9 Chapter Nine Compiled Web Server Programs. 9 Chapter Objectives Learn about Common Gateway Interface (CGI) Create CGI programs that generate dynamic

9 ChapterNine

Compiled Web

Server Programs

9 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

9 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

9 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

9 Common Gateway Interface

Figure 9-1: CGI processing architecture

9 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

9 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

9 Writing a CGI Program Using Visual Basic

• Visual Basic can be used to create CGI programs– VB doesn’t directly support reading from STDIN and

STDOUT

– To create CGI programs using VB, use a VB standard module named cgi.bas

Module contains declarations of Windows API functions that enable VB to read from STDIN and write to STDOUT

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

9

9 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


Figure 9-10: String concatenation error


• 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


• 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


FIGURE 9-13: Code window with execution paused at breakpoint


• 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


Figure 9-14: Query to display variable value in the Immediate window

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9 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 (&)


Figure 9-16: Code to create hyperlinks with URL parameter values


Figure 9-17: Item ID hyperlink URL parameter value

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9 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

9 Sharing Data Values with Other CGI Programs

• Cookie– Data file that is written on the user’s

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

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9 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


• 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


• 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

server’s 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

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9 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


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


• 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

9 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

9 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

9 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

9 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


• 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


Figure 9-41: Registry Editor


• 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

9 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

9 Retrieving Database Data in an ActiveX DLL

• When an ActiveX DLL is called from another program, the DLL remains in the Web server’s 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

9 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 DLL’s procedure as VB procedure parameters


• 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 user’s browser


• 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


• 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

9 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

9 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

9 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

Documents

9 Chapter Nine Compiled Web Server Programs. 9 Chapter Objectives Learn about Common Gateway Interface (CGI) Create CGI programs that generate dynamic