Problems With Well-Formed XML Documents
XML data forms the basis for many computing applications, but can pose a series of problems. XML data is well-formed if it obeys the general rules of XML syntax in terms of element and attribute structure. However, XML data can be well-formed and still cause difficulties within a wider application. Well-formed XML can create problems due to issues with XML Schema, validation, character encoding or the other technologies involved in an application.
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XML Schema
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Developers can use XML Schema Definitions to enforce rules on XML data. These XSD documents work in conjunction with the XML for an application to restrict the elements, attributes and structures in use. An XSD defines many aspects of XML data, including the names of elements and attributes, the data types within these and the number of children any particular elements may have. This means that a set of XML data can be well-formed but still breach the rules of a particular XML Schema, potentially causing problems within the application context.
Character Encoding
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Some XML documents can cause problems with character encoding. When saving XML files, developers need to choose an encoding method to suit the characters within the data. XML can generally store characters that are not within the ASCII range, but if such characters appear within a document that is not saved as Unicode, errors may crop up. This issue tends to arise when certain text editing programs such as Windows Notepad are involved, as the default encoding method does not provide enough storage for non-ASCII characters. Files that do not contain characters outside the ASCII range are less likely to encounter this problem.
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Application Processing
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Well-formed XML documents can encounter problems when the other technologies in an application attempt to process them. Most applications with XML data involve more than one technology. For example, Web applications commonly use XML in conjunction with PHP or ASP at server side, plus HTML and JavaScript at client side. If scripts in these languages attempt to process the XML data in a way that is not correct -- for example, by querying for incorrectly named elements -- this may cause the entire application to fail.
Validation
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Many developers do not use XML validation tools to verify that XML content is correctly structured. The XML can then have negative effects on any applications using it. Developers can avoid many XML problems using validation. An XML document can be well-formed but not valid in a particular context. XML data is valid if it complies with the rules of a Document Type Definition (DTD) or XML Schema (XSD). Developers can use Web validation services to check that their XML is both well-formed and valid, while detecting additional problems with the content.
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