Chapter Nine

1
ChapterNine

• Compiled Web
• Server Programs

2
Chapter Objectives

• Learn about Common Gateway Interface (CGI)
• Create CGI programs that generate dynamic Web
  pages using Visual Basic, and learn the
  advantages and disadvantages of using CGI
  programs to create dynamic Web pages
• Pass parameter values among CGI programs

3
Chapter Objectives

• Learn about ActiveX dynamic-link libraries (DLLs)
  and their advantages and disadvantages
• Create an ActiveX DLL that generates a Web page
• Call an ActiveX DLL from an Active Server Page

4
Common Gateway Interface

• Common Gateway Interface (CGI)
• Protocol that specifies how Web servers and
  compiled programs for processing user inputs that
  run on the Web server communicate with each other
• Developed as a protocol for programs that process
  data entered into HTML forms and use these inputs
  to generate dynamic Web pages

5
Common Gateway Interface
Figure 9-1 CGI processing architecture
6
Processing Inputs and Outputs in CGI Programs

• In the early days of mainframe computing
• Standard input (STDIN) referred to a memory
  location where user input from the keyboard was
  stored
• Standard output (STDOUT) referred to a memory
  location where output to be displayed on the
  monitor was stored

7
Processing Inputs and Outputs in CGI Programs

• The terms STDIN and STDOUT are no longer used to
  reference keyboard input and monitor output
• STDIN and STDOUT now define a standard way of
  communicating the locations of inputs and outputs
  between different programs

8
Writing a CGI Program Using Visual Basic

• Visual Basic can be used to create CGI programs
• VB doesnt directly support reading from STDIN
  and STDOUT
• To create CGI programs using VB, use a VB
  standard module named cgi.bas

9
Writing a CGI Program Using Visual Basic

• The cgi.bas module contains declarations of
  Windows API (Application Programming Interface)
  functions that enable Visual Basic to read from
  STDIN and write to STDOUT
• Windows API is a set of code libraries that
  developers can incorporate within Windows
  applications written in a variety of languages,
  to create standard Windows program components

10
Using the cgi.bas Standard Module
Table 9-1 cgi.bas procedures and functions
11
Using the cgi.bas Standard Module

• The InitializeCGI procedure must be called before
  using the other procedures in cgi.bas
• The InitializeCGI procedure can be called only
  once at the beginning of your program
• The stdOut function receives a string containing
  the program outputs as an input variable, and
  writes these outputs to STDOUT

12
Using the cgi.bas Standard Module

• The DisplayAll procedure is used in the first CGI
  program that you will write to display the names
  and values of all the form inputs and environment
  variables
• The GetInput function receives the name of an
  HTML form variable as an input parameter, and
  retrieves the value of the variable from STDIN

13
Debugging CGI Programs in Visual Basic

• The main source of errors in CGI programs
  developed in Visual Basic is concatenation
  errors, where the SQL or HTML commands are not
  correctly formed as text strings
• Commands with incorrectly formed text strings are
  usually easy to spot
• The VB editor displays
• Lines with syntax errors in red
• A message box describing the error when you enter
  the code

14
Debugging CGI Programs in Visual Basic
Figure 9-10 String concatenation error
15
Debugging CGI Programs in Visual Basic

• After eliminating the red errors, there still
  might be errors from text strings where the
  syntax is correct, but the content is not correct
• This usually happens when you concatenate
  variable values with text strings to create SQL
  queries

16
Debugging CGI Programs in Visual Basic

• Breakpoint
• A place in a program where execution is paused
  while the program is running, allowing the
  developer to examine variable values
• When program execution is paused at a breakpoint,
  place the mouse pointer on any reference variable
  in the program code, and the current value of the
  variable will be displayed in the ToolTip window

17
Debugging CGI Programs in Visual Basic
FIGURE 9-13 Code window with execution paused at
breakpoint
18
Debugging CGI Programs in Visual Basic

• Placing the mouse pointer on variable values to
  determine their current values works well for
  variable values that are fairly short
• When a query string is very long, some of the
  text might not appear in the ToolTip window
• Immediate window
• Testing area that is displayed when an execution
  is paused while a VB program is running in the VB
  Integrated Development Environment

