Web servers (l6)

1
Web Servers
• How do our requests
for resources on the
Internet get handled?
• Can they be located
anywhere? Global?

2
Web Servers
• Looking at ….
– Introduction to web servers
– HTTP request types
– 3-tier system architecture
– Accessing a web server (local and remote)
– Web server examples
• Apache
• Tomcat
• IIS
• PWS

3
What is a web server?
• Web server
– Specialized software that responds to client
requests by providing resources (web pages,
documents, etc..)
– Based on the client/server model
– When users enter URL into Web browsers, they
request specific documents from Web server
– Maps URL to file on server and returns requested
document to client
– Communicates with client using HTTP
• Protocol for transferring requests and files over the
Internet

4
HTTP Request Types
• Also known as request methods
• Most popular are get and post
– Retrieve and send client form data to Web server
– get request
• Sends form content as part of URL
• Retrieves appropriate resource from Web server
• Limits query to 1024 characters
– post request
• Updates contents of Web server (posting new messages to
forum)
• Has no limit for length of query
• Not part of URL and cannot be seen by user

5
HTTP Request Types
• get and post requests are sent to a server-
side form handler
• Client browsers cache (save on disk) Web
pages
– Allows for quick reloading
– Cache responses to get request
– Do not cache responses to post request

6
System Architecture
• A web server forms part of a multi-tier
application
– Divide functionality into separate tiers
• Logical groupings of functionality
• Can reside on same computer or on different
computers
• What does a 3-tier architecture look like?
– let’s have a look

7
System Architecture
ApplicationMiddle tier
Information tier
Client tier
Database

8
System Architecture
• Client tier
– Referred to as the top tier
– Application’s user interface
– Users interact with application through user
interface
– Interacts with middle tier to make requests and
to retrieve data from information tier
– Displays data to user

9
System Architecture
• Information tier
– Referred to as data tier or bottom tier
– Maintains data for application
– Stores data in relational database management
system
• Middle tier
– Implements business logic and presentation logic
– Controls interactions between application clients
and application data
– Acts as intermediary between data in information
tier and application clients

10
System Architecture
• Middle tier, cont.
– Controller logic
• Processes client requests from top tier
• Retrieves data from database
– Presentation logic
• Processes data from information tier
• Presents content to client
– Business logic
• Enforces business rules
– Dictates how clients can access application data and how
applications process data
• Ensures data validity before updating database

11
Accessing Web Server
• Requesting documents
– Must know machine name on which Web server
resides
– Through local Web servers or remote Web servers
– Through domain name or Internet Protocol (IP)
address
• Local Web server
– Resides on users’ machines
– Requests documents in two ways
• Machine name
• localhost
– Host name that references local machine

12
Accessing Web Server
• Remote Web server
– Resides on different machines
– Accessing resources on remote web server
requires
• Domain name
– Represents group of hosts on Internet
– Combines with how name (www) and top-level domain to from
fully qualified host name
• Fully qualified host name
– Provides user friendly way to identify site on Internet
• IP address
– Unique address for locating computers on Internet
• DNS (see notes)

13
Examples of Web Servers
• Apache (Apache group)
• Tomcat (Apache group)
• IIS (Microsoft Internet Information
Services)
• PWS (Microsoft Personal Web Server)

14
Apache web server
• Apache
– Maintained by Apache Software Foundation
– Currently most popular Web server
• Stable
• Efficient
• Portable
• (got it’s name from the original developers
who called it ‘a patchy server’)

15
Apache web server
• Open source
– Freely distributed on the Internet
– Huge amount of end-user support
– Has earned the reputation of being one of the
most reliable web server available
• Uses server modules to add functionality to
the core HTTP server
– i.e. extra services that it can provide (e.g.
security authentication)

16
Apache web server
• To launch Apache, successively select
– Start, Programs, Apache httpd Server, Control
Apache Server and Start

17
Tomcat web server
• Tomcat
– Similar to Apache and also maintained by
Apache Software Foundation
– Fast becoming a popular Web server
– Built in Servlet container (engine)
– Requires no add-ons to support Servlets

18
Tomcat web server
• To launch Tomcat, successively select
– Start, Programs, Apache Tomcat 4.x and Start
Tomcat

19
Internet Information Services
(IIS) web server
• Developed by Microsoft
• IIS 5.0
– Enterprise-level Web server
– Included with Windows 2000
– Allows computer to serve documents
• Uses an Internet Services Manager to
administrate
– see diagram

20
Internet Information Services
(IIS) web server
Internet Services Manager dialog.

21
Personal Web Server (PWS)
• PWS developed by Microsoft
• PWS
– Scaled-down version of IIS
– Intended for personal computers (PC)
– Ideal for educational institutions, small
businesses and individuals
– Does not require PC to be used exclusively as
Web server
– A Personal Web Manager is used to
administrate it

22
Personal Web Server
Personal Web Manager dialog.

23
References
• Check out
– http://www.apache.org
• Apache and Tomcat documentation
• Apache and Tomcat are free to download and install
– http://www.microsoft.com
• IIS and PWS documentation

24
Summary
• We have looked at:
– Introduction to web servers
– HTTP request types
– 3-tier system architecture
– Accessing a web server (local and remote)
– Web server examples

Web servers (l6)

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