The Client-Server Model for the Web

1
The Client-Server Model for the Web

1. A Web Client (usually in the form of a web
   browser) makes an HTTP request to a specific web
   server.
2. The Web Server receives the request and sends
   back the requested document (usually in the form
   of an HTML page).
3. The web client interprets the information
   returned by the server and displays it
   appropriately.

2
Serving Web Pages

• Have a computer to act as a server to send
  pages along when requested.
• Connect your computer on a network (e.g. the
  Internet or a local network such as a university)
• Install server software that can respond to
  requests for documents by serving them to the
  requesting computer.
• Create and upload the appropriate documents that
  you wish to make available on the web.

3
The Server

• Have a computer to act as a server to send
  pages along when requested
• A server can be any computer capable of
  connecting to the internet and running
  server-software.
• The busier the site, the more powerful the
  computer. However the server can be as simple as
  your Dell desktop, or as complex as a rack of
  high-end dedicated computers.
• Busy sites (e.g. Google, Microsoft) will have
  multiple redundant computers to serve their web
  pages.

4
Serving web pages Connecting to the Internet

• Most computers today are equipped with network
  cards that allow them to connect to a workplace
  network (e.g. school, business).
• These networks typically also connect the
  computer to the internet.

5
Serving web pages Server Software

• A computer program called a web server listens
  (constantly) for HTTP requests and responds by
  sending back HTTP responses.
• An example of an http request is
  http//www.nytimes.com/index.html
• An example of an http response is (usually) the
  web page document that is sent back to your
  computer
• Several software companies publish web server
  software
• Some of the better known server software programs
  include
• Apache HTTP Server
• Microsoft IIS (Internet Information Services)
• Googles GWS (Google Web Server)

6
Serving web pages Creating and uploading
information

• Good part of what we will do in the rest of this
  course.

7
Web clients

• Software that is capable of sending out HTTP
  requests and interpreting HTTP responses.
• Recall that HTTP responses are usually (but not
  always) HTML documents.
• The majority of this course will be about
  creating HTML documents.
• The most popular web-clients
• Internet Explorer (published by Microsoft)
• Firefox (published by Mozilla Corp.)
• Safari (published by Apple)
• Chrome (published by Google)

8
HTML

• HTML HyperText Markup Language
• HyperText
• text containing navigable links to other texts
• A Markup Language
• a method of adding information to the text
  indicating the logical components of a document
  and instructions for layout of the text on the
  page, which can be interpreted by some automatic
  system
• You can see the HTML of most web pages by
  selecting View ? Source on your web browser.

9
HTML The Language of the Web

• Web pages are text files written in HTML.
• HTML is easy to write and even nonprogrammers can
  learn to use it.
• HTML allows web page authors to create documents
  that can be displayed across different operating
  systems.
• HTML describes the format of web pages through
  the use of tags.
• Web browsers read the HTML file, interpret the
  HTML tags, and render (display) the web page
  accordingly
• Ideally, all browsers should display pages
  exactly the same. However, different web
  browsers frequently display pages differently!

10
Creating Web pages

• In order to create your own web pages in this
  course you need
• A text editor (not a web design application like
  FrontPage or Dreamweaver)
• (e.g. PSPad, NotePad, WordPad, Textpad)
• An web browser for testing the pages(e.g.
  Firefox, Internet Explorer, Safari, Opera, Chrome
  or test with ALL of them!)
• A web server to store your files and make them
  available online (students.depaul.edu)

11
Basic process for creating web pages

• Write HTML file
• Text file (i.e. do not write in Word)
• Assign a file extension (.html or .htm)
• View on your local machine (File ? Open)
• Upload to server
• All web pages must be inside a directory called
  public_html (you may have to create this folder)
• You may need to set permissions on folders and
  files (brief tutorial on Unix permissions here)
• Images must also be uploaded
• All images should be in either .gif or .jpg
  formats
• As with html files, you may need to set
  permissions on image files

12
File extensions

• HTML files must be saved with a .html or .htm
  extension.
• HTML editors like PSPad do this automatically
  (when you click on File ? Save As and choose
  Save as HTML)
• You must write the extension manually when using
  a text editor like Notepad, i.e., you must
  include the .htm extension when saving the file.
• To make sure your files have the right extension,
  make the file extensions visible in Windows
  Explorer (a.k.a. My Documents) by selecting
  Tools ?Folder Options ? View tab and unchecking
  the Hide extensions for known file types box.