML doing exactly what it was built for: letting a digital repository, a national library, and a university catalogue share the same record while each keeps its own internal systems. The data is structured, self-describing, and software-independent, which is precisely why XML-based metadata standards remain central to digital libraries and institutional repositories.

What do you think? As newer formats like JSON have become popular for web data exchange, do you think XML will remain the preferred choice for library metadata, or will it gradually be replaced? And in your view, what makes a data format truly “future-proof” for preserving cultural and bibliographic records over decades?

References

1. https://www.w3.org/XML/
2. https://www.w3.org/People/Raggett/Drafts/xml.html
3. https://www.w3.org/TR/REC-xml/
4. https://www.loc.gov/preservation/digital/formats/fdd/fdd000075.shtml
5. https://docs.oracle.com/en/database/oracle/oracle-database/19/adxdb/native-XML-DB-web-services.html
6. https://asistdl.onlinelibrary.wiley.com/doi/10.1002/bult.268
7. https://journals.ala.org/index.php/sln/issue/viewFile/409/109