Introduction to XML

XML is the universal format for data interchange.  It is supported by all major operating systems, programming languages and development platforms.  It is the standard platform for building interoperable distributed applications. 

The ability to invoke cross platform communications makes the reach of XML Technology unlimited and its use in Web Services compelling.

Web Services are the building blocks for constructing distributed Web-based applications. They are units of programmable application logic located on web servers that can be accessed remotely using standard internet protocols and data formats such as XML, HTTP and SOAP. Web services use XML-based messaging to send and receive data, which enables heterogeneous applications to interoperate with each other.

Basic concepts of XML

 XML represents structured content rather than visual display. A well-formed XML document contains an end tag for every begin tag. Well-formed XML documents can be modeled as trees. The tree has a single root node containing one or more child nodes. A node without a child is called leaf node. XML Document Object Model (XML DOM) is required to work with XML data.

Document Object Model (DOM) 

Document Object Model (DOM) is a representation of the XML document in memory.  It is used to read, write, and manipulate an XML document. DOM loads the entire XML document into memory.

Simple API for XML (SAX) 

Simple API for XML (SAX) is used to read data from XML documents. SAX parser reads the contents sequentially and generates events as it reads the XML document.  It does not load the entire XML document into memory. SAX is good for reading large XML Documents.

Limitations of SAX:  SAX does not maintain any data structures that are required to perform complex searches. They cannot be used to modify the XML document.

Document Type Definition (DTD)

Document Type Definitions (DTD) outlines elements, attributes & relationships allowed in an XML document. DTDs are written in their own syntax rather than XML syntax.

XML Schema Document (XSD)

XML Schema Documents describe and validate the structure of XML documents.  They restrict the value of elements and attributes in an XML document. They are written in XML syntax. An XML document that adheres to XML schema is called Valid XML document. The XML Schema documents have suffix xsd.