extensible Markup Language

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1 extensible Markup Language XML is rapidly becoming a widespread method of creating, controlling and managing data on the Web. XML Orientation XML is a method for putting structured data in a text file. XML looks a bit like HTML but isn't HTML. XML is text but is not (usually) meant to be read. XML is a family of technologies. XML is verbose, but that is not a problem. XML is new, but not that new. XML is license-free, platform-independent and wellsupported. 1

2 Structured Data For example, spreadsheets, address books, configuration parameters, financial transactions, technical drawings, etc. Programs that produce such data often also store it on disk, for which they can use either a binary format or a text format. The latter allows you, if necessary, to look at the data without the program that produced it. XML is a set of rules, for designing text formats for such data, in a way that produces files that are easy to generate and read, that are unambiguous, and that avoid common pitfalls, such as lack of extensibility, lack of support for internationalization/localization, and platform-dependency. 2

3 Not HTML XML makes use of tags (words in < and >) and attributes (name="value"), but while HTML specifies what each tag and attribute means, XML uses the tags only to delimit pieces of data, and leaves the interpretation of the data completely to the application that reads it. Not meant to be read XML files are text allowing experts (programmers) to more easily debug applications, and in emergencies, they can use a simple text editor to fix a broken XML file. The rules for XML files are much stricter than for HTML. A forgotten tag, or an attribute without quotes makes the file unusable, while in HTML such practice is often explicitly allowed, or at least tolerated. It is written in the official XML specification: applications are not allowed to try to second-guess the creator of a broken XML file; if the file is broken, an application has to stop right there and issue an error. 3

4 A family of Technologies There is XML 1.0, the specification that defines what tags and attributes are, but around XML 1.0, there is a set of optional modules that provide sets of tags and attributes, or guidelines for specific tasks. Xlink which describes a standard way to add hyperlinks to an XML file. XPointer and XFragments for pointing to parts of an XML document. (A bit like an URL, but instead of pointing to documents on the Web, it points to pieces of data inside an XML file.) CSS, the style sheet language, is applicable to XML as it is to HTML. XSL is the advanced language for expressing style sheets. The DOM is a standard set of function calls for manipulating XML (and HTML) files from a programming language. XML Namespaces is a specification that describes how you can associate a URL with every single tag and attribute in an XML document. XML Schemas 1 and 2 help developers to precisely define their own XML-based formats. 4

5 Verbosity Since XML is a text format, and it uses tags to delimit the data, XML files are nearly always larger than binary formats. Communication protocolslike modem protocols and HTTP/1.1 can compress data on the fly, saving bandwith as effectively as a binary format. Maturity In development since SGML, developed in the early 1980s, (ISO standard 1986), widely used for large documentation projects. HTML (SGML syntax), development started in The XML designers took the best parts of SGML and guided by the experience with HTML produced something that is as powerful as SGML, but more regular and simpler to use. Choice Opting for XML is like choosing SQL for databases: you still have to build your own database and your own programs to manipulate it, but there are many tools available. XML is license-free, you can build your own software around it without paying royalties. 5

6 XML the future of the Web HTML is a simple language well suited for the display of small and reasonably simple documents. HTML applications hardwire a small set of tags in conformance with a single SGML specification. Freezing a small set of tags allows users to leave the language specification out of the document and makes it much easier to build applications. The cost is Extensibility. HTML does not allow users to specify their own tags or attributes in order to parameterize or otherwise semantically qualify their data. Structure. HTML does not support the specification of deep structures needed to represent database schemas or objectoriented hierarchies. Validation. HTML does not support the kind of language specification that allows consuming applications to check data for structural validity. 6

7 XML Applications Divided into four categories. Applications that require the Web client to mediate between two or more heterogeneous databases. attempt to distribute a significant amount of processing from the Web server to the Web client. require the Web client to present different views of the same data to different users. tailor information discovery of individual users. The alternative to XML is proprietary code embedded in HTML documents and delivered using browser plug-ins or Java applets. XML derives from a philosophy that data belongs to its creators. Content providers are best served by a data format that does not bind them to particular delivery mechanism. 7

