Fundamentals of Web Applications Technology

Background for Contact Hour 2: To be discussed on Tuesday 31st January, 2012.

Lecturer: Dr Chris P. Jobling.

Setting the scene for the EG-259 Web Applications Technology module.

Fundamentals of Web Applications Technology

Setting the scene for the EG-259 Web Applications Technology module.

The slides used in this lecture are based on:

Chapter 1 of Robert W. Sebasta, Programming the World-Wide Web, 3rd Edition, Addison Wesley, 2006.
Chapter 2 of James F. Kurose and Keith W. Ross, Computer Networking: A Top-down Approach Featuring the Internet, Addison-Wesley, 2005.
Additional material from Jennifer Niederst Robbins, Web Design in a Nutshell, 3rd Ed. O'Reilly, 2006.

Contents of this Lecture

Learning Outcomes for this Lecture (1)

At the end of this lecture you should be able to answer this selection of lecture review questions:

What protocol is used by all computer connections to the Internet?
What is the task of a domain name server?
In what common situation is the document returned by a Web server created after the request is received?
What is meant by the terms document root, server root, virtual host when applied to a web server?

Learning Outcomes for this Lecture (2)

At the end of this lecture you should be able to answer this selection of lecture review questions:

What is the purpose of a MIME type specification in a request/response transaction between a browser and a server?
Prior to HTTP 1.1, how long were connections between browsers and servers normally maintained?
What is the purpose of the Common Gateway Interface?
Where is the code for JavaScript, Java Applet, Java Servlet, Perl CGI Script, and PHP script interpreted?

History of the Internet (Video)

<html> <object width=“400” height=“225”><param name=“allowfullscreen” value=“true” /><param name=“allowscriptaccess” value=“always” /><param name=“movie” value=“http://vimeo.com/moogaloop.swf?clip_id=2696386&server=vimeo.com&show_title=1&show_byline=1&show_portrait=0&color=&fullscreen=1” /><embed src=“http://vimeo.com/moogaloop.swf?clip_id=2696386&server=vimeo.com&show_title=1&show_byline=1&show_portrait=0&color=&fullscreen=1” type=“application/x-shockwave-flash” allowfullscreen=“true” allowscriptaccess=“always” width=“400” height=“225”></embed></object><p><a href=“http://vimeo.com/2696386”>History of the Internet</a> from <a href=“http://vimeo.com/picol”>PICOL</a> on <a href=“http://vimeo.com”>Vimeo</a>.</p> </html>

“History of the Internet” is an animated documentary explaining the inventions from time-sharing to filesharing, from Arpanet to Internet. The history is told using the PICOL icons on picol.org. You can already download a pre-release of all picol icons on blog.picol.org/downloads/icons/.

You can see the credits and more information on this movie on lonja.de/motion/mo_history_internet.html.

Origins of the Internet

ARPAnet – late 1960s and early 1970s
BITNET, CS net – late 1970s & early 1980s
NSF net – 1986
NSF net eventually became known as the Internet

Notes

ARPAnet:
- Network reliability
- For ARPA-funded research organizations
BITNET, CS net:
- email and file transfer for other institutions
NSF net:
- Originally for non-DOD funded places
- Initially connected five supercomputer centers
- By 1990, it had replaced ARPAnet for non-military uses
- Soon became the network for all (by the early 1990s)

What is the Internet?

A world-wide network of computer networks
At the lowest level, since 1982, all connections use the Internet Protocol (IP)
IP hides the differences among devices connected to the Internet

Applications and Application-Layer Protocols

Application: communicating, distributed processes
- e.g., e-mail, Web, P2P file sharing, instant messaging
- running in end systems (hosts)
- exchange messages to implement application
Application-layer protocols
- one piece of an application
- define messages exchanged by applications and actions taken
- use communication services provided by transport layer protocols (TCP, UDP)

