Saturday, October 27, 2007

Cache

(B) What is application object ?
Application object can used in situation where we want data to be shared across users globally.
(I)What’s the difference between Cache object and application object ?
The main difference between the Cache and Application objects is that the Cache object provides cache-specific features, such as dependencies and expiration policies.
(I)How can get access to cache object ?
The Cache object is defined in the System.Web.Caching namespace. You can get a reference to the Cache object by using the Cache property of the HttpContext class in the System.Web namespace or by using the Cache property of the Page object.
(A)What are dependencies in cache and types of dependencies ?
When you add an item to the cache, you can define dependency relationships that can force that item to be removed from the cache under specific activities of
dependencies.Example if the cache object is dependent on file and when the file data changes you want the cache object to be update.Following are the supported dependency
:-
√ File dependency :- Allows you to invalidate a specific cache item when a disk
based file or files change.
√ Time-based expiration :- Allows you to invalidate a specific cache item
depending on predefined time.
√ Key dependency :-Allows you to invalidate a specific cache item depending
when another cached item changes.
(P)Can you show a simple code showing file dependency in cache ?
Partial Class Default_aspx
Public Sub displayAnnouncement()
Dim announcement As String
If Cache(“announcement”) Is Nothing Then
Dim file As New _
System.IO.StreamReader _
(Server.MapPath(“announcement.txt”))
announcement = file.ReadToEnd
file.Close()
Dim depends As New _
System.Web.Caching.CacheDependency _
(Server.MapPath(“announcement.txt”))
Cache.Insert(“announcement”, announcement, depends)
End If
Response.Write(CType(Cache(“announcement”), String))
End Sub
Private Sub Page_Init(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Init
displayAnnouncement()
End Sub
End Class
Note :- Above source code can be obtained from CD in “CacheSample”
folder.”Announcement.txt” is in the same folder which you can play around to see the
results.
Above given method displayAnnouncement() displays banner text from Announcement.txt file which is lying in application path of the web directory.Above method first checks is the Cache object nothing , if the cache object is nothing then it moves further to load the cache data from the file.Whenever the file data changes the cache object is removed and
set to nothing.
(A) What is Cache Callback in Cache ?
Cache object is dependent on its dependencies example file based , time based etc.Cache items remove the object when cache dependencies change.ASP.NET provides capability to execute a callback method when that item is removed from cache.


(B) What are different types of caching using cache object of ASP.NET?
You can use two types of output caching to cache information that is to be transmitted to
and displayed in a Web browser:
√ Page Output Caching
Page output caching adds the response of page to cache object.Later when
page is requested page is displayed from cache rather than creating the
page object and displaying it.Page output caching is good if the site is
fairly static.
√ Page Fragment Caching
If parts of the page are changing, you can wrap the static sections as user
controls and cache the user controls using pagefragment caching.
(B) How can you cache different version of same page using ASP.NET
cache object ?
Output cache functionality is achieved by using “OutputCache” attribute on ASP.NET
page header.Below is the syntax
<%@ OutputCache Duration="20" Location="Server" VaryByParam="state" VaryByCustom="minorversion" VaryByHeader="Accept-Language"%>
√ VaryByParam :- Caches different version depending on input parameters send
through HTTP POST/GET.
√ VaryByHeader:- Caches different version depending on the contents of the
page header.


(A) How will implement Page Fragment Caching ?
Page fragment caching involves the caching of a fragment of the page, rather than the
entire page. When portions of the page need to be dynamically created for each user
request this is best method as compared to page caching.You can wrap Web Forms user
control and cache the control so that these portions of the page don’t need to be recreated
each time.
(B) What are ASP.NET session and compare ASP.NET session with
classic ASP session variables?
ASP.NET session caches per user session state.It basically uses “HttpSessionState” class.
Following are the limitations in classic ASP sessions :-
√ ASP session state is dependent on IIS process very heavily.So if IIS restarts
ASP session variables are also recycled.ASP.NET session can be independent
of the hosting environment thus ASP.NET session can maintained even if IIS
reboots.
√ ASP session state has no inherent solution to work with Web Farms.ASP.NET
session can be stored in state server and SQL SERVER which can support
multiple server.
√ ASP session only functions when browser supports cookies.ASP.NET session
can be used with browser side cookies or independent of it.
(B) Which various modes of storing ASP.NET session ?
√ InProc:- In this mode Session state is stored in the memory space of the
Aspnet_wp.exe process.This is the default setting.If the IIS reboots or web
application restarts then session state is lost.
√ StateServer:-In this mode Session state is serialized and stored in a separate
process (Aspnet_state.exe); therefore, the state can be stored on a separate
computer(a state server).
√ SQL SERVER:- In this mode Session state is serialized and stored in a SQL
Server database.
Session state can be specified in element of application configuration file.Using State Server and SQL SERVER session state can be shared across web farms but note this comes at speed cost as ASP.NET needs to serialize and deserialize data over network again and again.
(B) What are the other ways you can maintain state ?
Other than session variables you can use the following technique to store state :
√ Hidden fields
√ View state
√ Hidden frames
√ Cookies
√ Query strings
(B) What are benefits and Limitation of using Hidden fields ?
Following are the benefits of using Hidden fields :-
√ They are simple to implement.
√ As data is cached on client side they work with Web Farms.
√ All browsers support hidden field.
√ No server resources are required.
Following are limitations of Hidden field :-

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