8 Example Using a Web server built on a database, a table of contents (TOC) into a large collection of data that can be expanded with a mouse click to open up a portion of the TOC and reveal more detailed levels of the document structure can be presented to a user. Dynamic TOCs of this kind can be generated at run time directly from the hierarchical structure of the document. Unfortunately, the Web latency built into every expansion or contraction of the TOC makes this process sluggish in many user environments. A much better solution is to download the entire structured TOC to the client rather than just individual server-generated views of the TOC. Then the user can expand, contract, and move about in the TOC supported by a much faster process running directly on the client. Linking HTML implements a tiny amount of the functionality of a true hypertext system. Only the simplest form of linking is supported - unidirectional links to hardcoded locations. 8

9 Hypertext linking mechanisms: Location-independent naming. Bidirectional links. Links that can be specified and managed outside of documents to which they apply. N-ary hyperlinks (e.g., rings, multiple windows). Aggregate links (multiple sources). Transclusion (the link target document appears to be part of the link source document). Attributes on links (link types). 9

10 XML Infrastructure XML separates content from presentation. This is achieved using a Document Object Model (DOM) and a Document Type Definition (DTD). The DOM defines the logical structure of documents; the DTD defines the content. The DOM acts like a data extractor that facilitates the access and manipulation of data. In a DTD the author defines the rules that govern the relationships and interactions between elements. The display of the content is specified by a style sheet - XML uses XSL (extensible Style Sheet). 10

11 What Do XML Documents Look Like? Here is a simple XML document: <?XML version="1.0"?> <oldjoke> <burns> Say <quote>goodnight</quote>, Gracie. </burns> <allen> <quote>goodnight, Gracie.</quote> </allen> <applause/> </oldjoke> XML Markup XML documents are composed of markup and content. There are six kinds of markup that can occur in an XML document: elements, entity references, comments, processing instructions, marked sections, and document type declarations. Elements Elements are the most common form of markup. Delimited by angle brackets, most elements identify the nature of the content they surround. Some elements may be empty, in which case they have no content. If an element is not empty, it begins with a 11

12 start-tag, <element>, and ends with an end-tag, </element>. Attributes Attributes are name-value pairs that occur inside tags after the element name. For example, <div class="preface"> is the div element with the attribute class having the value preface. In XML, all attribute values must be quoted. Processing Instructions (PI) PIs provide information to an application. Like comments, they are not textually part of the XML document, but the XML processor is required to pass them to an application. Processing instructions have the form: <?name pidata?>. The name, called the PI target, identifies the PI to the application. Applications should process only the targets they recognize and ignore all other PIs. CDATA Sections In a document, a (character data) CDATA section instructs the parser to ignore most markup characters. A document might contain characters that the XML parser would ordinarily recognize as markup. In order to prevent this, a CDATA section can be used. 12

13 <![CDATA[ *p = &q; b = (i <= 3); ]]> Between the start of the section, <![CDATA[ and the end of the section, ]]>, all character data is passed directly to the application. Document Type Declarations A lot of the XML specification deals with various sorts of declarations that are allowed in XML. These declarations are the same as SGML DTDs (Document Type Definitions). One of the greatest strengths of XML is that it allows you to create your own tag names. But for any given application, it is probably not meaningful for tags to occur in a completely arbitrary order. An XML document may be syntactically correct but meaningless.if the document is to have meaning, there must be some constraint on the sequence and nesting of tags. There are four kinds of declarations in XML: element declarations, attribute list declarations, entity declarations, and notation declarations. 13

14 Element Declarations Element declarations identify the names of elements and the nature of their content. A typical element declaration looks like this: <!ELEMENT oldjoke (burns+, allen, applause?)> This declaration identifies the element named oldjoke. Its content model follows the element name. The content model defines what an element may contain. An oldjoke must contain burns and allen and may contain applause. The commas between element names indicate that they must occur in succession. The plus after burns indicates that it occurs at least once. The question mark after applause indicates that it is optional. A name with no punctuation, must occur exactly once. Declarations for burns, allen, applause and all other elements used in any content model must also be present for an XML processor to check the validity of a document. <!ELEMENT burns (#PCDATA quote)*> In addition to element names, the special symbol #PCDATA is reserved to indicate character data (parseable character data). 14

