State Management

The State or Cache Management is nothing but the way to storing the data in Client-Side and in Server-Side using preity small memory.
There are two major category of the above :

I. Server-Side State Management

Session State:

Its nothing but defined as a period of time shared between the web application and user. Every user has individual session. Items/Objects can be placed into the Session which would only define these object for that user. Session contains key variables which help to identify the related values. This can be thought of as a hash table. Each user would represent a different key node in the hash identifying unique values. The Session variables will be clear by the application which can clear it, as well as through the timeout property in the web config file. Usually the timeout is 20 minutes by default.

Session Variables are stored on the server, can hold any type of data including references, they are similar to global variables in a windows application and use HTTP cookies to store a key with which to locate user's session variables.

The collection of session variables is indexed by the name of the variable or by an integer index. Session variables are created by referring to the session variable by name. You do not have to declare a session variable or explicitly add it to the collection.

Lets get it cleared from following example:

Session[“firstName”] = “Gaurav” //User’s first name
Session[“lastName”] = “Arora” //User’s last name

// Clear the session variable
Session[“FirstName”] = null;

//Clear all Session variables
Session.Abandon();

InProc—Stores Session state in the same process as the ASP.NET process [aspnet_wp.exe].

StateServer—Stores Session state in a Windows NT process, which is distinct from the ASP.NET process[aspnet_state.exe].

SQLServer—Stores Session state in a SQL Server database.

Both in StateServer and SQLServer options, we need to ensure that the objects we cache are serializable as data storages are out-of-process systems. Both these options have impact on the application performance as data retrieval and saving operations take more time when compared to the InProc option. So based on our application requirement we should choose the option that best suits our requirement.

Note:
By default, ASP.NET session state is enabled for all ASP.NET applications.

ASP.NET application object

ASP.NET provides an object called Application object to store data that is accessible to all user requests. The life span of this object is tied to the application and it is re-created every time the application starts. Unlike ASP.NETSession object this object is accessible to all user requests. Since this storage is created and maintained in an application domain space, this should not be used for data storage in a web farm scenario. This option is very useful to store data like the application metadata (CONFIG files data) that can be loaded to the Application object during application start up and can be used during the life of the application without reloading it every time for each user request. But if there is a requirement to invalidate the cached data whenever there is any change to the CONFIG files while the application is running, this option should not be used as it doesn't provide any feature to expire the cached data. So in this case other options like the ASP.NET Cache object should be used, which is explained below.

Types of Cache –Dependencies

When anyone add an item to cache, he/she can define the dependency relationships that can force that item to be removed from the cache under specific activities of dependencies.

For example: If the cache is dependent on file and when the file data changes you want the cache object to be updated.

Following are the difference dependencies:
(a) File Dependency :
Allows invalidating a specific cache item when a disk based file or files change.

object errorData;
//Load errorData from errors.xml

CacheDependency fileDependency =
new CacheDependency(Server.MapPath("errors.xml"));
Cache.Insert("ERROR_INFO", errorData, fileDependency);

(b) Time based expiration :
Allows to invalidate a specific cache item depending on predefined time.

//Absolute Expiration

Cache.Insert("EMP_NAME", "Shubhabrata", null,
DateTime.Now.AddDays(1), Cache.NoSlidingExpiration);

//Sliding Expiration

Cache.Insert("EMP_NAME", "Shubhabrata", null,
Cache.NoAbsoluteExpiration, TimeSpan.FromSeconds(60));

string[] relatedKeys = new string[1];
relatedKeys[0] = "EMP_NUM";
CacheDependency keyDependency = new CacheDependency(null, relatedKeys);
Cache["EMP_NUM"] = 5435;
Cache.Insert("EMP_NAME", "Shubhabrata", keyDependency);
Cache.Insert("EMP_ADDR", "Bhubaneswar", keyDependency);
Cache.Insert("EMP_SAL", "5555USD", keyDependency);

.NET remoting

You might be thinking how .NET remoting can be used for data caching? The same question came to my mind when I heard about it for the first time. As you know the .NET remoting singleton object shares the same instance with multiple clients so singleton objects can be used to store and share data between different client invocations. Since .NET remoting can be used outside the process and machine, this option is very useful when we want to cache data and share it across servers and users particularly in a web farm scenario. In this approach we can store the data as member variables of singleton remoting object and provide methods to read and save data. But while implementing this we need to ensure that the remoting object used as cache is not destroyed by the garbage collector. For that we will have to set the remoting cache object's lease period to infinite so that the lease period never times out. We can do that by overriding the InitializeLifetimeService method of MarshalByRefObject and return null from this method. But the only issue with this approach is performance. As the analysis says the performance is very poor in this approach when compared to other approaches. Anyway it is up to the application designers/developers to analyze different options and use the best one that suits the requirement.

