XML Namespaces. Mario Arrigoni Neri

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1 XML Namespaces Mario Arrigoni Neri 1

2 Applications and vocabularies Tags are meta-data and are interpreted by a specific XML enabled application XML parser must provide each application with the information to detect and extract relevant submodels (model fragments) Classic approaches File extensions Proprietary metadata In XML we can use custom metadata for each application 2

3 Mixing tags (1) Simple elaboration scenario: document is elaborated by a single application (a web browser for example) <html> </html> Parser XML viewer 3 Complex scenario: different parts of the same document must be elaborated by different applications <html> <book/> </html> Parser XML viewer Book agent

4 Mixing tags (2) What if two applications uses omonimous elements? 4 <table> <tr> <td>name</td> <td>surname</td> </tr> <tr> <td>mario</td> <td>bianchi</td> </tr> <tr> <td>luca</td> <td>rossi</td> </tr> <table> <tr><td>order</td> <td>product</td></tr> <tr><td>1</td> <td> <table> <name>mytable</name> <price>100</price> <width um= cm >50</width> <length um= cm >80</length> </td></tr> <table> <name>mytable</name> <price curr= EU >100</price> <width u= cm >50</width> <length u= cm >80</length> <table> is recognized by HTML viewer or by furniture agent?

5 Namespaces (1) Each XML element name (tag) is prefixed by a namespace, making it unique (qualified tag) Tag structure is: prefix:tagname <h:table> <h:tr><h:td>order</h:td> <h:td>product</h:td></h:tr> <h:tr><h:td>1</h:td> <h:td> <fur:table> <fur:name>mytable</fur:name> <fur:price>100</fur:price> <fur:width u= cm >50</fur:width> <fur:length u= cm >80</fur:length> </fur:table> </h:td></h:tr> </h:table> 5

6 Namespaces (2) Some issues: Prefix may be not unique Different documents may identify the same namespace with different identifiers (due to different relationships between languages) Enforcing unique names produces long namespaces Local prefixes are introduced and bound to namespaces <h:table xmlns:h= xmlns:fur= > </h:table> < </ Element is identified by fully expanded qualified name: <h:table xmlns:h= > 6 <pre:table xmlns:pre= />

7 URI, URL and URN Uniform Resource Identifier (URI) is an unique name for general entities, usually on the web The most common specialization is the Uniform Resource Locator (URL), that identifies a web resource and provides a way to retrieve it Less common Uniform Resource Name (URN) XMLNS does not provide any meaning to namespaces URLs. It is simply a string we require to be unique 7 Why URIs? Legal an technological reasons Relative URIs violates the requirements

8 Default namespace When an XML document refers to a given namespace for the majority of its size, such a namespace can be defined as the default 8 <table xmlns= xmlns:fur= > <tr><td>order</td> <td>product</td></tr> <tr><td>1</td> <td> <fur:table> <fur:name>mytable</fur:name> <fur:price>100</fur:price> <fur:width u= cm >50</fur:width> <fur:length u= cm >80</fur:length> </fur:table> </td></tr>

9 Prefixes scope Prefixes are defined in initial and empty tags. Prefix to namespace relation holds in the whole element Prefixes redefinitions follow classic static scope rules (inner block overrides external one) 9 <table xmlns= > <tr><td>ordine</td> <td>prodotto</td></tr> <tr><td>1</td> <td> <table xmlns= > <name>mytable</name> <price>100</price> <width u= cm >50</width> <length u= cm >80</length> </td></tr>

10 Attributes Attributes can refer to specific applications too; so they can be qualified <fur:table fur:xmlns= > <fur:name>mytable</fur:name> <fur:price>100</fur:price> <fur:width fur:u= cm >50</fur:width> <fur:length fur:u= cm >80</fur:length> </fur:table> Each namespace contains three names partitions: Element Type Partition: contains element names Global Attribute Partition: names of qualified attributes Per-Element partition: unqualified attribute names applied to qualified elements <fur:width fur:u= cm >50</fur:width> <fur:width u= cm >50</fur:width> 10

11 Namespaces and DTD (1) DTD is part of XML 1.0 specification Namespaces was introduced later Adapting DTD to (partially) support namespaces can be dispendious strategy: Express qualified names as valid names, blocking the prefix to namespace correspondency Enforce the assignment of the right namespace to the choosed prefix, by defining a fix xmlns attribute <!ELEMENT fur:table (fur:name, fur:price, fur:width, fur:length)> <!ELEMENT fur:name (#PCDATA) > <!ATTLIST fur:table xmlns:fur CDATA #FIXED > 11

12 Namespaces and DTD (2) The definition of a namespace in DTD schema limits the flexibility of original namespace technology correct <fur:table xmlns:fur= > </fur:table> error: redefining a #FIXED attribute <fur:table xmlns:fur= > </fur:table> error: prefix is blocked by DTD <f:table xmlns:f= > </f:table> error: prefix definition is required <fur:table> </fur:table> 12

COMP9321 Web Application Engineering

COMP9321 Web Application Engineering Semester 2, 2017 Dr. Amin Beheshti Service Oriented Computing Group, CSE, UNSW Australia Week 4 http://webapps.cse.unsw.edu.au/webcms2/course/index.php?cid= 2465 1