xml - What is the difference between xsd and xsi?

Question

What exactly is the difference between XML Schema Document and XML Schema Instance ?

• xmlns:xsd="http://www.w3.org/2001/XMLSchema"
• xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

Please elaborate.

Answer 1

xsd and xsi Similarities

• Both are XML namespace prefixes, abbreviations for an XML namespace.
• Both are, as are all namespace prefixes, arbitrarily named; other namespace prefix abbreviations could equally well be used. However, both prefixes are conventional and therefore recommended. (An also-conventional alternative to xsd is xs.)

xsd and xsi Differences

• The xsd (or xs) prefix referring to the Schema Namespace (http://www.w3.org/2001/XMLSchema) is used in XML Schemas (XSDs) for the elements, attributes, and types of the W3C XML Schema Recommendation itself. (This is possible because XML Schema is itself XML.)

• The xsi prefix referring to the The Schema Instance Namespace http://www.w3.org/2001/XMLSchema-instance is used in XML document instances for several special attributes defined by the XML Schema Recommendation:

  • xsi:type allows an XML instance to associate element type information directly rather than through an XSD. See How to restrict the value of an XML element using xsi:type in XSD?

  • xsi:nil allows an empty element to be considered to be valid when the XSD might not otherwise have allowed it.

  • xsi:schemaLocation and xsi:noNamespaceSchemaLocation provide hints to the XML processor as to how to associate an XSD with an XML document. Use xsi:schemaLocation when there is a namespace; use xsi:noNamespaceSchemaLocation when there is no namespace.

See Also

• Namespace related attributes in XML and XML Schema (XSD)
• How to restrict the value of an XML element using xsi:type in XSD?