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Server-Side Web Programming

with Active Server Pages

ASP Applications

Introduction

•^

In Microsoft literature, an ASP application is oftendefined as one that uses the ASP Applicationand/or Session objects.

-^

ASP applications can consist of three parts:^ –

Application Object

: provides global variables,

constants, objects.

-^

Session Object

: provides variables, constants, and

objects specific to each user's session.

• Global.asa: Can initiate application and session

variables, constants and objects

ASP Application Object Example^ • In any ASP page:^ Application("TestVar") = "Test Value"^ … • In the same or a different ASP page:^ Response.write Application("TestVar")^ Returns:

"Test Value"

ASP Application Object

•^

An ASP Application object is instantiated the firsttime a client requests

ANY

ASP page in the ASP

application.

-^

IIS installs a default ASP application in the rootdirectory of the server.

-^

Using IIS, separate ASP applications can becreated using the Internet Services Manager.

Application Object Variables

What can be stored in the Application Object?

• Simple variables (stored as variants!)– Variant type arrays– Variable references to COM objects (e.g., ADO) -^

Common real-life uses:^ – Data caching of small data sets that are commonly used

but don't change often.

• User counters.– Critical values to all pages.

Application Object Variables

•^

Referencing a non-existent application variablereturns an empty value, not an error (similar toRequest collection)

-^

Avoid repeated calls of same applicationvariable—write to a local variable.

-^

Since all pages/users have access to theapplication object's variables, carefully managewhen/where variables are written.

-^

Use the Application.Lock and Application.Unlockmethods to avoid concurrency problems.

Application Object

•^

Limit your use of application level objects – theytake up server memory that won't be released.

-^

Application-scope variables can be instantiated inany ASP page or by adding code to theApplication_OnStart event of the ASPapplication's global.asa file.

-^

Global.asa file not required to use manyApplication object capabilites.

Application Object Initialization •^

Global.asa file includes an Application_OnEndevent that occurs when the ASP application isunloaded or wthe web service is gracefullystopped.

-^

No guarantee this code will run if web server orthe application crashes.

Global.asa File

•^

You must use IIS to transform a directory into anASP Application if you want to use a separateglobal.asa. Otherwise, application object willdefault to the root global.asa.

Sample Global.asa File

Example

•^

Using the previous global.asa in the

/demo

directory, you would get the result of "MyValue"for

response.write Application("MyVariable")

on

both

/demo1/default.asp

and

/demo1/subdir1/default.asp

-^

If the global.asa in subdir2 is changed to: Application("MyVariable")="NewValue"

, then

/demo1/subdir2/default.asp

would return

"NewValue", but the other pages outside of /demo1/subdir

would still return "MyValue"

ASP Session Object

•^

Each user session is assigned a unique SessionIDvalue that is passed via cookies.

-^

Different SessionIDs are assigned for differentASP applications.

-^

SessionIDs are unique for a single user/applicationsession, but may or may not change betweensessions based upon the client's cookie support.

ASP Session Object

•^

Advantages of sessions:^ – Simplify management of user state information (don't

need to mess with cookies)

• Above advantage especially useful in an Intranet with

finite users and known browser support.

•^

Disadvantages of sessions include:^ – Resource intensive for server (overhead to track sessions)– Don't scale well for large number of users (memory

needed for each session)

• Very difficult to scale in server farms.– Sessions don’t scale well on multi-processor machines.

(thread affinity)