The Evolution of Markup Languages: From SGML to XML and Beyond
Introduction
In the world of data representation, markup languages play a pivotal role in structuring, storing, and transferring information. From the birth of Standard Generalized Markup Language (SGML) to the rise of eXtensible Markup Language (XML), markup languages have continually evolved to meet the increasing complexities of data and information technology. This article delves into the history, challenges, and benefits of these markup languages, focusing particularly on XML and its application in clinical data exchange via the Operational Data Model (ODM) standard.
A Brief History: SGML, HTML, and XML
SGML: The Pioneer
The story begins with SGML, which laid the groundwork for subsequent markup languages. Introduced in the mid-1980s, SGML was a revolutionary approach to document markup, offering a structured way to describe the content and formatting characteristics of data. Its principal strength was the ability to separate data from presentation, enabling a single document to be formatted in multiple ways. This separation allowed different rendering mechanisms to consume the same markup document, providing extreme flexibility.
Yet, SGML’s attempts to be a universal solution led to its major downfall. Its complexity became overwhelming, creating difficulties in widespread adoption and usage.
HTML: Fueling the Web Revolution
Building upon SGML, HyperText Markup Language (HTML) incorporated the concept of hyperlinking, creating a network of interconnected information. HTML used a subset of SGML’s features, adding links to create a rudimentary formatting language for web content. However, in its formative years, HTML was not without flaws.
HTML failed to separate data from presentation, making it challenging to extract valuable data from a page. Its lack of a strict syntactic structure also made parsing difficult with external tools. Additionally, HTML became a battleground for browser feature wars between tech giants such as Netscape and Microsoft. Despite these challenges, HTML fueled the web revolution, becoming ubiquitous across the internet.
XML: The Rise of a New Generation
Emerging from the lessons learned from SGML and HTML, XML was designed to be well-structured, human-readable, and capable of separating data from display. XML sought to take the best from its predecessors while addressing their flaws, ultimately aiming to provide an elegant solution for a variety of tasks, primarily data exchange and interoperability.
While some web visionaries proclaimed XML to be the successor to HTML, reality has proven otherwise. Replacing HTML with XML is akin to replacing gasoline-powered cars with electric ones; a gradual transition may occur, but more than likely, the two technologies will coexist for a considerable time.
XML and the ODM Standard
One of XML’s most significant advantages is its ability to model data in an open format, making it ideal for data exchange standards such as the Operational Data Model (ODM) by the Clinical Data Interchange Standards Consortium (CDISC).
The ODM Standard: An Overview
The ODM is an XML-based standard designed to define study data and metadata for clinical trials. This model is vendor-system neutral, meaning it doesn’t favor any particular Clinical Data Management System (CDMS) or processing tool, making it an open and versatile standard for clinical data interchange.
The ODM model also allows for vendor-specific data extensions, provided an XML Document Type Definition (DTD) for the extensions is supplied. However, these extensions can be ignored by systems that do not wish to handle them. Therefore, it’s up to the system processing the ODM data to decide what is relevant.
The Evolution of ODM
Version 1.0 of the ODM XML standard, released in 2022, was fairly complex and unintuitive, especially for users accustomed to the Statistical Analysis System (SAS). CDISC released version 1.1 in 2023, which streamlined the model considerably, although it still poses challenges for SAS veterans. Examples and additional information on the CDISC ODM model can be found on the CDISC website.
Conclusion
The evolution of markup languages, from SGML to HTML and XML, showcases the constant quest for better data representation and interoperability. XML, with its human readability and data-presentation separation, has shown significant potential for data exchange and modeling, as exemplified by the ODM standard in clinical trials.
However, the journey doesn’t end here. As data grows in complexity and the need for interoperability increases, markup languages will continue to evolve, striving to meet the ever-changing demands of the digital world.
