ASP 3.0 The Complete Reference
Chapter 9
Creating a Context for ASP Pages
So far, all of the ASP examples we have shown have been
restricted to a page scope. This means that the pages were unable to share
information with other users of the application or with information specific to
a user. This restriction would make it extremely difficult to develop an
interactive web applications. Fortunately, ASP overcomes this, ASP by providing
the Application
and Session
objects to establish a context that goes beyond the scope of the page.
The Application Object
The Application object provides a common context for all
of the pages and users of an ASP application by providing access to shared data
(sort of like global variables) and providing event handlers for the start and
stop of the application. Shared resources are made available in two
collections: Application.Contents
and Application.StaticObjects.
Both of these can be accessed using the default method as in the Request collection. The Application.Contents
collection contains data elements that are assigned to the application during
runtime. The StaticObjects
collections contain object references that are created in the global.asa
file with the <object>
tag and set to application scope.
Although there is a global application in the default web
site, we can easily create an application space that is separate and customized
to the requirements of an individual application. There are two steps to
setting up an application: first by configuring IIS and second by implementing
a global.asa
file.
Whether you are using a virtual directory or a virtual web
site for your application, the configuration is the same. We will begin by
opening the properties dialog for either the virtual directory or the web site.
Under the directory tab you will see the following:
Figure 1:
IIS Configuration
If your application has not yet been configured, you will
see the create button in the dialog box. To create an application, click the
button and give the application a unique name. Once this is set, click the
configuration button to set other application parameters.
Figure 2:
IIS Configuration
Under the App options tab, you will find settings for
your application. For most applications, the default settings are fine.
However, if you are standardizing on JScript instead of VBScript, you will want
to change this setting.
Application variables can be written to and read from by
simply accessing the variable name in the Application objects default Contents collection. New variables can
be added to the application at any time. In the example below, we start with a
simple form that allows someone to post a news item for the day. An ASP page
receives the post and saves it to an Application variable.
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news_post.html - HTML Form
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<html>
<head><title>News
Post Form</title></head>
<body>
<b>News
Post Form</b><HR color=CCCCCC>
<form
id=myForm action="news_post.asp" method="post"
style="margin:0px;">
<textarea
cols=40 rows=4 name="news"></textarea>
<HR
color=CCCCCC>
<input
type="submit" value="submit"><input
type="reset" value="reset">
</form></body></html>
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Figure 3:
News Post Form
The form data is submitted to the following page and saved
in an application variable along with a date stamp.
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news_post.asp - VBScript ASP
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<%
Application("news") =
Request.Form("news")
Application("news_date")
= Date
Server.Transfer("news_display.asp")
%>
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Next, we jump to a second asp page that reads the current
news item from the application and displays it to the user.
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news_display.asp - VBScript ASP
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<html>
<head><title>Application
Variable Sample</title></head>
<body>
<b>News
Bulletin</b> - <%= Application("news_date")
%><hr color=CCCCCC>
<%=
Application("news") %>
<hr
color=CCCCCC>
</body></html>
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Figure 4:
News Bulletin from Application Variable
Of course, this is a rather simplistic example. You will
probably want to save application data to a more permanent location in addition
to the memory resident application variable. The reason being that if the
application is shutdown, all the application variables are lost.
All the basic datatypes of both VBScript and JScript are
fair game for Application variables. When considering storing objects however,
a number of warning alarms should go off. The good news is that JScript objects
can be used freely. Objects derived from VBScript classes, however, cannot be
saved in the application context. The reason for this is due to the fact that
these objects reside within the instance of the scripting engine serving the
page. When the page is complete so is the object. To protect you, IIS 5 will
balk if you attempt to save VBScript objects into the Application context.
Objects derived from COM components are a whole new issue.
Any of these objects can be saved to an application variable, however, if the
object has not been designed for this purpose, it can restrict your application
to only being able to handle one request at a time. For more on the issues that
contribute to this, see the threading section of the previous Scripting with
Objects chapter.
One final restriction, you cannot
save an intrinsic object to an application variable. For instance, the
following is illegal: Application(response)=Response.
Preventing concurrency problems.
Since we are in a multi-user application environment, it
is possible that two pages could be trying to access the same application
variable at the same time. Imagine the following. Two users hit a page in
separate threads just milliseconds apart. The first users page reads the value
from the application variable to increment it by 1. The second page reads the
same value and increments it. The first page now writes the incremented value
back to the variable and the second page writes the same value back to the
variable. So two users have hit the page, but only one hit has been recorded
because of the synchronicity and concurrency problem. For this reason, the Application object provides a Lock method. By using this method,
only the person that has locked the application can have access to the
application variables until the Application.Unlock
is called. The following demonstrates how the counter can work with locking.
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application_lock.asp - VBScript ASP
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<%
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