Chapter 33 CGI Technology for Dynamic Web Documents

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Chapter 33 CGI Technology for Dynamic Web
Documents

• There are two alternative forms of retrieving web
  documents.
• Instead of retrieving static HTML documents,
  these alternative forms allow computer programs
  to control the contents of a document
  dynamically.
• In one form, a Web server runs an auxiliary
  program (CGI program) to create a document
  whenever a request arrives from a browser.
• In the other form, a server provides the browser
  with a computer program (JAVA).

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Three types of Web documents

• Static, dynamic, and active
• categorized depending on when the information in
  the document changes

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Static Documents

• content of document is determined when document
  is written and remains unchanged until author
  revises document

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Dynamic Documents

• document does not exist in a predefined form.
• document is created by a web server whenever a
  browser requests the document.
• When a request arrives, the Web server runs an
  application program that creates the dynamic
  document.
• The server returns the output of the program as a
  response to the browser.
• The contents of a dynamic document can vary
  whenever a server receives a request for the
  document

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Active Document

• consists of a computer program that understands
  how to compute and display values.
• When a browser request an active document, the
  server returns a copy of the program that the
  browser must run locally.
• Information displayed can change after the
  document has been loaded into a browser.

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Comparison of Document Types
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Common Gateway Interface (CGI)

• Standard is used for building dynamic Web
  documents.
• specifies how a server interacts with an
  application program that implements a dynamic
  document.
• The application is called a CGI program.
• CGI does not specify a particular programming
  language ( eg. could be C, Perl , or Unix shell
  (fig 33.1))

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CGI program output

• can be an HTML document, a plain text file, or a
  graphics image.
• A header (eg. Content-type text/html )
  (Content-type text/plain) consists of line(s) of
  text describing the document type followed by a
  blank line, or a header can specify that the
  requested document is at a different location
  (eg. Location /new/bar.txt ltblank linegt)

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CGI application support

• A server that is capable of running CGI programs
  must be configured before it can invoke a CGI
  program or script.
• When the a browser contacts the server and
  request the specified URL, the server runs the
  CGI program

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Parameters and Environment Variables

• browser can pass parameters to a CGI program via
  URL.
• If the prefix of the URL corresponds to a CGI
  program, the server invokes the CGI program and
  passes the suffix of the URL as an argument.
• The question mark character (?) in the URL
  separates the prefix from the suffix.
• The server uses its configuration information to
  map the prefix to a particular CGI program.
• The server passes everything after the question
  mark as an argument to the CGI program via
  environment variables instead of via command line

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Environment Variables inherited by a CGI program

• SERVER_NAME
• contains domain name of computer running web
  server
• GATEWAY_INTERFACE
• contains version of CGI software server is using
• SCRIPT_NAME
• the path in the URL after the server name.
• QUERY_STRING
• contains information following ? in the URL
• REMOTE_ADDR
• contains IP address of the computer running the
  browser that sent the request.

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State Information

• Information preserved from one invocation of a
  CGI program to the next
• helps avoid having a user answer questions
  repeatedly when specifying additional requests.
• CGI program saves information between requests
  (state information) via one of two methods
• for info that must be maintained across
  invocations of a browser, the CGI program uses
  files for long term storage (fig 33.3)
• for info that must be kept only while a browser
  is running, a dynamic document program encodes
  such information in URLs that appear in the
  hypertext links in the document (fig 33.4)

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Server-Side Scripting Technologies

• Alternative dynamic document technologies used
  when only a small portion of web page needs to be
  changed.
• Interpreters built into web servers that can make
  small changes to a predefined template
  containing both HTML and scripts
• Regular HTML passes through unchanged
• Script information is replaced with output from
  interpreting the script
• ASP, JSP, PHP, Cold Fusion