networked applications chapter 11 copyright 2001 prentice hall

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


• Solution: an application server (webserver with intermediary software) mediates between the two

User with Browser Database ServerApplication Server


• Step 1: User types URL of data entry form

• Step 2: Webserver application sends form


URL

Form

Application Server


• 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


• 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


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

Application Server

WebserverApplicationSoftware

IntermediaryProgram

(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


StandardQuery

Application Server

(Bogo.exe)


• Step 5: Database server sends response in its standard format to the intermediary program, Bogo.exe


StandardResponse

Application Server

(Bogo.exe)


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

Application Server

IntermediaryProgram

(Bogo.exe)

WebpageWebpage


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

Application Server

WebserverApplicationSoftware

IntermediaryProgram

(Bogo.exe)

WebpageWebpage


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


WebpageWebpage

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


• CGI and Intermediary Program

– Intermediary program (Bogo.exe) does the actual translation work

WebserverApplication

Program


Program

IntermediaryProgram

(Bogo.exe)

IntermediaryProgram

(Bogo.exe)

DatabaseApplication

Program

DatabaseApplication

Program


• CGI and Other Programs– Webserver application programs have a

standardized way of moving data to and from other programs

• Common Gateway Interface (CGI) standard


Program


Program

IntermediaryProgram

(Bogo.exe)

IntermediaryProgram

(Bogo.exe)

DatabaseApplication

Program

DatabaseApplication

Program

CGI


• 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


Program


Program

IntermediaryProgram

(Bogo.exe)

IntermediaryProgram

(Bogo.exe)

DatabaseApplication

Program

DatabaseApplication

Program

CGI IntermediaryProgram

(Bogo.exe)

IntermediaryProgram

(Bogo.exe)

IntermediaryProgram

(Bogo.exe)

IntermediaryProgram

(Bogo.exe)


• 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


Program


Program

DatabaseApplication

Program

DatabaseApplication

Program

API


• Application Program Interfaces (APIs)

– Faster than CGI but proprietary

– Webserver applications do not support all database APIs


Program


Program

DatabaseApplication

Program

DatabaseApplication

Program

API


CGI API

Speed Slower Faster

Generality General Proprietary


• Client-Side Processing

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

– Client can then communicate directly with the database server


Webpage withJava applet

Subsequent Interactions

networked applications chapter 11 copyright 2001 prentice hall

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