XML Motivation (Cont.) Earlier generation formats were based on plain text with line headers indicating the meaning of fields Similar in concept to email headers Does not allow for nested structures,no standard"type"language Tied too closely to low level document structure(lines,spaces,etc) Each XML based standard defines what are valid elements,using XML type specification languages to specify the syntax DTD(Document Type Descriptors) XML Schema Plus textual descriptions of the semantics XML allows new tags to be defined as required However,this may be constrained by DTDs A wide variety of tools is available for parsing,browsing and querying XML documents/data Database System Concepts-5th Edition,Aug 22,2005. 10.6 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 5 10.6 ©Silberschatz, Korth and Sudarshan th Edition, Aug 22, 2005. XML Motivation (Cont.) Earlier generation formats were based on plain text with line headers indicating the meaning of fields Similar in concept to email headers Does not allow for nested structures, no standard “type” language Tied too closely to low level document structure (lines, spaces, etc) Each XML based standard defines what are valid elements, using XML type specification languages to specify the syntax  DTD (Document Type Descriptors)  XML Schema Plus textual descriptions of the semantics XML allows new tags to be defined as required However, this may be constrained by DTDs A wide variety of tools is available for parsing, browsing and querying XML documents/data