Hello INTRODUCTION TO XML. Neil Maycock. Pro-Bel Limited, UK ABSTRACT

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1 INTRODUCTION TO XML Neil Maycock Pro-Bel Limited, UK ABSTRACT As the broadcasting world becomes ever more digital, computers systems are becoming more prevalent in many forms. This is bringing new challenges to the broadcaster in the transfer of both control data and media metadata between computer systems, this raises the issue of system interoperability. The core of many proposed interoperability solutions is XML, but many of us know little about this technology or how it can help solve interoperability problems. This paper introduces XML from first principals and is aimed at giving a basic understanding for those of us who aren t computer programmers. Through examples it illustrates how XML can solve data transfer and communication problems. INTRODUCTION Interoperability for the purpose of this paper can be defined as, the ability of software and hardware on multiple machines from multiple vendors to communicate. As television converges with the computer world any broadcast or video facility will needs systems to communicate and share data. A lack of interoperability will almost always equate to some form of cost, either at the initial integration of systems or at some point in the future when upgrades are performed. To illustrate the problem of interoperability we shall look at an example completely divorced from the world of television, the Acme Cake Company who sells cakes with customised messages. Figure 1 shows Acme s manufacturing system; it consists of two parts, a robot for icing the cake, and a computer on which the icing message is designed. Hello Figure 1 Acme Cake Icing System

2 The communication between the design computer and the robot follows a traditional communication approach where messages are sent in a format, or protocol, understood by both the sending end and the receiving end. The following example message is an ASCII string for ease of understanding, but the same principal would apply if the message were numerically coded as a binary protocol. Message format: Start of message, Type of cake, Text, End of Message Example message: SOM,Chocolate,Happy Birthday,EOM In our example the Acme s business is successful and they wish to expand production by purchasing a second robot to be driven from the same design computer. Since their system was first installed the robot manufacturer has upgraded the icing robots to support different coloured icing, and the new robots now expect additional information from the design computer. New format: Start of message, Type of cake, Colour of icing, Text, End of message Clearly this is incompatible with the existing design computer and robot, so Acme now has to face an additional cost of upgrading the design computer to support two different message formats or upgrading the original robot to the new message format, assuming that either is even possible. Whilst Acme s problem may seem a little contrived it is a simple illustration of the real problem of interoperability. More real questions that might face a broadcaster could be, Why can t I transfer media from system A to system B? Why can t I control my video server after upgrading my automation system? WHAT IS XML? XML stands for extensible Mark-up Language, so perhaps the first question we should answer is what is a mark-up language? Mark-up languages date back over thirty years when they were first designed to be used in the print world to allow document formatting to be described in an open and portable way. The following dates track the evolution from the early print application through to the HTML which was one of the primary enabling technologies for the World Wide Web, the dates are taken from A Short History of XML (1) GML Generalised Mark-up Language 1985 SGML Standard Generalised Mark-up Language 1991 HTML Hypertext Mark-up Language 1998 XML Extensible Mark-up Language XML was developed as a technology for the internet but has found widespread applications in many areas; a concise summary of how XML came about is given in Happy Birthday XML (2). Essentially the objective was to define a light weight standard that would enable applications to share data across the Internet. In a very short time XML has grown way beyond these early ambitions and has spawned a whole generation of associated technologies.

3 WHAT DOES XML LOOK LIKE? For those of you that have seen the HTML code behind a web page XML may appear very familiar, and whilst following some similar conventions it is actually doing a quite different job. Both languages contain data and descriptive elements around the data, but HTML describes how to display data whereas XML described the data itself. The code segment in Figure 2 is an example piece of XML that describes an entry for a contact database. N.B. Throughout this paper XML examples have been formatted (indented and bold text) for legibility, but XML does not require any such formatting. <contact_info> <name>john Smith</name> <phone> </phone> <road>1 Acacia Avenue</road> <town>reading</town> <postcode>rg6 AB1</postcode> </contact_info> Figure 2 Example XML Code Segment Elements XML describes data in Elements, each element is defined by a pair of Tags in angle brackets. In Figure 2 the data John Smith is defined in an element called name the start of the element is defined by <name> and the end of the element is defined by </name>. The first advantage we gain with XML is that by identifying each piece of data separately the order in which data is sent is no longer critical. Over and above this the presence or absence of data can be handled, this is where the extensible nature of XML exists. XML implementations will ignore unknown elements, this contrasts with traditional communications where typically unexpected data would render a system unable to process an entire message. Also missing elements are handled by XML implementations as default values can be substituted and the remainder of the message is not misinterpreted. There are a number of basic rules in XML for the definition and usage of elements: Elements always must have an open tag and a corresponding close tag. A XML document that does not close an element is incorrect. Elements must be organised in a hierarchy with one primary element in an XML document or message. Elements can not overlap Element names are case sensitive and must not contain spaces Table 1 shows some correct and incorrect examples usage of XML elements with notes as to what is wrong with the code.