Network applications: some jargon

Process: program running within a host.
- within same host, two processes communicate using interprocess communication (defined by operating system).
- processes running in different hosts communicate with an application-layer protocol
User agent: interfaces with user “above” and network “below”.
- implements user interface & application-level protocol
  - Web: browser
  - E-mail: mail reader
  - streaming audio/video: media player

Application-layer protocol defines

Types of messages exchanged, e.g., request and response messages
Syntax of message types: what fields in messages and how fields are delineated
Semantics of the fields, ie, meaning of information in fields
Rules for when and how processes send & respond to messages
Public-domain protocols:
- defined in RFCs
- allows for interoperability
- e.g., HTTP, SMTP
Proprietary protocols:
- e.g., KaZaA

Client-Server Paradigm

Typical network application has two pieces client and server

Client:
- initiates contact with server (“speaks first”)
- typically requests service from server,
- Web: client implemented in browser

Server:
- provides requested service to client
- e.g., Web server sends requested Web page

Processes communicating across network

Process sends/receives messages to/from its socket

Socket analogous to door
- sending process pushes message out of the door
- sending process assumes transport infrastructure on other side of door which brings message to socket at receiving process

Addressing Processes

For a process to receive messages, it must have a globally unique identifier
Every node has a unique 32-bit IP address
Every process running on the host has a unique port number
Process identifier includes both the IP address and port number associated with the process on the host

—- Notes

Organizations are assigned groups of IP addresses for their computers
The new standard, IPv6, uses 128 bits (1998) for host addresses.

Domain names

Form: host-name.domain-names

First domain is the smallest; last is the largest
Last domain specifies the type of organization (in USA) or geographical region (rest of the world)

Fully qualified domain name

The host name and all of the domain names

DNS servers

Convert fully qualified domain names to IP addresses

The last domain is controlled by naming authorities associated with the top-level ISPs.
- Examples are com, org, edu, gov – assigned (mostly to) US commercial institutions, not-for-profit organizations, educational institutions, and government institutions respectively;
- Examples of Geographical domains are uk, fr, ie for countries in Europe, and us, za, ca for other locations.
The naming authorities assign lower level domain names to ISPs or to large institutions.
- Examples of ISP domains are ac.uk (the UK Joint Academic Network), ntlworld.com (a UK cable broadband supplier), blogspot.com (Google's blog hoster).
- Examples of institutional domains are swan.ac.uk (Swansea University), microsoft.com, swansea.gov.uk (City and County of Swansea).
Within an institutional domain, the ISP or naming authority will usually assign a group of IP addresses that can be freely used. The institution acts as its own naming authority and is free to assign host names has it wishes (in fact several host names may be assigned to a single IP address)
An institutional Domain Naming Service (DNS) server (called the Authoritative Name Server) is used to map local host names to IP addresses.

Transport Services

What transport service does an application need?

Data loss
Timing
Bandwidth

Data loss:
- some applications (e.g., audio) can tolerate some loss
- other applications (e.g., file transfer, telnet) require 100% reliable data transfer
Timing:
- some applications (e.g., Internet telephony, interactive games) require low delay to be “effective”
Bandwidth:
- some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective”
- other apps (“elastic apps”) make use of whatever bandwidth they get

Internet transport protocols services: TCP

Connection-oriented: setup required between client and server processes
Reliable transport between sending and receiving process
Flow control: sender won't overwhelm receiver
Congestion control: throttle sender when network overloaded
Does not provide: timing, minimum bandwidth guarantees
Web Uses TCP

Internet transport protocols services: UDP

Unreliable data transfer between sending and receiving process
Does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee
DNS uses UDP

Origins of the Web

Problem
- By the mid-1980s, several different protocols had been invented and were being used on the Internet, all with different user interfaces (Telnet, FTP, Usenet, email, Gopher)
Possible Solution
- The World-Wide Web is a possible solution to the proliferation of different protocols being used on the Internet.

The World Wide Web

Tim Berners-Lee at CERN proposed the Web in 1989
Document form: hypertext
Objects? Pages? Documents? Resources?
Hypermedia—more than just text—images, sound, etc.
Web or Internet?

