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New Message on MumbaiUserGroup
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From: soniashish
Message 1 in Discussion
What is Caching?
Caching is a technique of persisting the data in memory for immediate access to
requesting program calls. Many in the developer community consider caching as one of
the features available to improve performance of Web applications.
Why Caching?
Consider a page that has list of Employee name, contact numbers and mail-Ids of
existing employees of a company on an intranet accessible by all employees. This is
very useful information that is available throughout the company and could also be one
of the most accessed pages. The functionality of adding, updating or deleting is
usually less intensive compared to more transaction-based systems like Purchase
ordering, Voucher creation etc. Now in a normal scenario the process of querying
database for each request is not cost-effective in terms of server resources, hence is
lot better to cache or persist the data to avoid this costly loss of resources.
The .NET Advantage
ASP.NET provides the flexibility in terms of caching at different levels.
Page Level Output Caching
This is at the page level and one of the easiest means for caching pages. This
requires one to specify Duration of cache and Attribute of caching.
Syntax: <%@ OutputCache Duration="60" VaryByParam="none" %>
The above syntax specifies that the page be cached for duration of 60 seconds and the
value "none" for VaryByParam* attribute makes sure that there is a single cached page
available for this duration specified. * VaryByParam can take various "key" parameter
names in query string. Also there are other attributes like VaryByHeader, VaryByCustom
etc. Please refer to MSDN for more on this.
Fragment Caching
Even though this definition refers to caching portion/s of page, it is actually
caching a user control that can be used in a base web form page. In theory, if you
have used include files in the traditional ASP model then this caching model is like
caching these include files separately. In ASP.NET more often this is done through
User Controls. Initially even though one feels a bit misleading, this is a significant
technique that can be used especially when implementing "n" instances of the controls
in various *.aspx pages. We can use the same syntax that we declared for the page
level caching as shown above, but the power of fragment caching comes from the
attribute "VaryByControl". Using this attribute one can cache a user control based on
the properties exposed.
Syntax: <%@ OutputCache Duration="60" VaryByControl="DepartmentId" %>
The above syntax when declared within an *.ascx file ensures that the control is
cached for 60 seconds and the number of representations of cached control is dependant
on the property "DepartmentId" declared in the control. Add the following into an
*.ascx file. Please note the use of tag "Control" and the cache declaration. <%@
Control Language="C#"%> <%@ outputcache duration="60" varybycontrol="DepartMentId" %>
<script runat="server"> private int _Departmentid=0; public int
DepartMentId { get{return _Departmentid;} set{_Departmentid =value;} }
//Load event of control void Page_Load(Object sender, EventArgs e) {
lblText.Text = "Time is " + DateTime.Now.ToString() + " for Department id = " +
_Departmentid + "\n"; } </script> <asp:Label id="lblText"
runat="server"></asp:Label> Add the following to an *.aspx file. Please note the way
"Register" tag is used; the declaration of control using syntax
<[TagPrefix]:[TagName]>; Usage of property
"DepartMentId". Open the page in two browsers and closely watch the Base form timing
and the User control timing. Also note that the following page results in two copies
or representation of user control in the cache. <%@ Page Language="C#" Trace="true" %>
<%@ Register TagPrefix="CacheSample" TagName="Text" Src="CachingControl.ascx" %>
<script runat="server"> void Page_Load(Object sender, EventArgs e) {
this.lbltime.Text ="Base form time is " + DateTime.Now.ToString() + "\n"; }
</script> <html> <head> </head> <body> <form runat="server"> <table>
<tbody> <tr> <td> <asp:Label id="lbltime"
runat="server"></asp:Label> </td> </tr> <tr> <td>
<CACHESAMPLE:TEXT id="instance1" runat="Server" DepartMentId="0">
</CACHESAMPLE:TEXT> </td> </tr> <tr> <td>
<CACHESAMPLE:TEXT id="instance2" runat="Server" DepartMentId="1">
</CACHESAMPLE:TEXT> </td> </tr> </tbody> </table>
</form> </body> </html> Application Level Caching
With Page level Output caching one cannot cache objects between pages within an
application. Fragment caching is great in that sense but has limitations by using user
controls as means to do. We can use the Cache object programmatically to take
advantage of caching objects and share the same between pages. Further the
availability of different overloaded methods gives a greater flexibility for our Cache
policy like Timespan, Absolute expiration etc. But one of the biggest takes is the
CacheDependancy. This means that one can create a cache and associate with it a
dependency that is either another cache key or a file.
In almost all Web applications there could be numerous master tables that act as
lookups to application specific tables. For e.g. if you take up adding a Employee,
usually one has master tables like "tblQualification" to get list of qualifications,
"tblLocations" to get list of locations etc. These tables* are usually set during the
initial application configuration phase and could be modified once a month or even
less than that. Hence it makes sense for us to use them in our Cache rather than
making calls to database on each request. But then what Cache Policy do we adopt?
We cannot hold these objects in Cache for entire application instance, because if
anybody changes data in these tables one has to also refresh the cache. It is here
that CacheDependancy can be used.
<CLASS="SMALLESTBLACK">* Even though these tables are less frequently used for
updates, they are extensively used in our select statements through out the
applications.