15 <!ELEMENT oldjoke(burns+, allen,applause?)> <!ELEMENT burns (#PCDATA quote)*> <!ELEMENT allen (#PCDATA quote)*> <!ELEMENT quote (#PCDATA)*> <!ELEMENT applause empty> Attribute Declarations Attribute declarations identify which elements may have attributes, what attributes they may have, what values the attributes may hold, and what default value each attribute has. A typical attribute declaration looks like this: <!ATTLIST oldjoke name ID #required label CDATA #implied status ( funny notfunny ) 'funny'> In this example, the oldjoke element has three attributes: name, which is an ID and is required; label, which is a string (character data) and is not required; and status, which must be either funny or notfunny and defaults to funny if not specified. Each attribute in a declaration has three parts: a name, a type, and a default value. Names cannot be repeated on the same element. 15

16 Entity Declarations Entity declarations allow you to associate a name with some other fragment of the document. That construct can be a chunk of regular text, a chunk of the document type declaration, or a reference to an external file containing either text or binary data. <!ENTITY ATI "ArborText, Inc."> <!ENTITY boilerplate SYSTEM "/legalnotice.xml"> <!ENTITY ATIlogo SYSTEM "/logo.gif" NDATA GIF87A> There are three kinds of entities: Internal Entities - the replacement text is stored in the declaration. Using &ATI; anywhere in the document insert ìarbortext, Inc.î at that location. Internal entities allow you to define shortcuts for frequently typed text or text that is expected to change. The XML specification predefines five internal entities (e.g. & for an ampersand). External entities - allow an XML document to refer to an external file. External entities contain either text or binary data. If they contain text, the content of the external file is inserted at the point of reference and parsed as part of the referring document. Binary data is not parsed and may only 16

17 be referenced in an attribute. Binary data is used to reference figures and other non-xml content in the document. Parameter Entities Parameter entities can only occur in the document type declaration. A parameter entity is identified by placing ``%'' (percent-space) in front of its name in the declaration. The percent sign is also used in references to parameter entities, instead of the ampersand. Parameter entity references are immediately expanded in the document type declaration and their replacement text is part of the declaration, whereas normal entity references are not expanded. Including a Document Type Declaration If present, the document type declaration must be the first thing in the document after optional processing instructions and comments. The document type declaration identifies the root element of the document and may contain additional declarations. All XML documents must have a single root element that contains all of the content of the document. Additional declarations may come from an external definition (a DTD) or be included directly in the document, or both. 17

18 <?XML version="1.0"?> <!DOCTYPE chapter SYSTEM "dbook.dtd"> <chapter>...</chapter> } This example references an external DTD, dbook.dtd. In order to determine if a document is valid, the XML processor must read the entire document type declaration (both internal and external). But for some applications, validity may not be required, and it may be sufficient for the processor to read only the internal declaration. Validity There are two classes of XML document, well-formed and valid. Well Formed In short it obeys the syntax of XML. There are other restrictions including No attribute may appear more than once on the same starttag. Non-empty tags must be properly nested. Parameter entities must be declared before they are used. 18

19 Valid Documents A well-formed document is valid only if it contains a proper document type declaration and if the document obeys the constraints of that declaration (element sequence and nesting is valid, required attributes are provided, attribute values are of the correct type, etc.). 19

A tutorial report for SENG Agent Based Software Engineering. Course Instructor: Dr. Behrouz H. Far. XML Tutorial.

A tutorial report for SENG Agent Based Software Engineering. Course Instructor: Dr. Behrouz H. Far. XML Tutorial. A tutorial report for SENG 609.22 Agent Based Software Engineering Course Instructor: Dr. Behrouz H. Far XML Tutorial Yanan Zhang Department of Electrical and Computer Engineering University of Calgary