Notes

The original purpose of the world-wide web was to allow scientists to have access to many databases of scientific work through their own computers.

Objects? Pages? Documents? Resources? We'll call them documents.

Web or Internet? The Web uses the application protocol, HTTP, that runs on the Internet – there are many others (telnet, ftp, email, etc.)

Web Browsers

Mosaic – NCSA (Univ. of Illinois), in early 1993
Browsers are clients – always initiate, servers react (although sometimes servers require responses)
Most requests are for existing documents, using HyperText Transfer Protocol (HTTP)
But some requests are for program execution, with the output being returned as a document

Notes

Mosaic was the second browser to use a GUI (the first, a graphical browser that Time Berners-Lee developed at Cern, did not have a wide distribution) and led to explosion of Web use. Initially for X-Windows, under UNIX, but was ported to other platforms by late 1993

Web Servers

Provide responses to browser requests, either existing documents or dynamically built documents
Browser-server connection is now maintained through more than one request-response cycle
All communications between browsers and servers use Hypertext Transfer Protocol (HTTP)
Web servers run as background processes in the operating system

Notes

Web servers monitor a communications port on the host, accepting HTTP messages when they appear
All current Web servers came from either
1. The original from CERN
2. The second one, from NCSA

Web Server Configuration

Web servers have two main directories conventionally called the Document Root and the Server Root
Document root is accessed indirectly by clients
Virtual document trees
Virtual hosts
Proxy servers
Web servers now support other Internet protocols

Notes

Server root contains the server system software
Document root contains the servable documents
- Its actual location is set by the server configuration file
- Requests are mapped to the actual location
Virtual documents trees are “aliases” for part of the document tree that is not actually located in the document root.
A virtual host is a web server which appears to have a different host name from the primary host. Such a host will have its own document root. DNS is used to map the virtual host name to the IP address of the actual web server. The web server will then recognize the virtual host name from the HTTP request and serve documents from the right place.
- Such a facility is very useful if you are hosting many web sites as each virtual host can be administered separately from the main web server's configuration.
Proxy web servers are web servers which can serve documents which are in the document root of another web server. They are often used to reduce the traffic from an institution to and from the Internet. For example, see UWS Proxy Server.
Web servers now support other Internet protocols:
- For example ftp, Gopher, News and mail. Nearly all web servers can interact with database systems through the Common Gateway Interface (CGI) programs and server-side scripts.

Modern Web Servers

You are Invited to Update this Page by providing a link to the January 2012 results and updating the tables

Market Share for Top Servers Across All Domains November 1995 - January 2011

January 2011 Data

Data from Netcraft Server Share Statistics (Netcraft Survey January 2011)

Developer	December 2010	Percent	January 2011	Percent	Change	Last Year (August 2009)
Apache	151,516,152	59.35%	161,591,445	59.13%	-0.23	46.30%
Microsoft	56,723,544	22.22%	57,392,351	21.00%	-1.22	21.94%
nginx	16,910,205	6.62%	20,504,634	7.50%	0.88	5.09%
Google	14,933,865	5.85%	15,112,532	5.53%	-0.32	6.29%
lighttpd	1,308,935	0.51%	1,866,872	0.68%	0.17	0.90%

September 2009 Data

Data from Netcraft Server Share Statistics (Netcraft Survey August 2009)

Developer	July 2009	Percent	August 2009	Percent	Change	Last Year
Apache	113,019,868	47.17%	104,611,555	46.30%	-0.87	49.82%
Microsoft	55,918,254	23.34%	49,579,507	21.94%	-1.39	34.88%
qq.com	30,447,369	12.71%	30,278,988	13.40%	0.69	–
Google	14,226,904	5.94%	14,213,976	6.29%	0.35	5.94%
nginx	10,174,573	4.25%	11,502,109	5.09%	0.84	–
lighttpd	2,942,469	0.55%	2,025,521	0.90%	0.34	1.65%

September 2008 Data

Data from Netcraft Server Share Statistics (Netcraft Survey August 2008)

