Web programming II

Introduction to Server Side Programming

Overview

Internet and Web Client/Server Programming on the Web Server-side programming and Applications Server-Side includes.

3

Origins of Internet• US Department of Defense (DoD) wanted a new large-scale

computer network in 1960s.

• Purpose : Communications, file sharing, email and remote computer access for researchers working on defense related contracts. Network should be robust.

• ARPAnet not available for educational institutions.

• BITNET and CSNET developed for email facilities. NSFnet was created in 1986 connected five supercomputer centres at five universities. Available to all academicians & institutions. THUS INTERNET EVOLVED.

4

Internet• Huge collection of DIVERSE DEVICES connected in a

communications network.

• Single low-level protocol TCP/IP. It allows all devices connected to the Internet to appear exactly the same.

• Instead of connected every device to Internet, create a local network in the organization and connect one of the node’s to Internet. Thus Internet is network of networks rather than a network of computers.

• BUT HOW EACH DEVICE SHOULD BE UNIQUELY IDENTIFIED ON THE INTERNET ????

5

Internet Protocol Addresses

Internet nodes for People identified by names and for Computers identified by numeric

addresses.

IP addresses(32-bits) are four 8-bit digits separated by dot operator.

IPv6 expanded to 128 bits.

6

Domain Names

• Difficult for people to remember numbers of machines on the Internet so textual names used instead.

• Begins with host machine followed by progressively larger enclosing collection of machines called domains. Called as FULLY QUALIFIED DOMAIN NAME.

• NAME SERVERS : collection of machines on the Internet for converting fully qualified domain name to an IP address.

7

Domain Name Service (DNS)

8

Protocols that run on top of TCP/IP

• To support a variety of Internet uses. TELNET : to allow a user on one computer to

log on to and use another computer. FILE TRANSFER PROTOCOL (FTP) : to

transfer files among computers on the Internet. USENET : to serve as an electronic bulletin

board. MAILTO : to allow users on other computers

on the Internet.

9

SO MANY PROTOCOLS = TROUBLE

• Restricted the growth of Internet as : Each had their own user interface Useful for only the intended purpose it was

designed. Users were required to learn all different

interfaces to gain entire advantage of the Internet.

• THUS A BETTER APPROACH WWW.

10

WWW or Web• New protocol for the Internet and also a system of document

access.

• Designed to allow a user anywhere on the Internet to search for and retrieve documents from databases on any number of different document-serving computers.

• HYPERTEXT : links for nonsequential browsing.

• HYPERMEDIA : nontextual information.

• Vast document access through Web Browsers provided by Web Servers.

11

WEB OR INTERNET ?

• NOT THE SAME.

• INTERNET : collection of computers and other devices connected by equipment that allows to communicate with each other.

• WEB : collection of software and protocols that has been installed on most, if not all, of the computers on the Internet.

Overview

Internet and Web Client /Server Programming on Web Server side programming and Applications Server-side Includes

13

Web Browsers Web and many other systems operate in client/server

configuration.

Programs running on client machine (Browsers) where documents provided by servers on the web are requested.

Web supports a variety of protocols, the most common is Hyper Text Transfer Protocol (HTTP).

Examples : Microsoft Internet Explorer, Firefox, Netscape Navigator, Apple’s Safari, Opera.

14

Web Servers They act by providing documents to browsers only if request made by browsers running on other

computers on the Internet.

Apache, Internet Information Server (IIS).

Web browsers initiate network communications with servers by sending them URLs.

Primary Task : to monitor a communications port on its host machine, accept HTTP commands through the port, and perform the operations specified by the commands.

File structure : document root and server root.

Virtual Hosts : Support more than one site on a computer.

Proxy Servers : serve documents that are in the document root of other machines on the Web.

All Web Servers interact with database systems through Common Gateway Interface (CGI) programs and server-side scripts.

