Caching!!

What is Caching?

Caching is a technique of storing frequently used data/information in memory, so that, when the same data/information is needed next time, it could be directly retrieved from the memory instead of being generated by the application.

Caching is extremely important for performance boosting in ASP.Net, as the pages and controls are dynamically generated here. It is especially important for data related transactions, as these are expensive in terms of response time.

Caching places frequently used data in quickly accessed media like the random access memory of the computer. The ASP.Net runtime includes a key-value map of CLR objects called cache. This lives with the application and is available via the HttpContext and System.Web.UI.Page.

In some respect, caching is similar to storing the state objects. However, the storing information in state objects is deterministic, i.e., you can count on the data being stored there, and caching of data is nondeterministic.

The data will not be available if its lifetime expires, or the application releases its memory, or caching does not take place for some reason.

You can access items in the cache using an indexer and may control the lifetime of objects in the cache and set up links between the cached objects and their physical sources.

Types of Caching:

1. Output Caching

2. Fragment Caching

3. Data Caching

4. Substitution Caching.

Output Caching

Here the response of the request is cached for a predefined period and the same is reused for all subsequent requests for the same page.

Cache.aspx:

<%@ OutputCache Duration="10" VaryByParam="txtDemo" VaryByHeader="referer"

VaryByCustom="Browser" Location="ServerAndClient" NoStore="true"%>

<asp:Label ID="lblCurrentTime" runat="server"></asp:Label>

<asp:TextBox ID="txtDemo" runat="server"></asp:TextBox>

<asp:Button ID="btnDemo" runat="server" Text="Demo" />

Cache.aspx.cs

protected void Page_Load(object sender, EventArgs e)

{

    lblCurrentTime.Text = DateTime.Now.ToLongTimeString();

}

1. Duration and VaryByParam are the only required attributes and others are optional.

2. The value of "VaryByParam" can be either QueryString parameter, Cookie Name, Form element Name or Id of Control on the WebForm.

3. For every value of the parameter (parameters) mentioned in VaryByParam, a different cache copy of the response is maintained on the server and same is reused for those values of parameters for subsequent requests.

4. VaryByParam can be "none" or "*"(create separate cache copy if any parameter changes) or ";" separated list of parameters.

5. VaryByCustom="Browser" à For different browser types a different cache is created.

6. VaryByHeader = "Accept-language" à Different for every value of request header submitted along with the request.

7. Location:

Any Cache can be located on the browser (client) or Proxy or Server

None Cache is disabled everywhere

Downstream Cache can be located either on browser (client) or proxy.

Server Cache is located on the server

Client Cache is located on the browser (client).

ServerAndClient Cache on both Server and Client but not on Proxy

8. NoStore=”true” à page should not be cached on the client irrespective of the browser settings (By Default it is false).

Note: When Tracing is enabled on the page, response of the page is not cached

Note: Browser caches the response of the page based on the response header “Cache-Control” and it’s based on “Location” attribute of Output Cache directive.

To specify cache settings programmatically: (Comment the OutputCache directive in aspx page)

protected void Page_Load(object sender, EventArgs e)

{

    lblCurrentTime.Text = DateTime.Now.ToLongTimeString();

    Response.Cache.SetCacheability(HttpCacheability.ServerAndPrivate);

    Response.Cache.SetExpires(new DateTime(2009,12,31,10,20,30)); //For setting the absolute Data and            Time.

   //Response.Cache.SetMaxAge(new TimeSpan(0, 0, 10)); //For mentioning the relative Data and Time.    

    Response.Cache.VaryByHeaders["referer"] = true;

    Response.Cache.SetValidUntilExpires(true);

    Response.Cache.VaryByParams["txtDemo"] = true; //works only if SetValidUntilExpires is set to true

}

/Web.Config

<system.web>

<caching>

<outputCacheSettings>

<outputCacheProfiles>

<add name="CacheInConfig" enabled="true" duration="10" location="ServerAndClient"/>

</outputCacheProfiles>

</outputCacheSettings>

</caching>

</system.web>

In CacheDemo.aspx:

<%@ OutputCache VaryByParam="txtDemo" CacheProfile="CacheInConfig"%>

Changes made to profile(i.e OutputCacheProfile) are automatically effected to all the webforms.

Fragment Caching

Here instead of caching the complete response of the page, only a small portion of it will be cached.

Step1 Add the following to the webform

Server Time: <%= DateTime.Now.ToLongTimeString() %>

Step2: To the Project à Add New itemàWeb UserControl (CacheControl.ascx)

Step3: In CacheControl.ascx add the following

<%@ OutputCache Duration="10" VaryByControl="none" %>

Note: when the output Cache directive is being used in UserControl replace “VaryByParam” attribute with “VaryByControl” i.e for mentioning the Control Id, but for cookie / querystring / form element use “VaryByParam”

Step4: Add to UserControl

<%= DateTime.Now.ToLongTimeString() %>

Step5: Drag and drop UserControl in WebForm

Step6: Run the application and Refresh the webform to note that the time rendered from UserControl is not changing but the time rendered from webform is changing.

