Output Caching Overview

Output caching can be set programmatically using the HttpCachePolicy class, or declaratively by using the OutputCache page or control directive. The primary attributes for output caching are as follows:

Note that the Output directive requires two parameters: VaryByParam and Duration. If VaryByParam does not apply then set its value to None. Output caching at the client is retrieved only by the page’s URL and not the page’s version, thus VaryByCustom, VaryByHeader, and VaryByParam apply only to server-side cache.

Examples of using these attributes would be as follows:

<%@ OutputCache Duration="3600" Location="Downstream" VaryByParam="None" %>

Response.Cache.SetExpires(DateTime.Now.AddSeconds(3600));
Response.Cache.SetCacheability(HttpCacheability.Public);
Response.Cache.SetNoServerCaching();

<%@ OutputCache Duration="3600" VaryByHeader="User-Agent" VaryByParam="None" %>

Response.Cache.SetExpires(DateTime.Now.AddSeconds(3600));
Response.Cache.SetCacheability(HttpCacheability.Public);
Response.Cache.VaryByHeaders["User-Agent"] = true;

<%@ OutputCache duration="3600" VaryByParam="ddlControl"%>

Response.Cache.SetExpires(DateTime.Now.AddSeconds(3600));
Response.Cache.SetCacheability(HttpCacheability.Public);
Response.Cache.VaryByParams["ddlControl"] = true;

<%@ OutputCache duration="3600" VaryByParam="None" VaryByCustom="MyVersion""%>

Response.Cache.SetExpires(DateTime.Now.AddSeconds(3600));
Response.Cache.SetCacheability(HttpCacheability.Public);
Response.Cache.SetVaryByCustom("MyVersion");

public override string GetVaryByCustomString(HttpContext context, string arg) {
   if (arg == "MyVersion") {
      return "Version=" + SomeString;
   }
   return SomeOtherString;
}

<%@ OutputCache Duration="3600" VaryByHeader="User-Agent" VaryByParam="None" %>

Response.Cache.SetExpires(DateTime.Now.AddSeconds(3600));
Response.Cache.SetCacheability(HttpCacheability.Public);
Response.Cache.VaryByHeaders["User-Agent"] = true;

A High Performance Database Report

Assume your system resources are running low. You have determined that a majority of the problem is a database report that constantly runs with four different versions. You decide to implement server-side output caching so that each version of the report is only run once every hour.

STEP BY STEP

1.      Add a new Web Form to your Visual Studio C# application.

2.      Place a Label control (lblTimeLastGenerated), a DropDownList control (ddlCountry), a DataGrid control (dgReport), and a Button control (btnGetData) on the Web Form.

3.      Select the Items property on the ddlCountry and then click the ellipsis (…) button. Add the following properties in the Collection Editor Dialog: France, Brazil, Italy, and Germany.

4.      Now enter the following Page directive at the top of the Web Form’s HTML. This is the only line of code required for fragment caching.

<%@ OutputCache duration="3600" VaryByParam="ddlCountry" %>

Alternatively, use the HttpCachePolicy class in the code as follows:

Response.Cache.SetExpires(DateTime.Now.AddSeconds(3600));
Response.Cache.SetCacheability(HttpCacheability.Public);
Response.Cache.SetValidUntilExpires(true);
Response.Cache.VaryByParams["ddlCountry"] = true;

using System;
using System.Data.SqlClient;
 
private void Page_Load(object sender, EventArgs e)
{
   if (Page.IsPostBack == false)
         GetData();
}
 
private void btnGetData_Click(object sender, EventArgs e)
{
   GetData();
}
 
private void GetData()
{
   SqlConnection conn = new SqlConnection(
         "Data Source=(local); Initial Catalog=Northwind;" +
         " Integrated Security=SSPI");
 
   SqlCommand cmd = conn.CreateCommand();
   cmd.CommandType = CommandType.Text;
   cmd.CommandText = string.Format(
         "SELECT * FROM customers WHERE Country = '{0}'",
         ddlCountry.SelectedItem.Text);
   SqlDataAdapter da = new SqlDataAdapter();
   da.SelectCommand = cmd;
   DataSet ds = new DataSet();
   da.Fill(ds, "Customers");
   dgReport.DataSource = ds;
   dgReport.DataBind();
   lblTimeLastGenerated.Text = 
         string.Format("Last generated {0}, {1}",
         ddlCountry.SelectedItem.Text, DateTime.Now.ToString());
}

6.      Run the web page from several different computers and you will notice the same time reported for each country. This is because the data is being retrieved from server cache.

Caching at the client and Proxy

Client and proxy caches can only store a single version of the web page. Therefore you will need to remove the drop down list and button controls. Leaving these controls will allow the user to re-request possibly updated content from the server. However, when the client revisits the page he will view the previously cached data. This could cause a lot of confusion.

• To specify output caching at the client use the following Page directive:
  

  <%@ OutputCache Duration="3600" Location="Client" VaryByParam="None" %>

• To specify output caching at the client and proxy use the following Page directive:
  

   <%@ OutputCache Duration="3600" Location="Downstream" VaryByParam="None"%>

Revisiting a page cached at the client will display the original time that the client visited the page. Unlike server cache, visiting the page from another computer will display an entirely new time.