</CLASS>Find below the snippet that uses CacheDependancy. Here what I have done is to
provide a list view of existing employees. You need to create a Database in Sql
Server, setup some data before you can continue. The schema scripts are enclosed in
the article.
� Add database connection value in Web.Config and change the value as per your setup.
<appSettings> <add key="conn" value="Data
Source=vishnu;trusted_connection=yes;Initial Catalog=Users"/> </appSettings>
� First I get the dataset into which I fill the user list. But before this I check for
the cache initially if it exists I directly cast it to a dataset, if not create a
cache again.
daUsers.Fill(dsUsers,"tblUsers");
� I create the cache with "Users" as key using Cache.Insert* and link this with a file
"Master.xml". This "Master.xml" is a XML file that contains Master data of
"tblQualifications" and "tbllocations". I have used "Server.MapPath" to get the
physical path of the file on the server. The CacheDependancy instance will make sure
that any change in this dependency file means that you need to recreate your cache key
definition. This is a great feature to use since I can recreate my cache only when
required instead of caching the data at the page level.
Cache.Insert("Users",dsUsers, new
System.Web.Caching.CacheDependency(Server.MapPath("Master.xml")) ,
DateTime.Now.AddSeconds(45),TimeSpan.Zero); * For other overloaded parameters refer
MSDN. � Also note how we could use trace within to add my own statements.
HttpContext.Current.Trace.Write("from Database.."); <%@ Page Language="c#"
Trace="true" %> <%@ import Namespace="System" %> <%@ import Namespace="System.Data" %>
<%@ import Namespace="System.Data.SqlClient" %> <%@ import
Namespace="System.Configuration" %> <%@ import Namespace="System.Web" %> <%@ import
Namespace="System.Collections" %> <%@ import Namespace="System.IO" %> <script
runat="server"> void Page_Load(Object sender, EventArgs e) { DataSet dsUsers;
try { if(Cache["Users"]==null) { SqlConnection cn ; dsUsers = new
DataSet("new"); cn = new
SqlConnection(ConfigurationSettings.AppSettings.Get("conn")); SqlDataAdapter
daUsers; daUsers = new SqlDataAdapter("Select * from tblUsers",cn); cn.Open();
daUsers.Fill(dsUsers,"tblUsers"); //Update the cache object
Cache.Insert("Users",dsUsers, new
System.Web.Caching.CacheDependency(Server.MapPath("Master.xml")),
DateTime.Now.AddSeconds(45),TimeSpan.Zero);
HttpContext.Current.Trace.Write(DateTime.Now.AddSeconds(45).ToString() + " is expiry
time.."); cn.Close(); cn.Dispose(); HttpContext.Current.Trace.Write("from
Database.."); lblChange.Text ="From the database...."; } else {
HttpContext.Current.Trace.Write("From cache.."); lblChange.Text ="From the
cache...."; dsUsers= (DataSet) Cache["Users"]; } dlUsers.DataSource =dsUsers;
dlUsers.DataMember = dsUsers.Tables[0].TableName ; //lblChange.Text +=
Server.MapPath("Master.xml"); this.DataBind(); } catch(Exception ex) {
lblChange.Text = ex.Message; } } </script> <!DOCTYPE HTML PUBLIC "-//W3C//DTD
HTML 4.0 Transitional//EN" > <html> <head> <title>Cache Dependency Tester</title>
<meta content="Microsoft Visual Studio 7.0" name="GENERATOR" /> <meta content="C#"
name="CODE_LANGUAGE" /> <meta content="JavaScript" name="vs_defaultClientScript" />
<meta content="http://schemas.microsoft.com/intellisense/ie5"; name="vs_targetSchema"
/> </head> <body ms_positioning="GridLayout"> <form id="Form1" method="post"
runat="server"> <asp:DataList id="dlUsers" style="Z-INDEX: 101; LEFT: 44px;
POSITION: absolute; TOP: 104px" runat="server" Height="148px" Width="343px"
BorderWidth="1px" GridLines="Horizontal" CellPadding="4" BackColor="White"
ForeColor="Black" BorderStyle="None" BorderColor="#CCCCCC"> <SelectedItemStyle
font-bold="True" forecolor="White" backcolor="#CC3333"></SelectedItemStyle>
<FooterStyle forecolor="Black" backcolor="#CCCC99"></FooterStyle>
<HeaderStyle font-bold="True" forecolor="White" backcolor="#333333"></HeaderStyle>
<ItemTemplate> <table> <tr> <td>
<%#DataBinder.Eval(Container.DataItem,"UserId")%></td> <td>
<%#DataBinder.Eval(Container.DataItem,"FirstName")%></td> <td>
<%#DataBinder.Eval(Container.DataItem,"LastName")%></td> </tr>
</table> </ItemTemplate> </asp:DataList> <asp:Label id="lblChange"
style="Z-INDEX: 102; LEFT: 46px; POSITION: absolute; TOP: 63px" runat="server"
Height="28px" Width="295px"></asp:Label> <asp:Button id="btnMaster"
style="Z-INDEX: 103; LEFT: 50px; POSITION: absolute; TOP: 293px"
onclick="btnMaster_Click" runat="server" Text="Refresh Master"></asp:Button> </form>
</body> </html> We created the page that initiates and uses the Cache. For testing
purpose we need another page that will overwrite this "Master.xml" on click of a
button for which the code snippet is as follows. This ideally should be our master
maintenance page that adds/updates Master records in database and overwrites the XML.
