1. Web Servers (IIS and Apache)21
Web Servers (IIS and Apache)

2. 21.1 Introduction A web server responds to client requests
A web server and a client communicate using the platform-independent Hypertext Transfer Protocol (HTTP)

3. 21.2 HTTP Transactions
The HTTP protocol allows clients and servers to interact and exchange information.
HTTP uses URLs (Uniform Resource Locators) to identify data on the Internet.
indicates that the resource is to be obtained using the HTTP protocol.

4. 21.2 HTTP Transactions (Cont.)
A hostname is translated into an IP
Translation is performed by a domain name system (DNS) server
Virtual Directory:
For security reasons the path normally specifies the location of a virtual directory.
The server translates the virtual directory into a real location on the server.

5. 21.2 HTTP Transactions (Cont.)
HTTP method get indicates that the client wishes to obtain a resource from the server.
HTTP status code 200 indicates success. Status code 404 informs the client that the web server could not locate the requested resource.

6. Fig. 21. 1 | Client interacting with web server
Fig | Client interacting with web server. Step 1: The GET request.

7. Fig. 21. 2 | Client interacting with web server
Fig | Client interacting with web server. Step 2: The HTTP response.

8. 21.3 Multitier Application Architecture
Web-based applications are multitier applications that divide functionality into separate tiers.
The bottom tier maintains the application’s data.
The middle tier implements business logic to control interactions between the client and data. Business rules dictate how clients can and cannot access application data, and how applications process data.
The top tier is the application’s user interface. The client tier never directly interacts with the data tier.

9. Fig. 21.3 | Three-tier architecture.

10. 21.4 Client-Side Scripting versus Server-Side Scripting
Client-side scripting can be used to validate user input, to interact with the browser
Client-side scripting does have limitations, such as browser dependency.
Sensitive information, such as passwords or other personally identifiable data, should not be stored or validated on the client.
Server-side scripting languages can access the server’s file directory structure.

11. Software Engineering Observation 21.2
Properly configured server-side script source code is not visible to the client; only XHTML and any client-side scripts are visible to the client.

12. 21.5 Accessing Web Servers Localhost
Local web servers can be accessed through your computer’s name or through the name
Localhost
a hostname that references the local machine and normally translates to the IP address (also known as the loopback address).

13. 21.6 Microsoft Internet Information Services (IIS)
Microsoft Internet Information Services (IIS) is a web server. Installing IIS on a machine allows that computer to serve documents.
To install IIS 5.1 on Windows XP, you may need your original operating-system disk.

14. Fig. 21.4 | Internet Information Services window of IIS 5.1.

15. Fig. 21. 9 | Internet Information (IIS) Services Manager window (IIS 7

16. 21.7 Apache HTTP Server
The Apache HTTP Server is currently the most popular web server.
It is open source software that runs on UNIX, Linux, Mac OS X, Windows platforms.

17. Fig. 21. 11 | Apache Service Monitor
Fig | Apache Service Monitor. (Courtesy of The Apache Software Foundation, <

18. 21.8 Requesting Documents Static Web Pages
The server sends XHTML documents to the client as static web pages.
Dynamic Web Pages
For other types such as PHP, Ruby on Rails, ASP.NET and JavaServer Faces, the appropriate scripting engine first generates XHTML content, then transmits it to the client over HTTP.

19. Fig. 21.12 | Requesting test.html.