19
Debugging CGI Programs in Visual Basic
Figure 9-14 Query to display variable value in
the Immediate window
20
Creating Hyperlinks to Pass Parameter Values in
CGI Programs

• The hyperlink lists the name of the CGI program,
  followed by a question mark and the parameter
  list variable name/value pairs
• Each individual variable name/value pair is
  separated from the next by an ampersand ()

21
Creating Hyperlinks to Pass Parameter Values in
CGI Programs
Figure 9-16 Code to create hyperlinks with URL
parameter values
22
Creating Hyperlinks to Pass Parameter Values in
CGI Programs
Figure 9-17 Item ID hyperlink URL parameter value
23
Sharing Data Values with Other CGI Programs

• CGI programs, like Active Server Pages, can pass
  data values as URL parameters, as form
  parameters, or as cookies
• URL parameters are passed to a CGI program within
  a form ACTION tag by forming the ACTION parameter
  using the name of the CGI executable file and a
  question mark, and then listing the URL parameter
  variable name/value pairs, separated by ampersands

24
Sharing Data Values with Other CGI Programs

• Form parameter
• Passed to the Web server when an HTML form is
  submitted to the Web server by the user
• Form parameters are the names and associated
  values of form controls are written to STDIN when
  a form is submitted to the Web server

25
Sharing Data Values with Other CGI Programs

• Cookie
• Data file that is written on the users
  workstation by a program within a Web page
• Available to any Web page in an application,
  regardless of the order in which the Web page is
  selected and viewed

26
Advantages and Disadvantages of Using CGI Programs

• CGI programs can be written in any programming
  language that allows users to write values
  directly to STDIN and STDOUT on a Web server
• A CGI program can be used with most Web servers
  and operating systems
• In contrast, Active Server Pages run only on a
  Microsoft Web server

27
Advantages and Disadvantages of Using CGI Programs

• CGI programs are compiled programs
• The program source code is converted into machine
  language when you compile the program
• In contrast, script commands must be converted
  into machine language each time the script is run
• For this reason, compiled programs execute (run)
  much faster than scripts

28
Advantages and Disadvantages of Using CGI Programs

• The drawback of CGI programs is that they do not
  use Web server resources efficiently
• On a busy Web site using CGI programs, all of the
  Web servers main memory could be consumed trying
  to service multiple submissions of the same HTML
  form, and the Web server would be very slow in
  sending responses back to users
• To solve this problem, vendors are developing
  products to allow a single CGI program to service
  multiple submissions of the same form

29
Using ActiveX DLLs for Server-side Web Processing

• A dynamic link library (DLL) is not a stand-alone
  program, but it contains code that can be linked
  to, or used by, many different programs
• The code modules in a DLL are called libraries
  because their code can be checked out and used
  by many different programs
• A DLL is somewhat different from a conventional
  library in that the code from a specific DLL can
  be used simultaneously by many different programs

30
Using ActiveX DLLs for Server-side Web Processing

• ActiveX DLL
• Code module that is stored on the Web server
• Runs in the Web servers memory space
• Can be used only with Microsoft Web servers

31
Using ActiveX DLLs for Server-side Web Processing

• ActiveX DLLs have a significant advantage over
  CGI programs
• A single copy of an ActiveX DLL can service an
  unlimited number of user requests without
  starting additional copies of the program

32
Differences Between ActiveX DLLs and CGI Programs

• Unlike a CGI program, an ActiveX DLL
• Is not a stand-alone program
• Comprises one or more procedures that must be
  called from another program
• Must be entered into the Registry of the Web
  server where it will be used, so that other
  programs will be able to call the procedures
  within the DLL

33
Differences Between ActiveX DLLs and CGI Programs

• When an ActiveX DLL is called from an ASP, you
  must explicitly add the code to send the form
  inputs as parameters from the ASP to the DLL in
  the ASP command that calls the ActiveX DLL
  procedure
• When an ActiveX DLL is called from an ASP, it
  returns outputs to the calling ASP script either
  as a value returned by a function, or by changing
  the values of variables that were passed to the
  ActiveX DLL as input parameters from the calling
  program

34
Creating an ActiveX DLL Using Visual Basic

• A VB ActiveX DLL consists of a class module that
  contains multiple functions or procedures that
  can be called by programs that link to the DLL
• Class
• Template from which an object is created

