Introduction
- How web clients interact with web servers � by messages!
- I install web servers and write CGI scripts
- A CGI script or SOAP server can do anything
- Eg they can be a database client
Web Clients and Servers
Substitutions:
- A web client normally means an HTML client
- Here, a web client may be a SOAP client
- Clients talk to a web server, CGI script or SOAP server
- SOAP does not have to use HTTP
Topics of Discussion
What is a Protocol?
At the post office:
- I'd like to buy a book of stamps, thanx
- Ok, here they are. That'll be $5
- Here's $10
- And here's your change: $5
The customer and the server communicate via protocols. Protocols mediate the interaction, ie they make it work.
What is a Web Client?
- It�s a browser or a non-browser
- HTML are commands to a browser
- HTML is a rendering language
- A browser renders content in a window according to the HTML
- A non-browser does not care about HTML 'v' content. It�s all data
A real, live, Web Client
#!/usr/bin/perl use LWP::Simple; print get 'http://savage.net.au/index.html';
This 3 line program is a complete, tested, web client
A real, live, SOAP Server
#!/usr/bin/perl
# Name: server-1.cgi.
use SOAP::Transport::HTTP;
SOAP::Transport::HTTP::CGI -> dispatch_to('Demo') -> handle;
package Demo;
sub hi{return "Hello, world";}
This 6 line program is a complete, tested, SOAP server
A real, live, SOAP Client
#!/usr/bin/perl
# Name: server-1-client-1.pl.
use SOAP::Lite;
print SOAP::Lite -> proxy('http://127.0.0.1/cgi-bin/soap/server-1.cgi') -> uri('Demo') -> hi() -> result;
This 4 line program is a complete, tested, SOAP client
Output from that SOAP Client
DOS>perl server-1-client-1.pl Hello, world
That's all, folks!
OK, so there was no error checking. Next time!
Running a Script
<img src = 'web-servers-slide-10.png'>
The Request Loop
<img src = 'web-servers-slide-11.png'>
Finding the Destination
- http://savage.net.au/index.html is a message
- We wish to send it from a web client to a web server
- savage.net.au is a domain name, ie a symbolic address
- How does it get converted into a numeric address, like 222.44.55.66?
- A Domain Name System (DNS) server converts it
Special Destinations
- http://127.0.0.1/index.html means...
- the client and the server are in the same machine
- 127.0.0.1 is like a mirror around the PC. The message cannot leave
- 192.168.*.* is a mirror around a group of PCs
- We call this group a LAN!
- http://www.ietf.org/rfc/
127.0.0.1
- You can think of 127.0.0.1 as 'myself'
- Every person calls themselves 'myself'. Eg: I talk to myself
- Every TCP/IP stack calls itself 127.0.0.1
DNS under MS Windows
- For Win95/98/ME, see C:\Windows\hosts
- For WinNT/2000/XP, see C:\WinNT\system32\drivers\etc\hosts
- For WinNT/2000/XP, ignore C:\WinNT\hosts
- For Win*, ignore the sample C:\Windows\hosts.sam
- Explorer will lie to you, if you try to edit this file (when not logged on as Administrator)
The hosts file
# A hash starts a comment 127.0.0.1 ronnie.net.au 127.0.0.1 iris.ron.net.au 127.0.0.2 violet.net.au
http://ronnie.net.au/index.html is the same as http://127.0.0.1/index.html
Encoding the Message
- Client message Msg = http://savage.net.au/cgi-bin/script.cgi
- Each protocol encodes message
- CGI[Msg]
- HTTP{CGI[Msg]}
- TCP/IP(HTTP{CGI[Msg]})
- Modem<TCP/IP(HTTP{CGI[Msg]})>
- Just like Russian dolls
Finding the Server
- The message goes to savage.net.au, ie 222.44.55.66
- Remember: The DNS converts savage.net.au into 222.44.55.66
- How do we know which program gets the message?