Developer	July 2008	Percent	August 2008	Percent	Change	Last Year
Apache	86,845,154	49.49%	88,047,801	49.82%	0.33	50.48%
Microsoft	62,411,537	35.57%	61,646,837	34.88%	-0.69	34.94%
Google	10,001,763	5.70%	10,502,299	5.94%	0.24	4.90%
lighttpd	2,942,469	1.68%	2,914,867	1.65%	-0.03	1.12%

September 2007 Data

Data from Netcraft Server Share Statistics (September 2007)

Developer	August 2007	Percent	September 2007	Percent	Change	Last Year
Apache	65,153,417	50.96%	68,228,561	50.48%	-0.49	65.52%
Microsoft	43,861,854	34.31%	47,232,300	34.94%	0.63	30.13%
Google	5,702,456	4.46%	6,616,713	4.90%	0.43	-
Sun	2,195,495	1.72%	2,212,821	1.64%	-0.08	0.37%
lighttpd	1,500,126	1.17%	1,515,963	1.12%	-0.05	-

A lot of the change would appear to be due to the growth of blogging and community web sites such as Microsoft Live Spaces. Google used to host the Blogger service on Apache web servers, it now uses its own server. Live Spaces is hosted on the Microsoft's IIS. Google and Microsoft are also busy competing in the ISP space which is fueling a general growth in the number of web sites.

September 2006 Data

Data from Server Share Statistics (September 2006)

Server	August 2006	Percent	September 2006	Percent	Change	2005
Apache	57,906,817	62.52	59,699,872	61.64	-0.88	69.15%
Microsoft	27,905,439	30.13	30,272,249	31.26	-1.13	20.36%
Zeus	521,619	0.56	515,670	0.53	-0.03	0.82%
Sun	344,862	0.37	345,834	0.36	-0.01	2.61%

Some Important Web Servers

Apache (more later)
Microsoft Internet Information Server (IIS)
Apache Tomcat
lighttpd

Notes

Microsoft Internet Information Server (IIS)
- Standard on windows professional
- Windows only
- Standard web services (files and CGI)
- ASP technology provides server scripting in multiple languages
- FrontPage server extensions
- Key component of .NET
- Operation is maintained through a program with a graphical user interface (GUI)
Apache Tomcat
- A web server written in Java (runs on any platform that supports Java)
- Standard web services (files and CGI)
- A “servlet container” which uses Java as a web application programming language and Java Server Pages (JSP) for interactivity.
- Often runs behind (i.e. is fronted by) an Apache or IIS web server, so rarely appears in server statistics.
- Key component of Java Enterprise Edition (Java EE).
Lighttpd (pronounced “lighty”)
- “web server designed to be secure, fast, standards-compliant, and flexible while being optimized for speed-critical environments.” Wikipedia
- small memory footprint = low server impact
- can efficiently handle lots of connections
- full featured
- used for sites popular sites like YouTube and Wikipedia
- Open source: relased under BSD license.

Introducing the Apache Web Server

Still the most popular web server in use today

First web server was built by Tim Berners-Lee at CERN
First really popular web server was developed by NCSA and was available to all.
Apache was originally developed to fix bugs in NCSA Web Server version 1.3 in 1995.
It is open source and is developed and maintained by a group of volunteers.
Runs on most common platforms.

Notes

Apache market share:
- September 2006: 62% of the market, 59.6 Million hosts.
- September 2007: 50% of the market, 68.2 Million hosts.
- August 2008: just less than 50% of the market, 86.9 Million hosts.
- August 2009: around 46% of the market, 113 Million hosts.

The Apache Web Server

Official name httpd (HTTP daemon)
Open source, fast, reliable. Latest version 2.2.
Directives (operation control): ServerName, ServerRoot, ServerAdmin, DocumentRoot, Alias, Redirect, DirectoryIndex, UserDir
Available from http://httpd.apache.org.