4 Correct Incorrect Error <name>john</name> <Name>John</name> Mixed Case <person> <name>john</name> <age>35</age> </person> <list> <name>john</name> <name>fred</name> </list> <person> <name>john <age>35 </name></age></person> <name>john</name> <name>fred</name> Incorrect nesting Multiple top level elements Table 1 Correct and Incorrect Element Usage Attributes An attribute allows additional information about an element s data to be defined within the element definition. Attributes are strings placed in the opening element tag. The following example shows an XML element describing a time value; the element has an attribute to describe the format of the data, <time format= HH:MM >13:45</time> With the introduction of attributes new flexibility is introduced to how we can describe data within XML, the following code arguably achieves the same net result as our as the last example without the use of attributes, <time> <hours>13</hours> <minutes>45</minutes> </time> Both of the examples are valid use of XML and the most appropriate will be determined by the end application for the data. The key thing to remember about attributes is that they should be used for describing data not carrying data. This is illustrated below with a code segment that highlights the difference between describing data and carrying data, <name gender= male >John</name> At first appearances the XML may appear to be defining a name called John and that John is a male. This interpretation of the XML would be taking the attribute as data. In actual fact reading this XML correctly it is defining a name called John and that John is a male name. Making Elements Names Unique Because an XML author is free to define element names for their application it can be important to uniquely define the element names so that conflicts do not occur between two XML implementations. Having long names that we could be reasonably sure to be unique would be unwieldy and a big overhead in writing XML. To address this problem there is a mechanism called Name Spaces. This allows elements to share a common prefix in order to make them unique,

5 typically the prefix will be an Internet URL owned by the implementer as this is certain to be unique. A name space can be specifically defined for a group of elements with a prefix as shown in the following code segment, <person xmlns:p= > <p:name>chocolate</p:name> <p:age>happy Birthday</p:age> </person> Alternatively a name space can be defined for an element and all its child elements, <person xmlns= > <name>chocolate</name> <age>happy Birthday</age> </person> HOW DOES XML AID INTEROPERABILITY? To answer this question we return to the Acme Cake Company. We will now assume the original implementation of their system had employed XML based communications between the design computer and robot, e.g. <cake> <type>chocolate</type> <text>happy Birthday</text> </cake> When we consider their expansion problem with the new robot s additional capabilities the use of XML can provide solutions in two ways: a) The new robot could be controlled by the existing design computer with no modifications. Even though the new robot can process data on icing colour the absence of an XML element describing this in a message will not cause a problem. b) If the design computer is modified to add icing colour information to its messages then the original robot will simply ignore the additional element in the XML message that it does not recognise. XML = Unambiguous? In essence one of the fundamental things XML does for us is to help remove ambiguity from communications. However, the use of XML does not guarantee a message or document will be unambiguous, consider the following, <name>john Smith</name> Verses <firstname>john</firstname> <lastname>smith</lastname> Verses <name format= first last >John</name>

6 All three code segments are valid XML, however, the second two provide additional information about the persons name. Depending on the end application the ordering of a name or the ability to access the different parts of the name maybe important. When designing XML schemas thought must be given to how the data is going to be used or accessed. OTHER XML TECHNOLOGIES DTD Document Type Definition Whilst well written XML can be self documenting to a degree it may be necessary to formally validate XML. There is more than one mechanism for defining an XML schema, we will look at the Document Type Definition. The DTD is a series of statements that describe the format of the elements within an XML document, it can be embedded within the XML or saved as a separate file. The DTD defines the element names, attributes, hierarchy, and type of data held in the element. The following code segment shows a simple XML document including a DTD, <!DOCTYPE cakedoc [ <!ELEMENT cake (type,colour,text)> <!ELEMENT type (#PCDATA)> <!ELEMENT colour (#PCDATA)> <!ELEMENT text (#PCDATA)> ]> <cake> <type>choc</type> <colour>red</colour> <text>happy Birthday</text> </cake> The details of DTD syntax and usage are beyond the scope of this paper, but the fundamental principal of defining XML elements has been illustrated. Microsoft tm Internet Explorer tm Whilst not directly an XML technology Microsoft s Internet Explorer provides the XML novice with a readily available means to check the syntax of their XML code. Simply save the XML as a plain text file with the extension.xml and then open the file with Internet explorer. If the code is correct Internet Explorer will show a formatted display of the XML, should there be any errors Internet Explorer will highlight them. There are many other products available for advanced XML authoring and validation. XSL Extensible Stylesheet Language XSL is a language defining a suite of functionality for processing XML. It provides formatting functionality for displaying XML, and processing functionality such as sorting XML data. For example by adding XSL script to a XML document the document can be formatted into a human readable form with text formatting such as font and colour definition. A more detailed description of XSL is beyond the scope of this paper.

7 XML IN TELEVISION Many manufacturers are adopting XML to provide more open systems, and there are a number of standardisation initiatives based on XML. One widely adopted XML based technology in broadcast is MOS, the Media Object Server Protocol (3). MOS has been adopted by a number of manufacturers to implement integration between news room, automation, and other associated systems. XML is used to pass media information and play-lists between applications in a system. For example, many applications my receive the XML describing a play-list, but only understand segments of the play-list that they can enact. By basing the play-list on XML these applications can extract the segments they needs regardless of what other information is contained in the list. Within Pro-Bel one recent application of XML has been with our Procion tm PC control product range (4). Here XML is used to store user configuration data, such as screen layouts. The purpose of using XML is to ensure that configuration created by a user will be compatible across different generations of Procion software. As new functionality is added to the software this will add new elements to the XML configuration file, however, old files will still be readable by new software, and conversely new files will be readable by old software. CONCLUSIONS Within this document we have only just touched on a huge subject area, however, by covering the basic principals of XML the reader should have gained an understanding of how XML can be used to facilitate communication between devices and therefore aid system interoperability. True interoperability relies on many things including the efforts of standardising bodies and other organisations. XML does not achieve interoperability but is merely a tool that can help. REFERENCES 1. A Short /History of XML 2. Hollander D, and Sperberg-McQueen C. M., Happy birthday XML, 3. MOS, Media Object Server Protocol, 4. Procion, PC Control Software, ACKNOWLEDGEMENTS provides an excellent set of online tutorials on XML and other web technologies, and has been a valuable resource in writing this paper.

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Structured documents An overview of XML Structured documents Michael Houghton 15/11/2000 Unstructured documents Broadly speaking, text and multimedia document formats can be structured or unstructured.