Maarten Balliauw {blog}

ASP.NET MVC, Microsoft Azure, PHP, web development ...

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Creating an ASP.NET MVC OutputCache ActionFilterAttribute

In every web application, there are situations where you want to cache the HTML output of a specific page for a certain amount of time, because underlying data and processing isn't really subject to changes a lot. This cached response is stored in the web server's memory and offers very fast responses because no additional processing is required.

Using "classic" ASP.NET, one can use the OutputCache directive on a .aspx page to tell the ASP.NET runtime to cache the response data for a specific amount of time. Optionally, caching may vary by parameter, which results in different cached responses depending on the parameters that were passed in the URL.

As an extra feature, one can also send some HTTP headers to the client and tell him to load the page from  the web browser's cache until a specific amount of time has passed. Big advantage of this is that your web server will receive less requests from clients because they simply use their own caching.

Using the ASP.NET MVC framework (preview 3, that is), output caching is still quite hard to do. Simply specifying the OutputCache directive in a view does not do the trick. Luckily, there's this thing called an ActionFilterAttribute, which lets you run code before and after a controller action executes. This ActionFilterAttribute class provides 4 extensibility points:

  • OnActionExecuting occurs just before the action method is called
  • OnActionExecuted occurs after the action method is called, but before the result is executed (before the view is rendered)
  • OnResultExecuting occurs just before the result is executed (before the view is rendered)
  • OnResultExecuted occurs after the result is executed (after the view is rendered)

Let's use this approach to create an OutputCache ActionFilterAttribute which allows you to decorate any controller and controller action, i.e.:

[code:c#]

[OutputCache(Duration = 60, VaryByParam = "*", CachePolicy = CachePolicy.Server)]
public ActionResult Index()
{
    // ...
}

[/code]

We'll be using an enumeration called CachePolicy to tell the OutputCache attribute how and where to cache:

[code:c#]

public enum CachePolicy
{
    NoCache = 0,
    Client = 1,
    Server = 2,
    ClientAndServer = 3
}

[/code]

1. Implementing client-side caching

Actually, this one's really easy. Right before the view is rendered, we'll add some HTTP headers to the response stream. The web browser will receive these headers and respond to them by using the correct caching settings. If we pass in a duration of 60, the browser will cache this page for one minute.

[code:c#]

public class OutputCache : ActionFilterAttribute
{
    public int Duration { get; set; }
    public CachePolicy CachePolicy { get; set; }

    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        // Client-side caching?
        if (CachePolicy == CachePolicy.Client || CachePolicy == CachePolicy.ClientAndServer)
        {
            if (Duration <= 0) return;

            HttpCachePolicyBase cache = filterContext.HttpContext.Response.Cache;
            TimeSpan cacheDuration = TimeSpan.FromSeconds(Duration);

            cache.SetCacheability(HttpCacheability.Public);
            cache.SetExpires(DateTime.Now.Add(cacheDuration));
            cache.SetMaxAge(cacheDuration);
            cache.AppendCacheExtension("must-revalidate, proxy-revalidate");
        }
    }
}

[/code]

2. Implementing server-side caching

Server-side caching is a little more difficult, because there's some "dirty" tricks to use. First of all, we'll have to prepare the HTTP response to be readable for our OutputCache system. To do this, we first save the current HTTP context in a class variable. Afterwards, we set up a new one which writes its data to a StringWriter that allows reading to occur:

[code:c#]

existingContext = System.Web.HttpContext.Current;
writer = new StringWriter();
HttpResponse response = new HttpResponse(writer);
HttpContext context = new HttpContext(existingContext.Request, response)
{
    User = existingContext.User
};
System.Web.HttpContext.Current = context;

[/code]

Using this in a OnResultExecuting override, the code would look like this:

[code:c#]

public override void OnResultExecuting(ResultExecutingContext filterContext)
{
    // Server-side caching?
    if (CachePolicy == CachePolicy.Server || CachePolicy == CachePolicy.ClientAndServer)
    {
        // Fetch Cache instance
        cache = filterContext.HttpContext.Cache;

        // Fetch cached data
        object cachedData = cache.Get(GenerateKey(filterContext));
        if (cachedData != null)
        {
            // Cache hit! Return cached data
            cacheHit = true;
            filterContext.HttpContext.Response.Write(cachedData);
            filterContext.Cancel = true;
        }
        else
        {
            // Cache not hit.
            // Replace the current context with a new context that writes to a string writer
            existingContext = System.Web.HttpContext.Current;
            writer = new StringWriter();
            HttpResponse response = new HttpResponse(writer);
            HttpContext context = new HttpContext(existingContext.Request, response)
            {
                User = existingContext.User
            };

            // Copy all items in the context (especially done for session availability in the component)
            foreach (var key in existingContext.Items.Keys)
            {
                context.Items[key] = existingContext.Items[key];
            }

            System.Web.HttpContext.Current = context;
        }
    }
}

[/code]

By using this code, we can retrieve an existing item from cache and set up the HTTP response to be read from. But what about storing data in the cache? This will have to occur after the view has rendered:

[code:c#]

public override void OnResultExecuted(ResultExecutedContext filterContext)
{
    // Server-side caching?
    if (CachePolicy == CachePolicy.Server || CachePolicy == CachePolicy.ClientAndServer)
    {
        if (!cacheHit)
        {
            // Restore the old context
            System.Web.HttpContext.Current = existingContext;

            // Return rendererd data
            existingContext.Response.Write(writer.ToString());

            // Add data to cache
            cache.Add(
                GenerateKey(filterContext),
                writer.ToString(),
                null,
                DateTime.Now.AddSeconds(Duration),
                Cache.NoSlidingExpiration,
                CacheItemPriority.Normal,
                 null);
        }
    }
}

[/code]

Now you noticed I added a VaryByParam property to the OutputCache ActionFilterAttribute. When caching server-side, I can use this to vary cache storage by the parameters that are passed in. The GenerateKey method will actually generate a key depending on controller, action and the VaryByParam value:

[code:c#]

private string GenerateKey(ControllerContext filterContext)
{
    StringBuilder cacheKey = new StringBuilder();

    // Controller + action
    cacheKey.Append(filterContext.Controller.GetType().FullName);
    if (filterContext.RouteData.Values.ContainsKey("action"))
    {
        cacheKey.Append("_");
        cacheKey.Append(filterContext.RouteData.Values["action"].ToString());
    }

    // Variation by parameters
    List<string> varyByParam = VaryByParam.Split(';').ToList();

    if (!string.IsNullOrEmpty(VaryByParam))
    {
        foreach (KeyValuePair<string, object> pair in filterContext.RouteData.Values)
        {
            if (VaryByParam == "*" || varyByParam.Contains(pair.Key))
            {
                cacheKey.Append("_");
                cacheKey.Append(pair.Key);
                cacheKey.Append("=");
                cacheKey.Append(pair.Value.ToString());
            }
        }
    }

    return cacheKey.ToString();
}

[/code]

There you go! Now note that you can add this OutputCache attribute to any controller and any controller action you  have in your application. The full source code is available for download here: MvcCaching.zip (211.33 kb) (full sample) or OutputCache.zip (1.59 kb) (only attribute).

UPDATE: Make sure to read part 2, available here.

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