Alan Northam

Cold, Hard, Cache!

Don't get me wrong, I really like the Asp.Net Cache (or HttpRuntime.Cache or Uncle Daddy if you want to call it that... you're a little odd aren't you?), but sometimes it just can't be trusted. I mean, I just gave you (the cache) my precious object a second ago and now you claim you don't have it? Did you lose it? Did you sell it on eBay? Did you pawn it to support your habit? Please at least tell me you got a decent price for it.

One of the great things about the cache is also what makes it so untrustworthy. Objects stored there are referenced with WeakReference so if your sever starts getting low on memory GC will collect objects from the cache to free some up. Also take into account that if you put an object in Cache and set it to expire after X minutes, when you go back to retreive it, do you really know how long it's been?

So what we want is a way to tell the cache "I want this specific object (a yellow Tonka dumptruck, metal, not plastic) and if you don't have it, here are instructions on how to get it".

Something like this:

using System;
using System.Reflection;
using System.Web;
using System.Web.Caching;
using log4net;
public delegate bool Factory<T>( out T instance );
public class CacheHelper
{
    private static readonly ILog log =
        LogManager.GetLogger(
            MethodBase.GetCurrentMethod().DeclaringType );

private static readonly Cache cache = HttpRuntime.Cache;
    private CacheHelper(){}

public static T GetFromCache<T>(string key, int cacheTimeInMinutes, Factory<T> retrieveMethod) where T:class
    {
        T target = cache[key] as T;
        if(target == null)
        {
            log.Info( "Cache miss for key:" + key + "  type:" + typeof(T));
            if(retrieveMethod(out target))
            {
                cache.Insert( key, target, null, DateTime.Now.AddMinutes( cacheTimeInMinutes ),
                    Cache.NoSlidingExpiration, CacheItemPriority.Normal, OnRemove );
            }            
        }
        else
        {
            log.Info( "Cache hit for key:" + key + "  type:" + typeof(T));
        }
        return target;
    }

public static void OnRemove( string key, object cacheItem, CacheItemRemovedReason reason )
    {
        log.Info( "Object removed from cache: Key-" + key + ": Reason-" + reason);
    }

public static void RemoveFromCache(string key)
    {
        cache.Remove( key );
    }
}

Usage would look like this:

List<string> searchList = CacheHelper.GetFromCache<List<string>>( key, 5, delegate( out List<string> instance ) { return SomeClass.GetSearchList( key, out instance ); } );

I call it the code equivalent of "Trust but verify".

Published Mar 06 2008, 10:04 PM by anortham

Filed under: C#, ASP.NET, Cache, HttpRuntime.Cache

Comments

DotNetKicks.com said:

You've been kicked (a good thing) - Trackback from DotNetKicks.com

# March 7, 2008 8:06 AM

Will Sullivan said:

Heheh. I beat you to it. I've used this pattern for a long time.

statestreetgang.net/post.aspx

Its a great little pattern. You can also apply it to the session state, view state, or any other "bag" type storage. One of my fav uses is for session state-scoped singletons...

# March 7, 2008 11:20 AM

anortham said:

Will,

If I'd known we were in a race, I'd have kneecapped you with a pipe :)

# March 7, 2008 3:25 PM

Sam said:

I like your sense of humor. It's funnier than your code, but less useful.

# March 7, 2008 8:57 PM

Cold, Hard, Cache! - Alan Northam « Noocyte’s Weblog said:

Pingback from Cold, Hard, Cache! - Alan Northam « Noocyte’s Weblog

# March 11, 2008 1:45 PM

Alan Northam said:

Take a look at my previous post for the details on a cache helper method that I like to use. In this

# March 17, 2008 10:54 PM

Luka said:

Hi Alan,

I'm trying to store data table into cache, without much success.

I have a static method that suppose to return datatable in case that it is not already in cache (retrieve method). I admit, I don't know much with anonymous methods, but wander if this pattern should work for datatable types?

# May 23, 2008 3:06 PM