<PurchaseOrder OrderDate="2001-10-15"> <Contact Country="US"> in Visual Studio .NET

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<PurchaseOrder OrderDate="2001-10-15"> <Contact Country="US">
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<Name>Joe's Sports, Inc</Name>
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<Street>5555 High Street</Street>
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<City>Springfield</City>
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<State>CA</State>
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<Zip>95555</Zip>
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</Contact> <LineItem>
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<SupplierName>Skateboots</SupplierName>
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<ProductName>Special Red Color</ProductName>
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<Quantity>10</Quantity>
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<UnitPrice>15000</UnitPrice>
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</LineItem> <LineItem>
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<SupplierName>Skateboots</SupplierName>
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<ProductName>Cool Blue Boots</ProductName>
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<Quantity>5</Quantity>
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<UnitPrice>19500</UnitPrice>
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</LineItem> </PurchaseOrder>
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Both schemas and DTDs can be used to validate instance documents
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The purchase order shown in the example contains the PurchaseOrder root element well formed XML documents contain a single root element within which the rest of the elements are enclosed and several child elements of PurchaseOrder, including Contact and LineItem XML documents are represented using a hierarchical structure, in which elements are nested, as SupplierName is nested within LineItem The schema definition is:
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<xsd:schema xmlns:xsd="http: //wwww3org/2001/XMLSchema"
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xmlns:po="Skatebootscom/order"
elementFormDefault="qualified"
targetNamespace="Skatebootscom/order">
<xsd:element name="PurchaseOrder">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="Contact"
type="po:ContactType"/>
<xsd:element ref="po:LineItem"
maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="OrderDate"
type="xsd:date"/>
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Understanding Web Services- XML, WSDL, SOAP and UDDI
</xsd:complexType>
</xsd:element>
<xsd:complexType name="ContactType">
<xsd:sequence>
<xsd:element name="Name" type="xsd:string"/>
<xsd:element name="Street" type="xsd:string"/>
<xsd:element name="City" type="xsd:string"/>
<xsd:element name="State" type="xsd:string"/>
<xsd:element name="Zip"
type="xsd:positiveInteger"/>
</xsd:sequence>
<xsd:attribute fixed="US" name="Country"
type="xsd:NMTOKEN"/>
</xsd:complexType>
<xsd:element name="LineItem">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="SupplierName"
type="xsd:string"/>
<xsd:element name="ProductName"
type="xsd:string"/>
<xsd:element name="Quantity" type="xsd:positiveInteger"/> <xsd:element name="UnitPrice" type="xsd:decimal"/> </xsd:sequence> </xsd:complexType> </xsd:element> </xsd:schema>
The example contains the schema that validates the purchase order shown in the previous example, ensuring the proper data types for the elements and that the line-item element has the proper structure Such a schema ensures the document's conformance to the agreed-on format Without such agreement, it would be possible to receive documents that contained information that wasn't recognized or was ignored and therefore could not be processed correctly Complex types, such as in this example, define the names of elements permitted to appear in the associated document and use attributes to define additional constraint information for the elements The schema references, within the xsd:schema root element, the namespace for the May 2001 version of the XML schema specifications:
<xsd:schema xmlns:xsd="http://wwww3org/2001/XMLSchema" xmlns:po="Skatebootscom/order" elementFormDefault="qualified" targetNamespace="Skatebootscom/order">
The root element also contains an attribute declaring the namespace for the purchase order document itself xmlns:po="Skatebootscom/order" and specifies that the default format for elements is to be namespace qualified Qualified element names are also called QNames
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Understanding Web Services- XML, WSDL, SOAP and UDDI
The entire PurchaseOrder element is structured as a complex type enclosing sequences for ContactType and LineItem These element names are declared, referenced, and namespace qualified, using the po: prefix, within the top-level element, PurchaseOrder The specific simple types for other elements are declared with-in their specific sequences, such as xsd:string for Name and xsd:positiveInteger for Zip The fixed attribute of the ContactType element limits, or restricts, the customer information to a single country type, United States The corresponding DTD is:
<!ELEMENT <!ATTLIST <!ELEMENT <!ATTLIST <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT
PurchaseOrder (Contact, LineItem+)> PurchaseOrder OrderDate CDATA #IMPLIED> Contact (Name, Street, City, State, Zip)> Contact Country CDATA #FIXED "US"> Name (#PCDATA)> Street (#PCDATA)> City (#PCDATA)> State (#PCDATA)> Zip (#PCDATA)> LineItem (SupplierName, ProductName, Quantity, UnitPrice)> SupplierName (#PCDATA)> ProductName (#PCDATA)> Quantity (#PCDATA)> UnitPrice (#PCDATA)>
The example contains a DTD for the same purchase order instance document shown previously The DTD is a bit easier to define for use in validation than the schema, but schemas provide better flexibility for defining semantics The DTD is simpler to understand and use, especially for basic document validation The declaration of valid elements for the associated instance document is very straightforward, using the !ELEMENT tag in the DTD DTDs are easier for validation but limited for semantics
Also, basic structural information, such as the sequence of elements contained within another element, is very straightforward to define For example:
<!ELEMENT LineItem (SupplierName, ProductName, Quantity, UnitPrice)>
Alternatives to Schemas
XML schemas are relatively new and somewhat unusual among W3C specifications in that the W3C initiated the work from scratch Many other W3C initiatives, such as XML, SOAP, and WSDL, were started by small groups of individuals or companies seeking to develop a solution to a particular problem Later, the members of the group
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Understanding Web Services- XML, WSDL, SOAP and UDDI
submitted the material to the W3C for possible adoption Schemas, on the other hand have taken a relatively long time to develop and are fairly difficult to understand, at least in their entirety Many W3C members, including some of the original authors of XML, such as James Clark, feel that the result is much too complex, is difficult to learn and use, and did not meet the original goals for simplicity Several alternatives to schemas have been proposed, and DTDs remain in widespread use for validation, despite the fact that schemas are intended to replace them For applications other than document validation, however, schemas are the language of choice, and that's why Web services technologies are defined using schemas This element declaration indicates that SupplierName, ProductName, Quantity, and UnitPrice are required sub-elements of LineItem However, no data type information is available for document type definitions, namespaces are not used, and very limited structure information is available, making schemas required for most Web services applications[4] As mentioned previously, the one place a DTD is likely to be used is for simple document validation, when a Web service interaction sends a complete document such as this purchase order