THE XML REVOLUTION - TECHNOLOGIES FOR THE FUTURE WEB

Schemas and schema languages

A schema is a definition of the syntax of an XML-based language (i.e., it defines a class of XML documents).

A schema language is a formal language for expressing schemas.

Schema processing: Given an XML document and a schema, a schema processor

checks for validity, i.e. that the document conforms to the schema requirements
if the document is valid, a normalized version is output: default attributes and elements are inserted, parsing information may be added, etc.

The document being validated is called an instance document or application document.

Why bother formalizing the syntax with a schema?

a formal definition can provide a precise but human-readable reference
schema processing can be done with existing implementations
your own tools for your languages can benefit: by piping input documents through a schema processor, you can assume that the input is valid and defaults have been inserted

Schemas are similar to grammars for programming languages, however, context-free grammars are not expressive enough for XML.

The term "schema" comes from the database community.