Networked Applications

Networked
Applications

Chapter 11

Copyright 2001 Prentice Hall

The Primacy of Applications

•
Users only care about
applications

•
The other layers (transport, internet, data
link, physical) exist only to support
applications

Applications in this Chapter

•
There are many applications

•
We will only look at 3 networked
applications

–
Web
-
enabled database access

–
Internet E
-
mail

–
IP telephony for voice
-
data convergence

•
Each is important

•
Each has general lessons to teach

Web
-
Enabled Database Access

Web
-
Enabled Database Access

•
Problem: Database is on a “
database
server
”

–
Mainframe or client/server server

–
These need proprietary client software or a
terminal

•
Problem: Most users only have browsers

User with Browser

Database Server

Web
-
Enabled Database Access

•
Solution: an
application server

(webserver
with intermediary software) mediates
between the two

User with Browser

Database Server

Application Server

Web
-
Enabled Database Access

•
Step 1: User types URL of data entry form

•
Step 2: Webserver application sends form

User with Browser

Database Server

URL

Form

Application Server

Web
-
Enabled Database Access

•
Step 3: User types information in form, hits Send

–
Browser puts data in a request line

–
GET keyword

–
/cgi
-
bin is the absolute path to a directory holding an
intermediary program,
bogo.exe

GET /cgi
-
bin/bogo.exe?last=Lee&first=Pat

Web
-
Enabled Database Access

•
Step 3: User types information in form, hits
Send

–
Data separated from absolute path by ?

–
last

field contains the value
Lee

–
first

field contains the value
Pat

GET /cgi
-
bin/bogo.exe?last=Lee&first=Pat

Web
-
Enabled Database Access

•
Step 3: Application server webserver
software passes data form to the
intermediary program
, Bogo.exe, on the
webserver

Application Server

Webserver

Application

Software

Intermediary

Program

(Bogo.exe)

Web
-
Enabled Database Access

•
Step 4: Bogo.exe puts the data into a
query in the database server’s
standard
format

–
Sends the query to the server

User with Browser

Database Server

Standard

Query

Application Server

(Bogo.exe)

Web
-
Enabled Database Access

•
Step 5: Database server sends response in
its
standard format

to the intermediary
program, Bogo.exe

User with Browser

Database Server

Standard

Response

Application Server

(Bogo.exe)

Web
-
Enabled Database Access

•
Step 6: Application server intermediary
software (Bogo.exe)
creates a new HTML
webpage

containing the response

Application Server

Intermediary

Program

(Bogo.exe)

Webpage

Web
-
Enabled Database Access

•
Step 7: Application server intermediary
software passes this webpage to the
webserver application program

Application Server

Webserver

Application

Software

Intermediary

Program

(Bogo.exe)

Webpage

Web
-
Enabled Database Access

•
Step 8: Webserver application program
passes the webpage to the browser

User with Browser

Database Server

Webpage

Application Server

Transparency

•
Browser does not know that it is dealing with a
database server; it sends standard webserver
commands, gets webpages back

•
Database server does not know it is dealing with
a browser; receives standard database server
requests, sends standard database server
responses

•
No changes are needed in the browser or
database application software; process is
transparent

to both the browser and the
database server

Web
-
Enabled Database Access

•
CGI and Intermediary Program

–
Intermediary program

(Bogo.exe) does the
actual translation work

Webserver

Application

Program

Intermediary

Program

(Bogo.exe)

Database

Application

Program

Web
-
Enabled Database Access

•
CGI and Other Programs

–
Webserver application programs have a
standardized way of moving data to and from
other programs

•
Common Gateway Interface (CGI)

standard

Webserver

Application

Program

Intermediary

Program

(Bogo.exe)

Database

Application

Program

CGI

Web
-
Enabled Database Access

•
CGI

–
Starts a new copy of the intermediary program
every time the intermediary program is called

–
This is very slow

–
OK only for small applications

Webserver

Application

Program

Intermediary

Program

(Bogo.exe)

Database

Application

Program

CGI

Intermediary

Program

(Bogo.exe)

Intermediary

Program

(Bogo.exe)

Web
-
Enabled Database Access

•
Application Program Interfaces (APIs)

–
Database servers have
proprietary

ways of
communication with other application programs

–
Other application program sends an
application
program interface (API)

call to the database
program

Webserver

Application

Program

Database

Application

Program

API

Web
-
Enabled Database Access

•
Application Program Interfaces (APIs)

–
Faster

than CGI but
proprietary

–
Webserver applications do not support all
database APIs

Webserver

Application

Program

Database

Application

Program

API

Web
-
Enabled Database Access

CGI

API

Speed

Slower

Faster

Generality

General

Proprietary

Web
-
Enabled Database Access

•
Client
-
Side Processing

–
Webserver can download webpage with Java
or Active
-
X program

–
Client can then
communicate directly

with
the database server

User with Browser

Database Server

Webpage with

Java applet

Subsequent Interactions