Hi Folks,
As you know, a restaurant that offers a buffet style of dining lays out all foods on tables and then customers select the items they want. With a set menu style of dining the customer makes a
choice and that determines all the items the customer will receive.
Should an XML Schema be written like a buffet: create a long list of optional elements and then let the instance document author select whichever ones he desires?
Or should an XML Schema be written like a set menu: there is a choice and once the instance document author selects a choice, that determines all the elements for that choice?
Example: Data for a magazine includes name of the magazine, publication date, cost, edition number. Data for a book includes title of the book, publication date, cost, category (fiction, non-fiction),
binding (hardcover or softcover). Both magazines and books are publications. An XML Schema is created to model a publication. Here is a buffet style of designing the XML Schema:
<xs:element name="Publication">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="xs:string" minOccurs="0" />
<xs:element name="publication-date" type="xs:date" minOccurs="0" />
<xs:element name="cost" type="xs:string" minOccurs="0" />
<xs:element name="edition-number" type="xs:nonNegativeInteger" minOccurs="0" />
<xs:element name="title" type="xs:string" minOccurs="0" />
<xs:element name="category" minOccurs="0">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="fiction" />
<xs:enumeration value="non-fiction" />
</xs:restriction>
</xs:simpleType>
</xs:element>
<xs:element name="binding" minOccurs="0">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="hardcover" />
<xs:enumeration value="softcover" />
</xs:restriction>
</xs:simpleType>
</xs:element>
</xs:sequence>
<xs:attribute name="type">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="magazine" />
<xs:enumeration value="book" />
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
Notice that all the elements are optional (a buffet of elements!). All the elements for magazines and for books are declared within Publication.
Presumably, a person wanting to create an instance document for a magazine would select only the first four elements, e.g.,
<Publication type="magazine">
<name>Popular Mechanics</name>
<publication-date>1968-04-02</publication-date>
<cost>25 cents</cost>
<edition-number>430</edition-number>
</Publication>
The "type" attribute is used as a flag to indicate whether the person has selected elements appropriate for a magazine or for a book. In this case, type="magazine" indicates the intent is to
provide data about a magazine.
Here is a set menu style of designing the XML Schema:
<xs:element name="Publication">
<xs:complexType>
<xs:choice>
<xs:element name="magazine">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="xs:string" />
<xs:element name="publication-date" type="xs:date" />
<xs:element name="cost" type="xs:string" />
<xs:element name="edition-number" type="xs:nonNegativeInteger" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="book">
<xs:complexType>
<xs:sequence>
<xs:element name="title" type="xs:string" />
<xs:element name="publication-date" type="xs:date" />
<xs:element name="cost" type="xs:string" />
<xs:element name="category">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="fiction" />
<xs:enumeration value="non-fiction" />
</xs:restriction>
</xs:simpleType>
</xs:element>
<xs:element name="binding">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="hardcover" />
<xs:enumeration value="softcover" />
</xs:restriction>
</xs:simpleType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:choice>
</xs:complexType>
</xs:element>
Notice that two choices are provided – magazine or book – and once a choice is made, all the elements for the choice are specified.
A person wanting to create an instance document for a magazine would select the first choice:
<Publication>
<magazine>
<name>Popular Mechanics</name>
<publication-date>1968-04-02</publication-date>
<cost>25 cents</cost>
<edition-number>430</edition-number>
</magazine>
</Publication>
The “type” attribute is gone and is replaced by an explicit element: <magazine> or <book>.
Okay, those are two ways to design an XML Schema. Which design do you prefer?
/Roger
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