Erik Siegel - Schematron: a language for validating XML

Schematron

A Language for Validating XML

Erik Siegel

Computer systems process, read, exchange, and spit out data like the contents of this book, your tax form, electronic health records, social media messages, and much more. All such computer data must be in some kind of well-defined format, so it can be understood and processed correctly.

Computer data formats can be organized into families, according to the ground rules they follow. This is analogous to natural languages. In the western world we have a limited character set (A-Z, sometimes a few more), and we write from left to right. Other language families, for example Chinese, have a much more extended character set and a different writing orientation. If you speak a western language but dont understand French, you cant comprehend text in French, but you can still recognize constructs from the Western language family, such as characters, words, sentences, and paragraphs. For Chinese that will be much more difficult.

Computer data format families work the same way. Examples of data format families include EDI, CSV, JSON, and XML. Each has its own syntax rules, application domains, pros, cons, and fan club. And just as a French person and a Swedish person cannot easily communicate, even though their languages are in the same family, a computer system that talks XML language A cannot understand XML language B. People and computers both can perceive familiar language constructs, but they cannot understand what the other person/computer is saying unless they know the specific language being used.

In the natural language world we can sometimes partly solve that communication problem with a little improvisation, such as sign language or gestures. However, for computers thats not so easy. What goes in and what comes out must be correct and understandable. The process of establishing this correctness for computer data is called validation .

Validation is usually done in phases:

compares these phases for natural and computer languages:

The rules for computer data checks can be expressed in special formal computer languages called validation languages . Validation languages allow the computer system to validate whether the data it consumes or produces is correct and valid . Schematron, the subject of this book, is about validating XML, so lets focus on that.

This process of validating XML documents is explained in much more detail in .

A Schematron schema consists (mostly) of assertions. A simple example would be that, somewhere in an XML document, a given start date must always be before a given end date. You can write such an assertion in the Schematron language and then validate that its true for your data. If not, some error message is produced.

This book explains how to create Schematron schemas and use them to check XML data for compliance with your business rules.

This book is for anyone who wants to learn Schematron or expand their existing knowledge of the language.

I assume you have at least a basic knowledge of XML. That is, you know roughly what documents, elements, and attributes are, and you have seen data formatted like before:

Or like :