Memory-mapped files

You all know what a memory-mapped file is. It is basically about mapping a file on disk to a specific range of addresses in the application's process address space. This option allows different processes to use the same data by increasing the application performance. As using memory-mapped file is not very popular among .NET developers, I would personally not suggest this approach as implementing this involves a lot of complexities and also .NET Framework doesn't support this. But if anyone is very much interested in using this approach then they will have to develop their own custom solution as per their own requirement.

Static variables

We use static variables for storing data or objects globally so that it can be accessed during the life of the application. Similarly, in ASP.NET we can use static objects for caching data and we can also provide methods to retrieve and save data to the cache. As static variables are stored in the process area, performance wise it is faster. But since it is very difficult to implement expiration policies and dependencies incase of static variables, I generally prefer ASP.NET cache object over this option. Another problem is that the custom static cache object has to be thread-safe which has to be implemented carefully.

Example:

public class CustomCache
{
//Synchronized to implement thread-safe

static Hashtable _myCache =
Hashtable.Synchronized(new Hashtable());

public static object GetData(object key)
{
return _myCache[key];
}

public static void SetData(object key, object val)
{
_myCache[key] = val;
}
}

II. Client-Side State Management

Cookies

Cookie is a very familiar term in web development environment. Cookie is a client-side storage that is sent to the server for each request and also received as response back from the server. Because of its size limitation (4096 bytes) it should be used for storing small amount of data. Expiration policies can be set for cookies to invalidate the items after a certain period of time. The following example shows how Cookie can be used in an ASP.NET application:

if (this.Request.Cookies["MY_NAME"] == null)
{
this.Response.Cookies.Add(new HttpCookie("MY_NAME",
"Shubhabrata Mohanty"));
}
else
{
this.Response.Write(this.Request.Cookies["MY_NAME"].Value);
}

ViewState

ASP.NET ViewState is a new concept. Here the data related to the pages and controls are stored in ViewState which retains the values across multiple requests to the server. If you remember correctly, in VB-ASP applications we used to store data across multiple requests using hidden fields. ViewState is actually implemented internally as hidden fields in ASP.NET but here the data is hashed to improve security as against hidden fields. To see how ViewState is implemented, you can view the source of the page in which ViewState is used in the Internet browser. ViewState should not be used to store large amounts of data as it is passed to the server for each request

protected void Page_Load(object sender, EventArgs e)
{
if (this.ViewState["MY_NAME"] == null)
{
this.ViewState["MY_NAME"] = "Shuby";
}

//txtName is a TextBox control

this.txtName.Text = this.ViewState["MY_NAME"].ToString();
}

Hidden fields

Hidden fields are very popular among VB-ASP web developers. Hidden field is similar to any other control in a page but the visible state of this control is always false. Like ViewState we should not use it for storing large amounts of data.
Note:Similarly hidden frames can be used to cache data in the client side. But please note that hidden frames are not supported by all Internet browsers.

<asp:HiddenField ID="myHiddenField" Value="Shuby" runat="server" />


<input id="myHiddenField" type="hidden" value="Shuby" />

Query Strings

Query strings provide a simple but limited way of maintaining some state information.You can easily pass information from one page to another, But most browsers and client devices impose a 255-character limit on the length of the URL. In addition, the query values are exposed to the Internet via the URL so in some cases security may be an issue.

http://www.msdotnetheaven.com/?authorid=4

Example:

string authorid;
authorid=Request.Params[“authorid”];

Note: You can only use HTTP-Get method to post the web page, or you will never get the value from query strings.

.Net Stuff from My Desktop

Friday, January 30, 2009