Basics of Web Based Computing

The Architecture

The user’s system

A Web Server

What’s inside?

Server softwareApache or other

Resources to be accessible by means of the Web

Typical structure

Server is initiated at startup

File space is dividedSome default location for browser access

• Index.html (or index.htm) file in the root directory accessed through the server

Default location in the areas assigned to users• www.csc.villanova.edu/~yourid/

• Points to the html directory in your filespace

The protocolsHTTP

Hyper text transport protocolSpecifies the way the information is requested and providedIncludes invisible compression in some cases

Other protocolsftp://[user-id[:password]@]machine/path/filefile:[//machine]/path/fileServer may be configured to accept or not accept protocols other than http

The LanguageFiles accessible through the Web are retrieved by using http and are displayed. The software that displays the file contents depends on the format of the file data.Browsers can display information of several formats.

Browser may contain code to interpret and display the data Browser may invoke another program to display the data.

The lingua franca of the Web is HTML, now XHTML.

XHTML(X)HTML is a markup language

Information about the information (meta information) is imbedded in the documentHow the information is processed depends on the definition of the meta information and the program that processes it.HTML was designed to describe how a document should be displayed.

• Size and boldness of headers• Use of italics, underline, and bold in text• Spacing in tables and charts

HTML was also designed to allow parts of a document to be linked to other parts and to other documents

Other display requirements

Some things are hard to display using HTML and standard browsers

MusicMathematicsProtein structure, etc.

Other systems were needed to define ways to display these types of information, and others.

SGML and XMLSGML (Standard Generalized Markup Language) is a language for defining languages

HTML is an example languageSGML turned out to be burdensome

XML (eXtensible Markup Language) is derived from SGML

XHTML is HTML recast as an XML language

Both allow communities to define new markup languages appropriate to their needs

A quick introduction to XML(from http://www.w3c.org/XML/1999/XML-in-10-points)This material is slightly shortened and rephrased, but essentially from this site.

XML is a set of rules for designing text formats that allow you to structure your data

Extensible, platform independent, international support (unicode-compliant)

Uses tags that look like HTMLBrackets < > and Attributes <href=“…”>

HTML defines how the data will look; XML tags only delimit data. How the data will be used or presented is left up to the program that processes it.

XML continued

XML files are textHumans can read them, but that is not the idea

It is easy to write new programs to process the data, because the format of the file is not proprietary

XML 1.0 is the specification that defines what “tags” and “attributes” are. Beyond XML 1.0, “the XML family” is a growing set of modules that offer useful services to accomplish important and frequently demanded tasks. (quote)

XML and XHTML

XHTML is an XML application.

XHTML has slightly different format from HTML, to conform to XML rules.

XHTML determines what the tags mean and how they should be rendered.

The Semantic Web

Successor to the World Wide WebMore coordination of meaningThe W3C Resource Description Format (RDF) “integrates applications and agents into one Semantic Web”Please see the web page cited and additional material at w3c.org for more complete information.

Separating meaning from description

XML allows a community (or an individual, but that would be unusual) to define a set of tags to be imbedded in a collection of information.

A separate DTD (document type definition)

Example

Address book DTD<!– Example DTD addressbook.dtd--><!ELEMENT addressentry+ (name, address, otherinfo)>

<!ELEMENT name (lastname, firstname, nickname*)>

<!ELEMENT address ( street+ city, state, zip)>

<!ELEMENT otherinfo ( e-mail* )>

(+ = one or more, * = any number, ? = at most once)

The XML file An XML file to correspond to the addressentry dtd

<?xml version=“1.0”?><!– comments --><!DOCTYPE addressbook SYSTEM “addressbook.dtd”><addressentry><name>

<lastname> Ridley </lastname><firstname> Anna </firstname><nickname> Anne </nickname><nickname> Grandmom</nickname\>

</name><address>

<street>123 A Street</street><street> Apartment 3A </street><city> Willington </city><state> DE </state><zip> 19804 </zip>

</address></addressentry>

Summary

We have seen a brief overview of the architecture of the Web and an introduction to XML and DTD.

We will continue with discussion of forms and form processing