Apache configuration is usually done by editing configuration files with a text editor

Uniform Resource Identifier (URI)

A URI is used to uniquely identify a resource on the global Internet.
General form: scheme:object-address

Scheme

The scheme is often a communications protocol, such as telnet, ftp, http and https (secure HTTP)

Object-address

For the http protocol, the object-address is: fully qualified domain name/document path
For the file protocol, only the document path is needed

URI Object Address

Qualified domain name may include a port number, as in www.swan.ac.uk:80
URIs cannot include spaces or any of a collection of other special characters (semicolons, colons, …)
The document path may be abbreviated as a partial path
If the document path ends with /, it means it is a directory

Notes

Port 80 is the default port for a web server, so in www.swan.ac.uk:80 this is redundant and is usually abbreviated to www.swan.ac.uk.
Partial paths – the rest of the path is furnished by the server configuration. Partial paths are also known as relative paths.
Directory “object”: often the / character will be mapped by the web server to a file such as index.html, index.php or Default.asp.

Multipurpose Internet Mail Extensions (MIME)

Originally developed for email
Used to specify to the browser the form of a file returned by the server (attached by the server to the beginning of the document)
Type specifications
- Form: type/subtype
Server usually gets type from the requested file names suffix (.html implies text/html)
Browser gets the type explicitly from the server
Experimental types: subtype begins with x-: e.g., video/x-msvideo

Notes

Examples of type specifications: text/plain, text/html, image/gif, image/jpeg.
Experimental types require the server to send a helper application or plug-in so the browser can deal with the file.

The Hyper Text Transfer Protocol

The protocol used by ALL web communications

HTTP is web's application layer protocol
client/server model
- client: browser that requests, receives, “displays” documents
- server: Web server sends objects in response to requests
HTTP 1.0: RFC 1945
HTTP 1.1: RFC 2068

Key Facts about HTTP

HTTP Uses TCP
HTTP is “stateless”
HTTP Supports two modes of connection
- Non-persistent HTTP
- Persistent HTTP

Details

HTTP Uses TCP:
- client initiates TCP connection (creates socket) to server, port 80;
- server accepts TCP connection from client;
- HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server);
- TCP connection closed.
HTTP is “stateless”
- server maintains no information about past client requests.
HTTP Supports two modes of connection
- Non-persistent HTTP
  - At most one object is sent over a TCP connection.
  - HTTP/1.0 uses non-persistent HTTP.
- Persistent HTTP
  - Multiple objects can be sent over single TCP connection between client and server.
  - HTTP/1.1 uses persistent connections in default mode.

Response Time Modeling

Calculation of Response Time

Definition of Round-Trip Time (RTT): time for a small data packet to travel from client to server and back.
Response time:
- one RTT to initiate TCP connection
- one RTT for HTTP request and first few bytes of HTTP response to return
- file transmission time: total = 2 RTT + transmit time
Non-persistent HTTP issues:
- requires 2 RTTs per object
- operating system must work and allocate host resources for each TCP connection
- but browsers often open parallel TCP connections to fetch referenced objects
Persistent HTTP
- server leaves connection open after sending response
- subsequent HTTP messages between same client/server are sent over connection
  - Persistent without pipelining:
    - client issues new request only when previous response has been received
    - one RTT for each referenced object
  - Persistent with pipelining:
    - default in HTTP/1.1
    - client sends requests as soon as it encounters a referenced object
    - as little as one RTT for all the referenced objects

HTTP Request Phase

Form

HTTP method domain part of URL HTTP version

  Header fields
  blank line
  Message body

An example of the first line of a request:

GET /ugcourses/ HTTP/1.1

Notes

Most commonly used request methods:

GET – Fetch a document
POST – Execute the document, using the data in body
HEAD – Fetch just the header of the document
PUT – Store a new document on the server
DELETE – Remove a document from the server

Note that although servers support all these requests, browsers only issue GET and PUT. This has implications for so-called RESTful web applications which we will explore in a later lecture.

Four categories of header fields: General, request, response and entity.

