fall 2005csi 4118 – university of ottawa1 part 4 web technologies: http, cgi, php,java applets)
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FALL 2005 CSI 4118 – UNIVERSITY OF OTTAWA 1
Part 4
Web technologies: HTTP, CGI,PHP,Java applets)
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World Wide Web
Major application protocol used on the Internet
Simple interface Two concepts
Point Click
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Web Components
Browser Web server Hypermedia links Document representation Transfer protocol
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Browser
Application program User’s interface to Web Becomes Web client to fetch information from
Web server Displays information for user
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Web Server
Running program Stores set of Web documents Responds to request from browser by
sending copy of document
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Hypermedia Concept
Web document contains mixture of Text Images Selectable pointers to other Web pages
Known as hypermedia
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Hypermedia Link On A Web Document
Associated with object or area on screen Internally like a symbolic link Advantage
Can reference document on another computer Disadvantage
Can become invalid
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Web Document
Called a web page One web page per file Can contain
Binary image Text file
Text standard Readable representation ASCII Specifies contents and layout Known as Hypertext Markup Language
(HTML)
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Terminology
Markup language Gives general layout guidelines Does not specify exact placement or format
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Consequence of UsingA Markup Language
Web documents use the HyperText Markup Language representation. Instead of specifying a detailed document format, HTML allows a document to contain general guidelines for display, and allows a browser to choose details. Consequently, two browsers may display an HTML document differently.
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HTML Details
Document is free-format Embedded tags give display guidelines Tags often appear in pairs Tag format
Beginning tag
Ending tag<TAGNAME>
</TAGNAME>
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General Form of HTML Document
<HTML><HEAD>
<TITLE>text that forms the document title
</TITLE></HEAD><BODY>
body of the document appears here</BODY>
</HTML>
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Document Format
<HTML><HEAD><TITLE> text that forms the document title</TITLE></HEAD><BODY>body of the document appears here</BODY></HTML>
HTML source is free-formPrevious example equivalent to this
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Example HTML Tags
Begin paragraph
<P> Line break (force a new line)
<BR> Main heading (largest, boldest font)
<H1> … text … </H1> Next heading (next largest)
<H2> … text … </H2>
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Example of HTML Line Break
InputHello there.<BR>This is
an example<BR>of HTML.
OutputHello there.
This is an example
of HTML.
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Example of HTML Line Break (continued)
InputHello there.<BR><BR>This
shows<BR>HTML spacing.
OutputHello there.
This shows
HTML spacing.
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Example of HTML Headings
InputHello.<BR><H1>This is a
heading</H1><BR>Back to normal.
OutputHello.
This is a headingBack to normal.
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Other HTML Features
Numbered or unnumbered lists Images Links to other pages
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Images in HTML
Explicitly denoted as image Specified with image tag Can specify alignment with text Example image tags
<IMG SRC=“file_name”>
<IMG SRC=“file_name” ALIGN=MIDDLE>
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Images in HTML
Here is a picture. <IMG SRC=“file_name” ALIGN=MIDDLE>
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Links to Other Pages
Symbolic representation Embedded in HTML document Browser
Hides text of link from user Associates link with item on page Makes item selectable
Called Uniform Resource Locator (URL)
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General Form of URL
Only domain name required Defaults
Protocol is http Port is 80 Path is index.html
protocol :// domain_name : port / item_name
name of accessprotocol to use domain name of
server computer
protocol portnumber path name
of item
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Link in HTML
Link specified in <A> tag Applies to successive items Ends with </A> Called anchor
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Example of Anchor Tag in HTML
InputThe text is published by<A HREF=http://www.prenhall.com>Prentice Hall, </A> one of the larger publishers of ComputerScience textbooks.
ProducesThe text is published by Prentice Hall, one of the larger publishers of Computer Science textbooks.