But to make it easy I have just written an overwriting sample. <%@ Page Language="C#"
Trace="true"%> <%@ import Namespace="System" %> <%@ import Namespace="System.Data" %>
<%@ import Namespace="System.Data.SqlClient" %> <script runat="server"> void
btnMaster_Click(Object sender, EventArgs e) { //Call save function
this.Save(); } void Save() { try {
SqlConnection cn ; DataSet dsUsers = new DataSet("Users"); //I have used this
to get the Connectionstring from the //Configuration file. cn = new
SqlConnection(ConfigurationSettings.AppSettings.Get("conn")); SqlDataAdapter
daQualification; SqlDataAdapter daLocations; daQualification = new
SqlDataAdapter("Select * from tblqualifications",cn); daLocations = new
SqlDataAdapter("Select * from tblLocations",cn); cn.Open();
daQualification.Fill(dsUsers,"tblqualifications");
daLocations.Fill(dsUsers,"tblLocations");
HttpContext.Current.Trace.Write("Masters data up.."); //Overwrite the XML file. Also
please read MSDN on the overloaded parameters for WriteXml
dsUsers.WriteXml(HttpContext.Current.Server.MapPath("Master.xml"),XmlWriteMode.WriteSchema);
cn.Close(); cn.Dispose(); } catch(Exception ex)
{ throw new Exception(ex.Message); } } </script> <html> <head>
</head> <body> <form runat="server"> <span> <table> <tbody>
<tr> <td> <label id="lblRefresh" runat="server">Rewrite the XML
File by clicking the button below.</label> </td> </tr>
<tr align="middle"> <td> <asp:Button id="btnMaster"
onclick="btnMaster_Click" runat="server" Text="Write XML"></asp:Button>
</td> </tr> </tbody> </table> </span> </form> </body> </html>
Now once you have created the above pages i.e. one that implements caching and other
that overwrites the dependency file, create two instance of browser and open the cache
implementation page and note for trace, label text; open the other instance of browser
with the page which overwrites the XML. Note the former, the first time it fetches
data from the database and the subsequent request will be from cache till your
expiration time of 45 seconds is reached or anyone overwrites or changes the
"Master.xml" file. Also give a look on Timespan parameter since you have a concept of
Sliding expiration that can also be implemented. Keep refreshing the first page and
you will see that trace indicates the cached page retrieval. Click the overwrite XML
button on the latter page that would overwrite the XML and again refresh the former
page to note that the data is retrieved from database. Though in this example I have
not shown any direct relation between the cached data and the dependency file (like
get values from dependency file and merge with cached object etc) in terms of
integrated usage, this could very easily be designed and implemented. Dependency
caching is a powerful technique that .NET supports and should be utilized wherever
applicable.
DBScripts.sql ************* CREATE DATABASE [Users] GO use [Users] GO CREATE
TABLE [dbo].[tblLocations] ( [LocationId] [int] IDENTITY (1, 1) NOT NULL , [Name]
[varchar] (60) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL , [Description] [varchar]
(100) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ) ON [PRIMARY] GO CREATE TABLE
[dbo].[tblQualifications] ( [QualificationId] [int] IDENTITY (1, 1) NOT NULL , [Name]
[varchar] (60) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL , [Description] [varchar]
(100) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ) ON [PRIMARY] GO CREATE TABLE
[dbo].[tblUsers] ( [UserId] [int] IDENTITY (1, 1) NOT NULL , [FirstName] [varchar]
(60) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL , [LastName] [varchar] (60) COLLATE
SQL_Latin1_General_CP1_CI_AS NOT NULL , [QualificationId] [int] NULL , [LocationId]
[int] NULL ) ON [PRIMARY] GO ALTER TABLE [dbo].[tblLocations] ADD CONSTRAINT
[PK_tblLocations] PRIMARY KEY CLUSTERED ( [LocationId] ) ON [PRIMARY] GO ALTER
TABLE [dbo].[tblQualifications] ADD CONSTRAINT [PK_tblQualification] PRIMARY KEY
CLUSTERED ( [QualificationId] ) ON [PRIMARY] GO ALTER TABLE [dbo].[tblUsers] ADD
CONSTRAINT [PK_tblUsers] PRIMARY KEY CLUSTERED ( [UserId] ) ON [PRIMARY] GO ALTER
TABLE [dbo].[tblUsers] ADD CONSTRAINT [FK_tblUsers_tblLocations] FOREIGN KEY (
[LocationId] ) REFERENCES [dbo].[tblLocations] ( [LocationId] ), CONSTRAINT
[FK_tblUsers_tblQualifications] FOREIGN KEY ( [QualificationId] ) REFERENCES
[dbo].[tblQualifications] ( [QualificationId] ) GO
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