15

Client/Server Architecture Numerous applications run in a client/server environment, this means

that client computers (computers forming part of the network) contact a server, generally a very powerful computer in terms of input/output, which provides services to the client computers. These services are programmes which provide data such as the time, files, a connection, etc.

The services are used by programs client programs which run on client computers. Designed to run on a client computer, capable of processing data received from a server (in the case of the FTP client we are dealing with files whereas for the email client we deal with email).

16

Client/Server system operation The client sends a request to the

server using its IP address and the port, which is reserved for a particular service running on the server.

The server receives the request and responds using the client IP address and port.

17

Advantages of Client/Server Model Centralised resources: given that the server is the centre of the network, it can

manage resources that are common to all users, for example: a central database would be used to avoid problems caused by redundant and inconsistent data.

Improved security: as the number of entry points giving access to data is not so important.

Server level administration: as clients do not play a major role in this model, they require less administration.

Scalable network: thanks to this architecture it is possible to remove or add clients without affecting the operation of the network and without the need for major modification.

18

Disadvantages of Client/Server Model Increased cost: due to the technical complexity of the server.

A weak link: the server is the only weak link in the client/server network, given that the entire network is built around it! Fortunately, the server is highly fault tolerant (primarily thanks to the RAID system).

19

Anatomy of a Web Application

20

Web Server handling request for Web content

Each time you enter a Web address into your browser’s address field and press Enter, the web

browser sends an HTTP request for a file over the Internet to the Web server.

After it receives the HTTP request for a file, the web server retrieves the requested file and sends

the Web page to the Web browser.

The Web browser analyses the Web page file to determine if there are any inserted files (such

graphics, animations, sounds, and so on) that the browser needs from the Web server.

The Web browser sends multiple HTTP requests (one for each file the browser needs) to the

Web server.

As the Web server receives the HTTP requests for files, the server finds each file and sends the

files (one file per HTTP request) to the Web browser.

The Web browser takes the original Web page files, follows the instructions given by the HTML

tags in the file to combine the Web page and the inserted file contents into a Web document the

browser displays onscreen.

21

2 Tier Architecture 2-tier architecture is used to

describe client/server systems where the client requests resources and the server responds directly to the request, using its own resources.

The server does not call on another application in order to provide part of the service.

21

22

3 Tier Architecture In 3-tier architecture, there is an

intermediary level, meaning the architecture is generally split up between:

A client, i.e. the computer, which requests the resources, equipped with a user interface (usually a web browser) for presentation purposes

The application server (also called middleware), whose task it is to provide the requested resources, but by calling on another server

The data server, which provides the application server with the data it requires

22

23

Comparing 2 & 3 Tier Architectures 2-tier architecture is a client-server architecture where the

server is versatile, i.e. it is capable of directly responding to all of the client's resource requests.

In 3-tier architecture however, the server-level applications are remote from one another, i.e. each server is specialized with a certain task (for example: web server/database server).

3-tier architecture provides: A greater degree of flexibility Increased security, as security can be defined for each service, and at each

level Increased performance, as tasks are shared between servers

23

24

Multi Tier Architecture In 3-tier architecture, each server

(tier 2 and 3) performs a specialized task (a service).

A server can therefore use services from other servers in order to provide its own service.

As a result, 3-tier architecture is potentially an n-tiered architecture

24

Client-side programming

Overview

Internet and Web Client/Server Programming on web Server-side programming and Applications Server Side Includes

Server-side programming May be useful to send data back to

server, and run a program there. Program might store user details in

database or file, or do some calculation and return result to user.

A number of technologies available: Servlets, JSP, CGI, ASP, PHP.

Server-side Programming

Server side programming Short history

CGI – separate programs launched by web server They produce an HTML document as output They receive arguments as input Strong isolation, bad performance

Programs embedded inside web page (php, ASP, JSP) Program executed inside web server process

Separate “code-behind” file for the code (ASP.NET) What are dynamic pages used for?