Common request fields:

Accept: text/plain

     Accept: text/*
     If-Modified-Since: date

Can communicate with HTTP without a browser:

    telnet www.swan.ac.uk http
    GET /ugcourses/ HTTP/1.1
    Host: www.swan.ac.uk<Return>
    <Return>

Linux users have access to a useful command line tool cURL that can issue any web server request from the command line and gives full programmatic access to the header fields. cURL is very useful addition to the web developer's toolbox and is available for windows via cygwin (see Partical 0). cURL supports other protocols besides HTTP.

HTTP Response Phase

Form:

  Status line
  Response header fields
  blank line
  Response body

Status line format:

  HTTP version   status code   explanation

Example:

  HTTP/1.1  200  OK

Notes

Status code is a three-digit number; first digit specifies the general status:

    1 => Informational
    2 => Success
    3 => Redirection
    4 => Client error
    5 => Server error

The header fields, Content-type, and Content-length are required:
- The Content-type field's value is a content-type specification in MIME format
- The Content-length field's value is the size of the returned document in bytes.
Common header response fields:

   Content-length: 488
   Content-type: text/html

An example of a complete response header:

   HTTP/1.1  200  OK
   Date: Tues, 18 May 2004 16:45:13 GMT
   Server: Apache (Red-Hat/Linux)
   Last-modified: Tues, 18 May 2004 16:38:38 GMT
   Etag: "841fb-4b-3d1a0179"
   Accept-ranges: bytes
   Content-length: 364
   Connection: close
   Content-type: text/html, charset=ISO-8859-1

Both request headers and response headers must be followed by a blank line

Web Programming

Concerned with three “layers” of the current web standards stack

The Structural Layer

Marked up document which is the foundation for the other layers.
Current standards are:
- HTML 4.1
- XML 1.0 (Extensible Markup Language). See XML.
- XHTML 1.0 (Extensible Hypertext Markup Language) and XHTML 1.1. See Mark Up.
- HTML5 See HTML5: A vocabulary and associated APIs for HTML and XHTML

HTML

HTML describes the general form and layout of documents
Tools for creating HTML documents
HTML editors – make document creation easier
WYSIWYG HTML editors
Plug-ins
Filters

Notes

An HTML document is a mix of content and controls
- Controls are tags and their attributes
  - Tags often delimit content and specify something about how the content should be arranged in the document
  - Attributes provide additional information about the content of a tag
HTML editors – make document creation easier by providing shortcuts to typing tag names, spell-checker,
WYSIWYG HTML editors are useful in that developers need not know HTML to create HTML documents
Plug-ins are often integrated into tools like word processors, effectively converting them to WYSIWYG HTML editors
Filters convert documents in other formats to HTML
- Advantages of both filters and plug-ins:
  - Existing documents produced with other tools can be converted to XHTML documents
  - Use a tool you already know to produce HTML
- Disadvantages of both filters and plug-ins:
  - HTML output of both is not perfect – must be fine tuned
  - HTML may be non-standard
  - You have two versions of the document, which are difficult to synchronize

XML

A meta-markup language
Used to create a new markup language for a particular purpose or area
Because the tags are designed for a specific area, they can be meaningful
No presentation details
A simple and universal way of representing data of any textual kind

The Presentation Layer

Provides instructions on how a document should look on the screen, sound when it is read aloud and look when it is printed

Current CSS standards are:
- Cascading Style Sheets (CSS) Level 1
- CSS Level 2.1
- CSS Level 3

Notes

CSS Level 1 has been a Recommendation since 1996 and is now fully supported by current browsers. Level 1 contains rules that control the display of text, margins and borders.

CSS Level 2 is best known for the addition of absolute positioning of web page elements. Level 2 reached Recommendation status in 1998, and the 2.1 revision is currently a Candidate Recommendation. Support for CSS 2.1 is inconsistent in current browsers.

CSS Level 3 builds on level 2 but is modularized to make future expansion simpler and to allow different devices to support logical subsets. This version is still in development but browsers are gradually supporting more and more of the standard, often with the use of browser-specific attributes.

