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How ASP.NET can efficiently control the client cache

Published on 6/12/2013
Categories: Web Design
How ASP.NET can efficiently control the client cache

In this article we will see how to precisely control the cache of the client (the browser of your PC, for example) in order to avoid the transmission of data from the server when the client already owns them. The ASP.NET server already has systems for data caching, for example to avoid repeating the same database query; would not it be nice not to avoid sending these data to avoid bandwidth consumption?
The technique is called conditional GET and uses the 304 HTTP status code and in practice implements the following dialogue between client and server:

Client: "Hello server, I need some data. The last time you gave them to me, you told me they were update to the following date/time"

At this point, the server checks the provided date/time and, whether the data are changed, it responds:

Server: "Hello client, the data have changed since then, here's the new ones. If you need them again in future, note that data are update to the following date/time."

or

Server: "Hello client, the data have not changed since then, so I avoid send them again, you have them in your cache."

This type of optimization is particularly useful when data are dynamically generated on the server side and change during the time.
Let us transform this dialogue in C# code, using the example of an HttpHandler that generates a RSS file dynamically (that's what happens when you subscribe to the RSS feeds of this blog). In this example we do not address the topic of ther dynamic generation of RSS feeds, as this would be beyond the scope of the article, but we'll just see how to implement the conditional caching.
Let's assume that the method GetLastBlogUpdateDateTime() returns the UTC date/time of the last blog update.

public class BlogRssHandler : IHttpHandler, IRequiresSessionState
{
    public virtual void ProcessRequest(HttpContext Context)
    {
        // If the client already has the updated RSS, do not send it again
        if (BuildResponseHeader(Context) == false)
            return;
        else
        {
            // Generate the RSS feed
            ...
        }
    }
 
    // Returns false if contents can be retrieved from the browser's cache
    // (HTTP response 304 optimization)
    private Boolean BuildResponseHeader(HttpContext Context)
    {
        // Get the last modification date/time stored in the database
        // NOTE: the date/time must be UTC
        DateTime serverLastUpdateUTC = GetLastBlogUpdateDateTime();
 
        // Get the last modification date provided by the browser (if any)
        String ifModifiedSinceHeaderText = Context.Request.Headers.
            Get("If-Modified-Since");
        if (!String.IsNullOrEmpty(ifModifiedSinceHeaderText))
        {
            DateTime clientLastUpdateUTC = DateTime.Parse(ifModifiedSinceHeaderText);
            clientLastUpdateUTC = clientLastUpdateUTC.Value.ToUniversalTime();
 
            // If the modification date/time of the client is equal to the server one
            if (serverLastUpdateUTC <= clientLastUpdateUTC)
            {
                // ...tell the browser that contents have not changed and return
                Context.Response.ClearContent();
                Context.Response.StatusCode = (Int32)
                    System.Net.HttpStatusCode.NotModified;
                Context.Response.StatusDescription = "Not Modified";
                Context.Response.SuppressContent = true;
                return false;
            }
        }
 
        Context.Response.ContentType = "text/xml";
        // Tell the browser to cache the new contents
        Context.Response.Cache.SetCacheability(HttpCacheability.Private);
        Context.Response.Cache.SetRevalidation(HttpCacheRevalidation.AllCaches);
        Context.Response.Cache.SetLastModified(DateTime.SpecifyKind(
            serverLastUpdateUTC.Value, DateTimeKind.Utc));
        // Force the browser to not use it's current cache
        Context.Response.Cache.SetMaxAge(new TimeSpan(0, 0, 0));
        Context.Response.Cache.SetExpires(DateTime.Now.ToUniversalTime());
        return true;
    }
 
    ...
}

When the server sends the date/time to the client it is better to round it removing milliseconds.
It 'also important to specify that the date/time is UTC (DateTime.SpecifyKind method), this thing is not mentioned in some examples around the web, but it is very important.

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