- The 'http' says it goes to an HTTP server
- That is, a program which volunteers to accept HTTP-type messages
- Default: 222.44.55.66:80
What is port 80?
- Port 80 is the default port for HTTP
- What is a port?
- Hardware 'v' Software ports
- Mouse: COM1, Modem: COM2
- Printer: LPT1
- 0 .. 65535 software (logical) ports
- A port is the address of a server
Other Ports
- FTP: 23
- SMTP: 25 (Mail out)
- POP3: 110 (Mail in)
- A port is dedicated to a protocol
- Admin ports are 0 .. 1023
Load Balancing
- A web server on port 80 can redirect
- Static images and pages use 127.0.0.1:8080, a small, fast server
- Dynamic pages use 127.0.0.1:8081, a fancy server, eg using templates
- Manage this via Virtual Hosts
Decoding the Message
- Message = modem<TCP/IP(HTTP{CGI[Msg]})>
- TCP/IP(HTTP{CGI[Msg]})
- HTTP{CGI[Msg]}
- CGI[Msg]
- Server message Msg = http://savage.net.au/cgi-bin/script.cgi
Processing the Message
- The web server may handle the message (Apache):
- http://127.0.0.1/server-status
- The web server may find a real or virtual file:
- http://127.0.0.1/index.html
- The web server may run a script:
- http://127.0.0.1/cgi-bin/script.cgi
Processing a URI
- http://deakin.edu.au/comp-sci/courses/scripting101.html
- http is a protocol (not https)
- deakin.edu.au is a host
- / is a directory
- comp-sci/ is a directory
- courses/ is a directory
- scripting101.html is a file
/ is a directory
- / is the root directory of the URI
- / is not the root directory of the hard disk
- / is the root directory for that host
- The web server�s config file translates / into a real directory
- This makes the real / secure
A CGI script
#!/usr/bin/perl
use CGI;
my($q) = CGI -> new();
print $q -> header(),
$q -> start_html(),
$q -> start_form({action => $q -> url(), name => 'a_form'}),
'Email address: ',
$q -> textfield({name => 'email', size => 40}),
$q -> submit(),
$q -> end_form(),
$q -> end_html();
This 4 line program is a complete, tested, CGI script.
Output from a CGI Script
And this is the output from that CGI script, slightly simplified.
<html><head>�</head><body> <form action = 'http://127.0.0.1/cgi-bin/script.cgi', name = 'a_form'> Email address: <input type='text' name='email' size='40' /> <input type='submit'> </form></body></html>
Why output that form?
- The web server passes the data to the CGI script
- But, the very first time the CGI script runs, there is no data!
- So, the CGI script knows it must output a blank form this time
- The CGI script exits!
Interpretation
- When Submit is clicked:
- A) The data is encoded using the CGI protocol
- B) The message might look like:
- http://127.0.0.1/cgi-bin/script.cgi?email=me@here.net
- C) The action tells the client which server to send the message to
When the CGI script gets data
- It knows this is the second time the script is being run
- The scripts do not have to be the same
- But, expect to write scripts which are ALWAYS run twice
- Script processes the data (eg db)
- Script outputs a Thank You page
Resources
- http://savage.net.au/Ron/html/cgi-scripting.html (lecture 2001)
- http://savage.net.au/Ron/html/web-servers.html (lecture 2002)
- The slides: http://savage.net.au/Ron/web-servers.tgz
- The slides: http://savage.net.au/Ron/web-servers.zip
Author
Ron Savage 
.
Home page: http://savage.net.au/index.html
- Version: 1.01 01-Jun-2006
This version disguises my email address.
- Version: 1.00 13-May-2002
Original version.
Licence
Australian Copyright © 2002 Ron Savage. All rights reserved.
All Programs of mine are 'OSI Certified Open Source Software'; you can redistribute them and/or modify them under the terms of The Artistic License, a copy of which is available at: http://www.opensource.org/licenses/index.html