Using cache in ASP.NET

Question

I'm trying to use the cache in an ASP.NET MVC web application to store some list data that rarely updates. I insert this data into the cache in an UpdateCache() method like this:

HttpContext.Current.Cache.Insert("MyApp-Products", products, null, DateTime.Now.AddYears(99), Cache.NoSlidingExpiration);

Then in my model I retrieve it:

public static List<string> GetProducts()
{
    var cachedProducts = HttpContext.Current.Cache["MyApp-Products"];
    if (cachedProducts == null)
    {
        UpdateCache();
        cachedProducts = HttpContext.Current.Cache["MyApp-Products"];
    }
    return ((List<string>)cachedProducts );
}

The first time I visit the page, UpdateCache() is called as expected. If I refresh, the data comes from the cache and I do not need to call UpdateCache() . However, after maybe 15 mins, I come back to the app and the cached values are gone. My understanding was that this cache was per application, not session, so I would have expected it to still be there for myself or another user.

Is there something wrong with the way I'm storing the cache? Or is there something I'm not understanding about how the Cache in ASP.NET works in a web app?

Answer 1

My understanding was that this cache was per application, not session, so I would have expected it to still be there for myself or another user.

While the cache is per application, ASP.NET doesn't provide you any guarantees that if you stored something into the cache you will find it back there. The cache could be evicted under different circumstances such as for example your server starts running low on memory and so on. You can subscribe to an event and get notified when an item is evicted from the cache. You can also define a priority when caching an item. The higher the priority, the lower the chance of this item getting evicted.

Also since the cache is stored in the memory of the web server (by default) you should not forget the fact that your application domain could be recycled at any point by IIS. For example after a certain amount of inactivity or if it starts running low on memory or even if a certain CPU threshold usageis reached, ... and everything that is stored in the memory including the cache will simply disappear into the void and the next request in your application will start a new AppDomain.

But in any cases make sure that you check if the item is present in the cache before using it. Never rely on the fact that if you stored something into it, you will find it.

All this blabla to come to the really essential point which is something very worrying with your code. It looks like you are storing a List<string> instance into the cache. But since the cache is per application, this single instance of List<string> could be shared between multiple users of the application and of course this could happen concurrently. And as you know List<T> is not a thread safe structure. So with this code you will, at best, get an exception and at worst you will get corrupt data. So be very careful what you are caching and how you are synchronizing the access to the structure especially if you are caching a non thread-safe class.

Answer 2

IF this is on a full IIS, and happens around every 15minuntes. Remember to check the Idle timeout value.

That being said, if this list "never" changes, why not store it in a static array instead.

Answer 3

I can see how storing data in general in database is better than in static object. For one, in case data does change, it is easier to update DB than the application.

Try explicitly setting absolute expiration when caching your object:

HttpRuntime.Cache.Insert("MyApp-Products", cachedProducts, null, DateTime.Now.AddDays(20), TimeSpan.Zero );

Note HttpRuntime is used instead of HttpContext for performance reasons even though the difference is minor.