Multiple Approaches to State Management

1
Managing State

• Multiple Approaches to State Management

2
HTTP, Web are Stateless

• Remember that once the server generates the HTML
  for a single page and sends it to the client to
  render, the server-side app ends leaving no
  program running behind the page.
• Thus, nothing is running to save the state of
  the application. State describes the set of
  values of variables, properties, and other
  entities that the program uses to manage its
  functionality.

3
Web is Stateless

• The stateless nature of the web is intentional in
  a sense. Web servers are intended to be scalable
  to be able to serve requests for large numbers
  of users without significant performance
  degradation
• Not devoting extensive resources (memory,
  processor time, etc.) to managing state for all
  client allows for greater scalability. A web
  server may have thousands of clients at a time.

4
Managing State in a Stateless World

• It is up to the developer with help from
  ASP.NET - to preserve whatever state is
  essential.
• Web Form controls have built-in state management.
  The state of a control is preserved over a
  post-back to the same Page.
• ASP.NET supports four different approaches for
  the developer to manage other types of state such
  as the value of a counter or name of a file.

5
State Management in ASP.NET

• ASP.NET supports 4 approaches to state management
• ViewState Page maintains state of its .NET
  controls not useful for cross-page state
• SessionState individual user information
  relating to entire session
• ApplicationState useful for read-only global
  application settings. Typically better to use
  configuration files or cache instead.
• Cookies maintained on client machine by client
  browser may be transient or permanent

6
Application State

• Application State is designed to be used to
  manage state that applies to the entire
  application across all of the users of the
  application.
• Since there could be thousands of users, saving
  and retrieving application state can be resource
  intensive.
• It is also complex to ensure that the state is
  valid with so many users updating it.
• May be initialized in the Application_Start event
• Must use Application.Lock/Unlock when changing
  state (changing Application State is strongly
  discouraged)
• Dangerous because many clients may be accessing
  same item at one time.
• Inefficient and care required to prevent
  corrupted data.
• Application State is not maintained across
  machines/processors in web farm or web garden.
• Better to use cache than Application State.

7
Session State

• Session state is used to maintain per user
  information across a session
• The session state is maintained with an ID that
  is difficult to guess.
• The session ID is passed via cookies by default.
• Can be accessed via the Session property of each
  Page.
• Can be initialized in Session_Start event handler
  if appropriate.

8
Using Session State

• The Session State is maintained as a bag (a
  collection object) of ordered pairs.
• One item of each pair is the name assigned to
  the value.
• The other item is the value associated with the
  name.
• Syntax is
• Session (Name) value
• Since each value in the Session State collection
  is of type System.Object, one must cast the value
  to the appropriate type when it is retrieved
  before it can be used.
• nCount CType (Session (Counter), Integer)

9
Session State in Code
10
Session State in ASP.NET

• Can be configured to be maintained across
  multiple server machines in a web farm.
• Can be maintained across application
  shutdown/startup.
• Session ID is managed with cookies or without
  cookies based on configuration set in web.config
  in the sessionState section.

11
Session State Options
12
Sharing Session ID

• The ID of the session can either be passed from
  client to server and back to the server using
• a cookie
• URL mangling

13
Using URL Mangling to Pass Session Key
14
Session Key Management Comparisons

• Cookies are slightly more efficient because they
  avoid some round trips to the server.
• Cookies may not be supported by all clients.
• Mangled URLs work with all clients
• Mangled URLs only work with relative URLs
  rather than absolute URLs.
• An absolute URL is a fully qualified URL
  including the protocol (http, ftp, etc.) and
  all segments of the address starting with the
  server name or IP address
• A relative URL gives only the part of the address
  necessary relative to the current virtual
  directory (e.g. default.aspx) or the root
  directory of the application (e.g.,
  /app/subapp/default.aspx) for example.

15
Out-of-Process Session State

• It is possible to maintain the session state
  somewhere other than in memory within ASP.NET.
• In a system service (the ASP.NET State Server
  service) designed for the purpose
• In a SQL Server database
• In a byte stream
• Requires a round-trip to another server if not
  stored locally.

16
Out of Process Storage DB Server
17
Using Cookies

• Cookie is a small file stored on the client
  machine by the client browser
• Server requests client browser to set a cookie
  with a specific value in a response from the
  server to client
• Can be used to store preferences or user-specific
  information
• Client sends cookie info in subsequent requests
• Cookies may be set to expire in a given time
  period (say 5 minutes) or may be persistent for
  permanent record
• Cookies are small only 4096 bytes guaranteed by
  standards, so amount of data is limited
• Clients may disable cookies or prohibit
  persistent cookies.

18
Example Using a Cookie in ASP.NET
19
ViewState

• ViewState is a hidden field on each Page that
  stores page-specific information for a client
  across Post-Backs to same Page
• Data is not preserved between Pages
• Types in ViewState must be serializable

20
ViewState

• ViewState is similar to Session State in that it
  is a collection of name-value pairs, and the
  syntax of using it is similar to that for using
  Session State.
• ViewState hidden field is base64 encoded to
  minimize memory requirements.

21
ViewState Example

• Private nCount As Integer 0
• Private Sub Page_Load (ByVal sender As
  System.Object, _
• ByVal e As System.EventArgs) Handles
  MyBase.Load
• Me.lblHello.ForeColor System.Drawing.Color.Fro
  mArgb (rRan.Next)
• If Me.IsPostBack Then
• nCount CType (Me.ViewState ("Counter"),
  Integer)
• End If
• End Sub
• Private Sub btnColor_Click (ByVal sender As
  System.Object, _
• ByVal e As System.EventArgs) Handles
  btnColor.Click
• nCount 1
• Me.lblClickCounter.Text _
• String.Format ("The 'Click Me' button has been
  clicked 0 times.", nCount)

22
Comparison of State Types