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ASP Tutorial: ASP Session Object

ASP Session Object

The Session object is used to store information
about, or change settings for a user session. Variables stored in the Session
object hold information about one single user, and are available to all pages in
one application.



The Session object


When you are working with an application, you open it, do some
changes and then you close it. This is much like a Session. The computer knows
who you are. It knows when you start the application and when you end. But on
the internet there is one problem: the web server does not know who you are and
what you do because the HTTP address doesn't maintain state.


ASP solves this problem by creating a unique cookie for each
user. The cookie is sent to the client and it contains information that
identifies the user. This interface is called the Session object.


The Session object is used to store information about, or
change settings for a user session. Variables stored in the Session object hold
information about one single user, and are available to all pages in one
application. Common information stored in session variables are name, id, and
preferences. The server creates a new Session object for each new user, and
destroys the Session object when the session expires.



When does a Session Start?


A session starts when:



  • A new user requests an ASP file, and the Global.asa file
    includes a Session_OnStart procedure
  • A value is stored in a Session variable
  • A user requests an ASP file, and the Global.asa file uses
    the <object> tag to instantiate an object with session scope



When does a Session End?


A session ends if a user has not requested or refreshed a page
in the application for a specified period. By default, this is 20 minutes.


If you want to set a timeout interval that is shorter or
longer than the default, you can set the Timeout property.


The example below sets a timeout interval of 5 minutes:









<%
Session.Timeout=5
%>

To end a session immediately, you may use the Abandon
method:









<%
Session.Abandon
%>

Note: The main problem with sessions is WHEN they
should end. We do not know if the user's last request was the final one or not.
So we do not know how long we should keep the session "alive". Waiting
too long for an idle session uses up resources on the server, but if the session
is deleted too soon the user has to start all over again because the server has
deleted all the information. Finding the right timeout interval can be
difficult!


Tip: If you are using session variables, store SMALL
amounts of data in them.



Store and Retrieve Session Variables


The most important thing about the Session object is that you
can store variables in it.


The example below will set the Session variable username
to "Donald Duck" and the Session variable age to
"50":









<%
Session("username")="Donald Duck"
Session("age")=50
%>

When the value is stored in a session variable it can be
reached from ANY page in the ASP application:









Welcome <%Response.Write(Session("username"))%>

The line above returns: "Welcome Donald Duck".


You can also store user preferences in the Session object, and
then access that preference to choose what page to return to the user. 


The example below specifies a text-only version of the page if
the user has a low screen resolution:









<%If Session("screenres")="low" Then%> 
  This is the text version of the page
<%Else%> 
  This is the multimedia version of the page
<%End If%>





Remove Session Variables


The Contents collection contains all session variables.


It is possible to remove a session variable with the Remove
method.


The example below removes the session variable
"sale" if the value of the session variable "age" is lower
than 18:









<%
If Session.Contents("age")<18 then 
  Session.Contents.Remove("sale")
End If 
%>

To remove all variables in a session, use the RemoveAll
method:









<%
Session.Contents.RemoveAll()
%>





Loop Through the Contents Collection


The Contents collection contains all session variables. You
can loop through the Contents collection, to see what's stored in it:









<%
Session("username")="Donald Duck"
Session("age")=50

dim i
For Each i in Session.Contents
  Response.Write(i & "<br />")
Next
%>

Result:









username
age

If you do not know the number of items in the Contents
collection, you can use the Count property:









<%
dim i
dim j
j=Session.Contents.Count
Response.Write("Session variables: " & j)
For i=1 to j
  Response.Write(Session.Contents(i) & "<br />")
Next
%>

Result:









Session variables: 2
Donald Duck
50





Loop Through the StaticObjects Collection


You can loop through the StaticObjects collection, to see the
values of all objects stored in the Session object:









<%
dim i
For Each i in Session.StaticObjects
  Response.Write(i & "<br />")
Next
%>



Posted by JONAS