Cache Examples and Overview - ASP.NET

Problem. Caching with HTTP headers is important to every web site, but there are many complicated rules and conflicts among the headers. You want to simplify and improve your HTTP cache headers. Solution. Here are lots of notes about HTTP headers and their methods in ASP.NET.

Example: implement Expires on ASP.NET requests

ASP.NET websites may use ASPX files or ASHX handlers to serve static content like images. Keeping with Yahoo's guidelines, use this ASP.NET C# code to set Expires in the far future.

using System;
using System.Web.UI;

public partial class _Default : Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Set this ASPX response to expire in one year
        // on browsers, essentially 'never'.
        Response.Cache.SetExpires(DateTime.Now.AddYears(1));
    }
}

IIS7 will set the Cache-Control header automatically when you specify SetExpires. No other caching logic is necessary for static resource caching on client browsers.

Information: what Yahoo recommends for static files

Almost all big websites have certain images that never change, such as logos, shopping cart images, rounded borders and gradients, and navigation bars.

Yahoo says to use "the Expires header in the HTTP response to tell the client how long a component can be cached." [Best Practices for Speeding Up Your Web Site - developer.yahoo.com]

Yahoo introduces this example: "This is a far future Expires header, telling the browser that this response won't be stale until April 15, 2010."

Expires: Thu, 15 Apr 2010 20:00:00 GMT

Note that the time format is very specific and you will need to use a special DateTime format in C# to create or test it. [See section on DateTime]

Information: what Yahoo recommends for dynamic pages

Yahoo recommends the Cache-Control header for dynamic pages. There are several variants of this you can use.

Yahoo's best practices: "For dynamic components: use an appropriate Cache-Control header to help the browser with conditional requests."

Using Cache-Control gives you overriding power on the cache setting, allowing you to specify options for proxies and the server.

Example: how to implement Cache-Control headers on dynamic pages

The implementation of caching on Response.Cache is very complex and confusing in ASP.NET, and some options will trigger other options. Setting a page for 1 hour of caching is done like this:

using System;
using System.Web;
using System.Web.UI;

public partial class _Default : Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Set cache for 1 hour on all computers and servers.
        // Proxies, browsers, and your server will cache it.
        Response.Cache.SetCacheability(HttpCacheability.Public);
        Response.Cache.SetMaxAge(new TimeSpan(1, 0, 0));
    }
}

As Yahoo states, Cache-Control gives you more control and "helps browsers" with conditional requests.

Information: using Fiddler to debug HTTP headers

When developing ASP.NET applications with caching, you should use Fiddler, or another equivalent tool, to look at the HTTP headers. I have written material on Fiddler. [Fiddler Tool for HTTP Debugging - dotnetperls.com]

Information: Cache methods in ASP.NET using C#

These methods and properties are used to control the HTTP cache settings on your ASP.NET response. They must be called on the HttpResponse object, using the Response.Cache...() style syntax.

Information: HttpCacheability enumeration values

You need to call SetCacheability on the Response.Cache to set the main Cache-Control header. This header controls the location and general behavior of the cache.

You need to combine this setting with other Cache class method calls to achieve many behaviors. However, these enums define the general setting.

Question: what is meant by 'private' caching?

Private is a term that means the web browser on your users' computers. By using private, your users won't download or re-request the pages they have viewed.

This reduces the work your server does and the bandwidth you use, but it isn't optimal for optimization. It refuses proxy servers and your own server caches, meaning you still have much CPU use and bandwidth use.

Question: what is the difference between HTTP Expires and Max-Age?

Max-Age, part of the Cache-Control, is relative to the current time, making it easier to use in many cases. It "specifies the maximum amount of time that an representation will be considered fresh."

"Similar to Expires, this directive is relative to the time of the request, rather than absolute." [Caching Tutorial - mnot.net]

Information: Max-Age overrides Expires

From the HTTP spec: "If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header." [HTTP 1.1 Specification]

Information: public proxy caches, such as Squid

I am not certain how common proxy caches are, but when available, they can reduce your bandwidth and improve access times. Basically, they can serve up your pages for you, for free.

The Squid software "reduces bandwidth and improves response times by caching and reusing frequently-requested web pages. [It] optimises the data flow between client and server." [squid: Optimising Web Delivery - squid-cache.org]

Information: the Vary header

If you look at the HTTP headers of MySpace, for example, you see the header "Vary: Accept-Encoding". This means that the caches will be separate for each encoding. This improves localization. You can use SetVaryByCustom to change this.

Question: how can I set caching options in a Handler.ashx file?

Handler.ashx is the default name for an ASP.NET generic handler. You need to use the HttpContext parameter and access the Response that way.

<%@ WebHandler Language="C#" Class="Handler" %>

using System;
using System.Web;

public class Handler : IHttpHandler {

    public void ProcessRequest (HttpContext context) {
        // Cache this handler response for 1 hour.
        HttpCachePolicy c = context.Response.Cache;
        c.SetCacheability(HttpCacheability.Public);
        c.SetMaxAge(new TimeSpan(1, 0, 0));
    }

    public bool IsReusable {
        get {
            return false;
        }
    }
}

Information: removing the Cache-Control header in ASP.NET

When you use the Expires header on static resources such as JavaScript, CSS, and images, you do not need the Cache-Control header. Cache-Control and Expires are alternatives and are not both needed.

You can disable the HTTP Cache-Control header entirely in Web.config. Note however that this setting is defeated if you change cache headers during execution. [httpRuntime Element - MSDN]

<?xml version="1.0"?>
<configuration>
    <appSettings/>
    <connectionStrings/>
    <system.web>
        <httpRuntime sendCacheControlHeader="false"/>
    <!-- etc. -->

Question: how can I convert a DateTime into an HTTP date?

You can use a special format string, after converting the date into the current timezone. HTTP has a rigid definition of date formats, which is good.

using System;
using System.Web.UI;

public partial class _Default : Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        DateTime d = DateTime.UtcNow;
        string s = d.ToUniversalTime().ToString("r");
        Response.Write(s);
    }
}

For more information on HTTP date formats, such as the RFC1123 pattern, see the Microsoft site. [Standard Date and Time Format Strings - msdn.microsoft.com]

Question: what other ASP.NET caching methods are there?

There is a Cache object you can use to store data programmatically. This is separate from the methods shown here, and has many different options. This document focuses on HTTP headers.

Question: how is the Response.Cache class accessed?

The examples above show how you can access it in the Page class and also from generic handlers. However, Cache is an intrinsic object, and my research shows what happens when you access it. [ASP.NET - HttpContext and Request Property - dotnetperls.com]

My research indicates that accessing the Response.Cache property in each method call may not be ideal, and storing the HttpCachePolicy as a variable is better. [ASP.NET - Response.Write Improvement - dotnetperls.com]