Web Data Interchange Formats

Web Data Exchange

• Data exchange is the process of transforming structured data from one format to another to facilitate data sharing between programs

• A data exchange language is a language that is capable of expressing general purpose data

• We will look at two data exchange languages that are commonly used to exchange data on the web:

  • Extensible Markup Language (XML)

  • JavaScript Object Notation (JSON)

Extensible Markup Language (XML)

• XML is a markup language that uses a textual data format to represent arbitrary data structures

• Schema systems exist for XML that enable XML based languages to be defined and validated

• The file extension for XML files is .xml

• The MIME type for XML text is application/xml

XML vs HTML

• XML was designed as a data exchange format

• HMTL was designed to display data to the user

• XML tags are not predefined

• XML was designed to be extensible

Basic XML Syntax Rules

• An XML document begins with an optional declaration, for example:
  

  <?xml version="1.0" encoding="UTF-8"?>

• XML documents must have a root element that is the parent of all other elements

• XML elements must have a closing tag

• XML tags are case sensitive

• XML elements must be properly nested

• XML attributes must be quoted

XML Elements

• An XML element is a logical document component with the following basic syntax:

  <element attribute="value">content</element>

• Tags begin with < and end with >

  • start-tag: <element>

  • end-tag: </element>

  • empty-element-tag: <element />

• Attribute values must be in quoted (single or double quotes)

XML Element Naming Rules

• Element names are case sensitive

• Element names must start with a letter or an underscore

• Element names cannot start with the letters xml (in any combination of upper and lower case)

• Element names can contain letters, digits, hyphens, underscores, and periods

• Element names cannot contain spaces

XML Entities

• There are five pre-defined entity references

  • < less than (<)

  • > greater than (>)

  • & ampersand (&)

  • ' apostophe (’)

  • " quotation mark (")

XML Comments

• Comments in XML begin with <!– and end with –>

  <!-- This is a comment -->

• Two dashes – are not allowed in a comment

• Comments cannot be nested as a consequence of the previous rule

XML Namespaces

• XML elements are not predefined, so there is a chance that two different XML documents use the same element name

• XML Namespaces are a way to handle element name conflicts

• A namespace declaration has the following syntax
  xmlns:prefix="URI"

• Example:

  <p:ul xmlns:p="https://example.com/p">
    <p:li>Item 1</p:li>
    <p:li>Item 2</p:li>
  </p:list>

Accessing XML with JavaScript

• The XML DOM defines properties and methods for accessing and editing XML

• XML text data can be converted into an XML DOM object using the DOMParser object

  var parser = new DOMParser();
  x = parser.parseFromString(t, "text/xml");
  // t is variable containing an XML string

Valid XML Documents

• A well formed XML document follows the XML syntax rules

• A valid XML document must be well formed and conform to a document type definition

• There are two main schema systems that can be used with XML

  • Document Type Definition (DTD)

  • XML Schema

• The purpose of a schema is to define the structure of an XML document including:

  • The elements and attributes that can appear in a document

  • The number and order of child elements

  • The data types for elements and attributes

  • The default and fixed values for elements and attributes

JavaScript Object Notation (JSON)

• JSON is designed to be a lightweight data exchange language

• JSON is data is plain text

• The file extension for JSON files is .json

• The MIME type for JSON text is application/json

JSON Syntax

• JSON syntax is similar to the syntax of defining literal objects in JavaScript:

  • Data is in name/value pairs of the form "name":value

  • Data is separated by commas

  • Curly braces hold objects

  • Square braces hold arrays

JSON Data Types

• Number: a signed decimal number

• String: a sequence of zero or more unicode characters delimited by by double quotes

• Boolean: a value of true or false

• Array: an ordered list of zero or more values separated by commas and delimited by square brackets

• Object: an unordered collection of name/value pairs where pairs are separated by commas and delimited by curly braces

• Null: the empty value indicated by the word null

Accessing JSON with JavaScript

• JSON.parse(): convert a JSON string into a JavaScript type

• JSON.stringify(): convert a JavaScript object into JSON text

  • JavaScript Date objects are converted into strings

  • JavaScript functions are removed since they are not valid

XML vs JSON

• Similarities

  • “self describing” (human readable)

  • hierarchical (nested values)

  • can be parsed by many programming languages

  • can be fetched with XMLHttpRequest

• Differences

  • JSON does not use end tags

  • XML is more difficult to parse than JSON

  • JSON has an array type

  • JSON cannot be validated by a schema (yet)