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Use of Client-Server Paradigm
Web server Makes set of pages available Uses port 80
Web client Called a browser Creates TCP connections to server Sends requests for items
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Use of Client-Server Paradigm
Primary protocol used between browser and server known as HyperText Transfer Protocol (HTTP)
HTTP requests sent as text (ASCII) GET: request an item from the server HEAD: request status information about an
item POST: send data to the server
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Use of Client-Server Paradigm
Response from server begins with ASCII header Status code (200 = handled request)
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Inside a Browser
Main controller Receives input from user Invokes client and interpreter
Clients One or more built into browser Uses network to fetch items
Interpreter One or more built in Displays items
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Illustration of a Browser
Browser contains many components
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Alternative Protocol Example
File transfer service Protocol is FTP Example URL
ftp://ftp.cs.purdue.edu/pub/comer/netbook/client.c
Can be used in anchor tag
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Other Markup Languages
Extensible Markup Language (XML) does not specify layout
Tag names can be created as needed Example for a corporate phone book:
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Caching in Browsers
Cache for recently accessed HTML pages Images
Item normally fetched from cache User can override HTTP can verify timestamp before fetching
new copy
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Types of Web Pages
Static Stored in file Unchanging
Dynamic 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)
Active Executed at client Consists of a computer program Can interact with user Use Java technology
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Summary of Web Document TypesWeb 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
URL specifies Location of Web server CGI program on that server Arguments to program
Web server Uses TCP for communication Accepts HTTP request from client Runs specified CGI program Returns output to client
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CGI Technology (cont)
CGI program 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
Stops at first blank line Identifies
Encoding used Type of document
Format keyword: information
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CGI Header Examples
HTML document header
Content Type: text/html Text document header
Content Type: text/plain Redirection header
Location: /over_here/item4
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Example CGI Script
Generates document Document contains three lines of text
Header Blank line Document creation date
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Parameters
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
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
Program lifetime CGI program invoked by server Program exits after generating output
To maintain persistent data Write to file on disk Read from file on disk
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Long-Term State Information
Client’s IP address in environment variable Check if address in file Respond to client
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Short-Term State Information
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
Argument encodes number of times executed
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Example of Script Execution
Initial documentContent-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 displayThis is the initial page.
Click here to refresh the page.
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Example of Script Execution
Generated outputContent-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 displayYou have displayed this page 2 times.
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
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 Needed for
Animations Rapid updates (e.g., stock prices)
Achieved with two mechanisms Server push Active document
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Server Push Technology
Client forms connection Server sends updates repeatedly Impractical
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Active Document Technology
Server Sends computer program to client
Client Runs program locally
Program Controls display Interacts with user
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Active Document Representation
Desire Platform independence Efficient execution High-speed data transmission Late binding
Consequence Compact representation Interpretive execution
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Active Document Translation
Compiler produces machine-independent binary Browser interprets binary
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Java Technology
Developed by Sun Microsystems Used for
Conventional applications Active documents (applets)
Includes Programming language Run-time system Class library
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Java Language Characteristics
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
Interpretative execution Automatic garbage collection Multi-threaded execution Internet access Graphics support
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Java Library
Classes for 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
Window with two items Text area Button
Change text when button clicked
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Illustration of Applet Display
Initial
After user clicks button
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Example Applet Code
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Applet Invocation
Available in HTML Uses applet tag Specifies
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
HTML interface Controls display Interacts with user
HTTP interface Accesses remote Web documents Invokes other applets
Exceptions 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
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
Web is major application in Internet Client
Called browser Fetches and displays document
Web documents Stored on servers Standard representation is HTML
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Summary (continued)
HTML Markup language Uses tags embedded in text
URL components Protocol Domain name of server Protocol port number Path of item Only domain is required
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Summary (continued)
Static web page Unchanging
Dynamic web page Output from a program on the server
Active web page Runs in browser Consists of a computer program
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Summary (continued)
Dynamic web page technology Known as CGI CGI program usually a script Document begins with header line URL can contain arguments
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Summary (continued)
Active web page technology Known as Java Programming language plus runtime support Document called applet