The Behavioural Layer

The scripting and programming of the behavioural layer adds interactivity and dynamic affects to a site.
Client-Side
- Document Object Models
- JavaScript
Server Side
- Java
- Perl
- PHP
- Ruby on Rails

Document Object Models

Document Object Model (DOM) allows scripts and applications to access and update the content, structure and style of a document.
Achieved by formally naming each part of the document, its attributes, and how the document may be manipulated.
Originally specified incompatibly by each browser, now standardized by the W3C.
- Document Object Model (DOM) Level 1 (Core)
- DOM Level 2

Notes

Document Object Model (DOM) Level 1 (Core) covers core HTML and XML documents as well as document management and manipulation. See DOM 1.
DOM Level 2 includes a style sheet object model, making it possible to manipulate style information. See DOM 2.

Scripting in JavaScript

A client-side HTML-embedded scripting language
Only related to Java through syntax
Dynamically typed and not object-oriented
Provides a way to access elements of HTML documents and dynamically change them
Current Standard: JavaScript 1.5/ECMAScript 262

Notes

Netscape introduced its web scripting language, JavaScript, with its Navigator 2.0 browser. It was originally called “LiveScript” but was later co-branded by Sun, and “Java” was added to the name. Microsoft countered with its own JScript while supporting some level of JavaScript in its Version 3.0 browser. The need for a cross-browser standard was clear!

The W3C is developing a standardized version of JavaScript in coordination with ECMA International, an international industry association dedicated to the standardization of information and communication systems. According to the Mozilla site, Netscape's JavaScript is a superset of the ECMAScript standard scripting language, with only mild differences from the published standard. In general practice, most developers simply refer to “JavaScript” and the standard implementation is implied.

Java

General purpose object-oriented programming language
Based on C++, but simpler and safer
Client-side: Applets – compiled Java programs that are downloaded from a web server and execute in the browser
Server-side: Servlets – Java programs that execute in the server. Essentially manipulate the HTTP request and return an HTTP response. JSP template mark up allows Java code to be embedded in HTML pages (similar to Microsoft's ASP)
Sebasta's book covers both applets and servlets, but we will not have time to cover Java in this module.

Perl

Provides server-side computation for HTML documents, through CGI
Perl is good for CGI programming because:
- Direct access to operating systems functions
- Powerful character string pattern-matching operations
Access to database systems
Perl is highly platform independent, and has been ported to all common platforms
Perl is not just for CGI

Perl is useful general purpose system administrator's tool. In fact that was Larry Wall's intention, when he originally developed the language. It can be used to manage server configuration and server logs. In many ways it is the ultimate shell programming tool with the advantage that it works on systems other than Unix!

PHP

A server-side scripting language
An alternative to CGI
Similar (in programing style) to JavaScript
Great for form processing and database access through the Web

Ruby on Rails

Ruby is a scripting language
Rails is a web applications development framework written in Ruby
Rails exploits Ruby's features to make web app development as easy as possible
Supports a RESTful development style “out of the box”

Summary of this Lecture

Learning Outcomes for this Lecture (1)

At the end of this session you should be able to answer this selection of review questions:

What protocol is used by all computer connections to the Internet?
What is the task of a domain name server?
In what common situation is the document returned by a Web server created after the request is received?
What is meant by the terms document root, server root, virtual host when applied to a web server?

Learning Outcomes for this Lecture (2)

At the end of this lecture you should be able to answer this selection of lecture review questions:

What is the purpose of a MIME type specification in a request/response transaction between a browser and a server?
Prior to HTTP 1.1, how long were connections between browsers and servers normally maintained?
What is the great advantage of XML over XHTML for describing data?
What is the purpose of the Common Gateway Interface?
Where is the code for JavaScript, Java Applet, Java Servlet, Perl CGI Script, and PHP script interpreted?

After writing up your notes for this lecture, you also should be able to answer all the Review Questions. You should also try the Homework Exercises.

What's Next?

The Structural and Presentation Layers

Revision

New material

XHTML 2 and HTML 5
CSS 3
Watch the video before the session!

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Fundamentals of Web Applications Technology