4.0-CGI&WebDocumentT..

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Transcript 4.0-CGI&WebDocumentT.. 

Part 4
Web technologies: HTTP, CGI,
PHP,Java applets)
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Types of Web Pages
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Static
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Stored in file
Unchanging
Dynamic
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Formed by server
Created on demand
Output from a program
Use Common Gateway Interface (CGI)
technology
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Types of Web Pages (cont)
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Active
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Executed at client
Consists of a computer program
Can interact with user
Use Java technology
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Summary of Web Document
Types
Web documents can be grouped into three
categories depending on when the information
in the document changes. The information in a
static document remains unchanged until the
author revises the document. The information
in a dynamic document can change whenever
a server receives a request for the document.
Information displayed by an active document
can change after the document has been
loaded into a browser.
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CGI Technology
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URL specifies
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Location of Web server
CGI program on that server
Arguments to program
Web server
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Uses TCP for communication
Accepts HTTP request from client
Runs specified CGI program
Returns output to client
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CGI Technology (cont)
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CGI program
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Performs arbitrary computation
Often written in a scripting language
Produces output file when run
Starts output with header
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Header in CGI Output
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Stops at first blank line
Identifies
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Encoding used
Type of document
Format
keyword: information
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CGI Header Examples
HTML document header
Content Type: text/html
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Text document header
Content Type: text/plain
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Redirection header
Location: /over_here/item4
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Example CGI Script
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Generates document
Document contains three lines of text
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Header
Blank line
Document creation date
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Parameters
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CGI programs can be parameterized
URL can contain arguments that are passed
to CGI program
Question mark separates CGI path from
arguments
Server places information following question
mark in environment variable
QUERY_STRING
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Example Environment
Variables
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Encoding Information in a URL
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Information that program stores between
invocations: state information
Information passed to browser in form of
cookie
Cookie consists of name/value pair
Long term (persistent) or short term (session)
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Long-Term State Information
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Program lifetime
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CGI program invoked by server
Program exits after generating output
To maintain persistent data
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Write to file on disk
Read from file on disk
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Long-Term State Information
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Client’s IP address in environment variable
Check if address in file
Respond to client
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Short-Term State Information
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Each invocation of a dynamic document
program can produce a document containing
a new set of URLs
Use new arguments in new URLs
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Short-Term State Information
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Argument encodes number of times executed
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Example of Script Execution
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Initial document
Content-type: text/html
<HTML>
This is the initial page.<BR><BR>
<A HREF=“http://www.nonexist.com/cgi/ex4?1”>
Click here to refresh the page.</A> </HTML>
Resulting display
This is the initial page.
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Click here to refresh the page.
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Example of Script Execution
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Generated output
Content-type: text/html
<HTML>
You have displayed this page 2 times.<BR><BR>
<A HREF=“http://www.nonexist.com/cgi/ex4?2”>
Click here to refresh the page.</A> </HTML>
Resulting display
You have displayed this page 2 times.
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Click here to refresh the page.
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Generated URL Values
When it generates a document, a dynamic
document program can embed state
information as arguments in URLs. The
argument string is passed to the program for
the URL, enabling a program to pass state
information from one invocation to the next.
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Server-Side Scripting
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ASP (Active Server Pages)
JSP (Java Server Pages)
PHP (Perl Helper Pages)
ColdFusion
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Server-Side Scripting
Server-side scripting technologies allow a
dynamic page to be created from a template or
skeleton that contains embedded commands or
program scripts. Instead of using a computer
program to generate an entire page, an
interpreter copies the page and replaces only
the scripting commands.
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Continuously Changing
Information
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Needed for
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Animations
Rapid updates (e.g., stock prices)
Achieved with two mechanisms
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Server push
Active document
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Server Push Technology
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Client forms connection
Server sends updates repeatedly
Impractical
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Active Document Technology
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Server
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Client
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Sends computer program to client
Runs program locally
Program
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Controls display
Interacts with user
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Active Document
Representation
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Desire
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Platform independence
Efficient execution
High-speed data transmission
Late binding
Consequence
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Compact representation
Interpretive execution
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Active Document Translation
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Compiler produces machine-independent
binary
Browser interprets binary
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Java Technology
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Developed by Sun Microsystems
Used for
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Conventional applications
Active documents (applets)
Includes
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Programming language
Run-time system
Class library
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Java Language Characteristics
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High level
General purpose
Similar to C++
Object oriented
Dynamic
Strongly typed
Statically type checked
Concurrent
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Java Run-Time Environment
Characteristics
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Interpretative execution
Automatic garbage collection
Multi-threaded execution
Internet access
Graphics support
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Java Library
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Classes for
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Graphics manipulation
Low-level network I/O
Interaction with a Web server
Run-time system access
File I/O
Conventional data structures
Event capture
Exception handling
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Choice of Graphics Interface
Java includes an extensive graphics toolkit that
consists of run-time support for graphics as well as
interface software. The toolkit allows a programmer
to choose a high-level interface, in which the toolkit
handles details, or a low-level interface, in which the
applet handles details.
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Example Java Applet
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Window with two items
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Text area
Button
Change text when button clicked
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Illustration of Applet Display
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Initial
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After user clicks button
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Example Applet Code
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Applet Invocation
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Available in HTML
Uses applet tag
Specifies
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Codebase (machine and path)
Code (specific class to run)
Example
<applet codebase=“www.nonexist.com/pth”
code=“bbb.class”>
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Java Functionality
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HTML interface
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HTTP interface
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Controls display
Interacts with user
Accesses remote Web documents
Invokes other applets
Exceptions
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Indicate unanticipated circumstances
Can be caught and handled
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Example Applet Code
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Example Applet Code
(continued)
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Illustration of Applet Display
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JavaScript Technology
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Alternative to Java technology
Provides scripting language
Browser reads and interprets script in source
form
JavaScript can be integrated with HTML
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JavaScript Example
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JavaScript Example
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Summary
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Web is major application in Internet
Client
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Called browser
Fetches and displays document
Web documents
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Stored on servers
Standard representation is HTML
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Summary (continued)
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HTML
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Markup language
Uses tags embedded in text
URL components
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Protocol
Domain name of server
Protocol port number
Path of item
Only domain is required
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Summary (continued)
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Static web page
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Dynamic web page
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Unchanging
Output from a program on the server
Active web page
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Runs in browser
Consists of a computer program
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Summary (continued)
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Dynamic web page technology
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Known as CGI
CGI program usually a script
Document begins with header line
URL can contain arguments
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Summary (continued)
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Active web page technology
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Known as Java
Programming language plus runtime support
Document called applet
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