Image from HttpHandler won't cache in browser

Question

I\'m serving up an image from a database using an IHttpHandler. The relevant code is here:

public void ProcessRequest(HttpContext context)
{
    context.Resp         


        

Answer 1

This is how it's done in Roadkill's (a .NET wiki) file handler:

FileInfo info = new FileInfo(fullPath);
TimeSpan expires = TimeSpan.FromDays(28);
context.Response.Cache.SetLastModifiedFromFileDependencies();
context.Response.Cache.SetETagFromFileDependencies();
context.Response.Cache.SetCacheability(HttpCacheability.Public);

int status = 200;
if (context.Request.Headers["If-Modified-Since"] != null)
{
    status = 304;
    DateTime modifiedSinceDate = DateTime.UtcNow;
    if (DateTime.TryParse(context.Request.Headers["If-Modified-Since"], out modifiedSinceDate))
    {
        modifiedSinceDate = modifiedSinceDate.ToUniversalTime();
        DateTime fileDate = info.LastWriteTimeUtc;
        DateTime lastWriteTime = new DateTime(fileDate.Year, fileDate.Month, fileDate.Day, fileDate.Hour, fileDate.Minute, fileDate.Second, 0, DateTimeKind.Utc);
        if (lastWriteTime != modifiedSinceDate)
            status = 200;
    }
}

context.Response.StatusCode = status;

Thomas's answer about IIS not supplying the status code is the key, without it you just get 200s back each time.

The browser will simply send you a date and time for when it thinks the file was last modified (no no header at all), so if it differs you just return a 200. You do need to normalize your file's date to remove milliseconds and ensure it's a UTC date.

I've gone for defaulting to 304s if there's a valid modified-since, but that can be tweaked if needed.

Answer 2

If you have source file on disk you can use this code:

context.Response.AddFileDependency(pathImageSource);
context.Response.Cache.SetETagFromFileDependencies();
context.Response.Cache.SetLastModifiedFromFileDependencies();
context.Response.Cache.SetCacheability(HttpCacheability.Public);

Also, make sure that you test using IIS, not from Visual Studio. ASP.NET Development Server (aka Cassini) always sets Cache-Control to private.

See also: Caching Tutorial for Web Authors and Webmasters

Answer 3

AFAIK, you are responsible for sending 304 Not Modified, meaning I am not aware of anything in the .Net framework that does it for you in this use case of you sending "dynamic" image data. What you will have to do (in pseudo code):

• Check for the If-Modified-Since header in the request and parse out the date (if it exists).
• Compare it to the last modification date of your original image (dynamically generated) image. Tracking this is probably the most complex part of the solution to this problem. In your current situation, you are re-creating the image on every request; you don't want to do that unless you absolutely have to.
• If the date of the file the browser has is newer or equal to what you have for the image, send a 304 Not Modified.
• Otherwise, continue with your current implementation

A simple way to track last modified times on your end is to cache newly generated images on the file system and keep an in-memory dictionary around that maps the image ID to a struct containing the file name on disk and the last modification date. Use Response.WriteFile to send the data from disk. Of course, every time you restart your worker process, the dictionary would be empty, but you're getting at least some caching benefit without having to deal with persisting caching information somewhere.

You can support this approach by separating the concerns of "Image Generation" and "Sending Images over HTTP" into different classes. Right now you're doing two very different things in the same place.

I know this may sound a little complex, but it's worth it. I just recently implemented this approach and the savings in processing time and bandwidth usage were incredible.

Answer 4

Do you have any response buffering happening? If so you might want to set the headers before you write to the output stream. i.e. try moving the Response.OutputStream.Write() line down to below the Cache setting lines.