35
Registry Changes Made When an ActiveX DLL is
Compiled

• Normally, Registry entries are made by programs
  when they are installed or modified
• Registry stores information such as
• The location of program files
• User preferences such as the toolbars that are
  displayed in a program

36
Registry Changes Made When an ActiveX DLL is
Compiled

• To view Registry entries on a Windows95,
  Windows98, Windows NT, or Windows 2000
  workstation, use a utility named REGEDIT
• REGEDIT enables users to view and change Registry
  entries
• Normally, you never need to modify the Registry
  manually

37
Registry Changes Made When an ActiveX DLL is
Compiled
Figure 9-41 Registry Editor
38
Registry Changes Made When an ActiveX DLL is
Compiled

• Key
• Parent item that can be a folder, or variable
  that has an associated value
• Value
• Text string that is associated with a key
  variable
• A key folder can have a default value
• Value that is associated with the key folder if a
  query does not specify the name of a key within
  the key folder

39
Retrieving Database Data in an ActiveX DLL

• You can an ActiveX DLL to perform operations that
  you cannot perform in an ASP
• You can also link an ActiveX DLL to an ASP to
  perform database operations
• While database operations can be performed
  directly within the ASP script code, you can
  enhance the execution speed of the ASP if you
  perform some operations using a DLL, since the
  DLL is compiled prior to execution

40
Retrieving Database Data in an ActiveX DLL

• When an ActiveX DLL is called from another
  program, the DLL remains in the Web servers
  memory until the Web server is rebooted
• You cannot recompile the DLL, because the DLL is
  loaded into memory, and is marked as being in use
  by the operating system
• To recompile a DLL that has been called by a
  program, you must reboot your computer and
  recompile the DLL

41
Passing Form Input Parameters to an ActiveX DLL

• ISAPI enables you to create DLLs that can be
  called directly by the Web server
• When you call an ActiveX DLL from an ASP, the
  ActiveX DLL has no built-in way to directly read
  form inputs
• The alternative is to use the Request.Querystring
  property in the ASP to retrieve the form inputs
  in the ASP, and then send the parameters to the
  ActiveX DLLs procedure as VB procedure parameters

42
Passing Form Input Parameters to an ActiveX DLL

• When creating ActiveX DLLs in VB to generate
  dynamic Web pages
• Create functions within the ActiveX DLL that
  return
• A text string containing all HTML commands
• Formatted texts
• Retrieved data values to be displayed by the
  users browser

43
Passing Form Input Parameters to an ActiveX DLL

• When creating ActiveX DLLs in VB to generate
  dynamic Web pages (cont.)
• Create an Active Server Page to
• Call functions within an ActiveX DLL
• Pass input parameter values to the DLL functions
• Display the function return value, which is a
  text string that represents a formatted Web page

44
Passing Form Input Parameters to an ActiveX DLL

• When creating ActiveX DLLs in VB to generate
  dynamic Web pages (cont.)
• To retrieve database data in an ActiveX DLL
• Create an ADODB connection object to reference a
  database
• Use the recordset Executive method to manipulate
  data using SQL commands

45
Chapter Summary

• Common Gateway Interface (CGI) is a protocol that
  specifies how a Web server communicates with
  programs running on the Web server
• CGI is usually used to process data that are
  entered into HTML forms using STDIN and STDOUT
• STDIN and STDOUT are memory locations on a
  computer that define a standard way for different
  programs to share the values of inputs and outputs

46
Chapter Summary

• A CGI program can be written in any language
  that can be read from STDN, write data to STDOUT,
  and read environment variables
• CGI programs can use URL parameters, form
  parameters, or cookies to share data values with
  other CGI programs
• CGI programs can run on most Web servers, and on
  a variety of operating systems

47
Chapter Summary

• The main drawback of CGI programs is that they do
  not use Web server resources efficiently
• Dynamic-link libraries (DLLs) are collections of
  compiled code modules that are called from other
  programs
• ActiveX DLLs have a significant advantage over
  CGI programs, since a single copy of an ActiveX
  DLL can service an unlimited number of user
  requests without starting additional copies of
  the program