Personalizing based on user identity Interacting with databases (e.g. on-line banking) Web applications (e.g. web based email)

Separate database keeps persistent data

Server-side Vs Client-side

Common server side application

32

The Evolution to the Object Web

On the Client

JavaScript, ActiveX Controls, Java Applets

On the Server

Perl, C, Unix Shell, Java Servlets, Active Server Pages

(ASP), Coldfusion, Hypertext Preprocessor (PHP), Java

Server Pages (JSP), XML Server Pages (XSP), and other

proprietary languages (e.g., Lotus Domino’s scripting

language).

Client-Side versus Server-Side Programming:

33

The Evolution to the Object Web Static Web Pages =

HTML

Web Pages with CGI scripts = HTML + Programming Language X (e.g., Perl or C)

Dynamic HTML = XML + CSS + Programming Language X (e.g., JavaScript) + DOM

Database-Driven Web Pages = HTML + Programming Language X + ODBC/JDBC

Web Services = Java-Based Initiatives: XML + SOAP + WSDL + UDDI + Java + J2EE Microsoft’s .Net = XML + SOAP + WSDL + UDDI + C# + .NET

Overview

Internet and Web Client/Server Programming on Web Server-side programming and Applications Server Side Includes

Server Side Includes (SSI)

SSI (Server Side Includes) are directives that are placed in HTML pages, and evaluated on the server while the pages are being served.

They let you add dynamically generated content to an existing HTML page, without having to serve the entire page via a CGI program, or other dynamic technology.

Server Side Includes (SSI)

The decision of when to use SSI, and when to have your page entirely generated by some program, is usually a matter of how much of the page is static, and how much needs to be recalculated every time the page is served.

SSI is a great way to add small pieces of information, such as the current time. But if a majority of your page is being generated at the time that it is served, you need to look for some other solution.

37

Server-Side Includes (SSI) Configuring server to permit SSI To permit SSI on your server, you must have mod_include installed and enabled.

Additionally, you must have the following directive either in your httpd.conf file, or in a .htaccess file:

Options +Includes This tells server that you want to permit files to be parsed for SSI directives. Note that

most configurations contain multiple Options directives that can override each other. You will probably need to apply the Options to the specific directory where you want SSI enabled in order to assure that it gets evaluated last.

Not just any file is parsed for SSI directives. You have to tell server which files should be parsed. There are two ways to do this. You can tell server to parse any file with a particular file extension, such as .shtml, with the following directives:

AddType text/html .shtml AddHandler server-parsed .shtml

38

Server-Side Includes (SSI) Syntax of SSI Commands The document is parsed as an HTML document, with special commands embedded as

SGML comments. A command has the syntax: <!--#element attribute=value attribute=value ... --> It is formatted like an HTML comment, so if you don't have SSI correctly enabled, the

browser will ignore it, but it will still be visible in the HTML source. If you have SSI correctly configured, the directive will be replaced with its results. The value will often be enclosed in double quotes; many commands only allow a single

attribute-value pair. Note that the comment terminator (-->) should be preceded by whitespace to ensure that it isn't considered part of an SSI token. Note that the leading <!--# is one token and may not contain any whitespaces.

The element can be one of a number of things, and we'll talk some more about most of these in the next installment of this series. For now, here are some examples of what you can do with SSI

39

Server-Side Includes (SSI) Parsed HTML.

Short directives embedded in an HTML document to indicate files to be read and included in the final output.

To indicate to the server that the page contains SSI commands, use the .shtml extension.

Eg : If a copyright message has to be attached to all web pages dynamically, then create a file called footer.html and then include it dynamically in all the other web pages by adding HTML comment with a command include in all the web pages.<!--#include file=“footer.html” ->

Disadvantages : Security (it might be possible to insert a command to launch a remote session) and Performance (All SSI files have to be parsed, they can cause a performance hit).

40

Questions?