Substitution Caching

If the response of the page is cached, after first request, for subsequent requests the cached output is rendered. If the webform has a Substitution control, even if the cached copy is rendered, the return value of the MethodName of the substitution control is used by ASP.NET for rendering latest content.

Step1: Cache the response of the page (Default.aspx) using

<%@ OutputCache Duration="10" VaryByParam="none" %>

Step2: Add the following to the webform

Server Time: <%= DateTime.Now.ToLongTimeString() %>

Step3: Drag and Drop Substitution control from ToolBox onto the WebForm.

Step4: Set its MethodName property = "GetServerTime"

Step5: Add the method as below to the aspx.cs file

static string GetServerTime(HttpContext context)

{

    return DateTime.Now.ToLongTimeString();

}

The return value of the above function is rendered in position of Substitution Control.

Step6: Run the application to view the webform in webbrowser. Refresh the page to note that overall response of the page is cached but latest time (return value of GetServerTime) is rendered in place of substitution control.

Data Caching

Instead of caching the response of the page we now cache the data used in making the response of the page. The cached data can be then reused for subsequent requests by any client for any WebForm i.e. the cached data is global.

When the memory is insufficient the Items in data cache automatically expires.

Step1: Add to the project a new webform - DataCacheDemo.aspx

Step2: Add to .aspx file the following two lines:

Server Time: <%= DateTime.Now.ToLongTimeString() %> <br />

Time From Cache: <asp:Label ID="lblTime" runat="server"/>

Step3: In Page_Load add the following:

if (Cache["dt"] == null)

Cache["dt"] = DateTime.Now;

DateTime dt = (DateTime)Cache["dt"];

lblTime.Text = dt.ToLongTimeString();

Step4: Run the webform and refresh it multiple times…

Note that the Server time is changing but the cached time is not changing.

To set expiration policy to expire the data cache

a) Cache Item dependency on file.

Step5: Append to .aspx file the following

<asp:Label ID="lblDemo" runat="server"/>

Step6: Project à Right clickà Add New itemàText File (Demo.txt) – Add some text to it

Step7: Top of .aspx.cs file add the following

using System.Web.Caching;

using System.IO;

Step8: Append the following to Page_Load

if (Cache["file"] == null)

{

    lblDemo.Text = "Fetched from file: ";

    string pp = MapPath("~/Demo.txt"); //To convert Virtual path to physical path

    StreamReader sr = new StreamReader(pp);

    CacheDependency dep = new CacheDependency(pp);

    Cache.Insert("file", sr.ReadToEnd(), dep); //The cache item is dependent on file and if file changed the      cached

    item immediately expires.

    sr.Close();

}

else

    lblDemo.Text = "Fetched from Cache: ";

    lblDemo.Text += (string)Cache["file"];

    Step9: Run the webform, first time the content if fetched from file. On refresh it fetched from Cache.

    Step10: In Studio, open Demo.txt and modify…

    Step11: Refresh the webform to note that again the content is fetched from file.

b) Cache Item dependency on Absolute Time.

Step12: Add the following to .aspx file

<asp:Label ID="lblTime1" runat="server"></asp:Label>

Step13: Append the following to Page_Load

if (Cache["dt1"] == null)

Cache.Insert("dt1", DateTime.Now, null, DateTime.Now.AddSeconds(10), Cache.NoSlidingExpiration);

lblTime1.Text = ((DateTime)Cache["dt1"]).ToLongTimeString();

Step14: Run the WebForm, Keep refreshing…Note that every 10 seconds the label shows new server time.

c) Cache Item dependency on Sliding Expiration

If the data cache remains unused for a pre defined period the data must expire

Step15: Add the following to .aspx file

<asp:Label ID="lblTime2" runat="server"></asp:Label>

Step16: Append the following to Page_Load

if (Cache["dt2"] == null)

Cache.Insert("dt2", DateTime.Now, null, Cache.NoAbsoluteExpiration, new TimeSpan(0,0,10));

lblTime2.Text = ((DateTime)Cache["dt2"]).ToLongTimeString();

Step17: Run the WebForm, Keep refreshing…Note that even after 10 seconds the label shows cached time.

Step18: Refresh the form after waiting for 10 secs…this time the label shows server time because after 10 seconds from last request the cached item “dt2” expired.

Additional Methods:

Response.AddCacheItemDependency("dt"); //Response of the page in output cache expires if the data item “dt” expires

Response.AddCacheItemDependencies; // Cache item is dependent on Array of data items

Response.AddCacheDependency;

Response.AddFileDependencies; // Output Cache dependent on array of files

Response.AddFileDependency; // Output Cache item is dependent on single file

About Author
Rachna Bagwe works with Systems Plus and is working on Dot Net technology projects. She can be contacted at:rachna.b@spluspl.com