ASP.NET Runtime Cache - Clone Objects to Preserve Cached Data (Part II)

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Introduction

This article is strengthening a previous article I've written, ASP.NET Runtime Cache - Clone Objects to Preserve Cached Data, by presenting you with better code that is available for you to download from this page and use in your applications.

Allow me to re-iterate. My previous article explained that the ASP.NET runtime cache is shared between website requests and that the cache is unprotected, which means that a user can request an object in the cache and then modify the object's data. A second user can make a request for the same cached object and get my modifications, or worse still, half of my modifications.

Pre-Requisites

Knowledge of the ASP.NET technology, an understanding of data caching, an understanding of shallow vs. deep copying, and knowledge of .NET generics is required for this article.

Lock the Cache

One method of resolving this conflict is to introduce locking around the cache, but this can be expensive, certainly when a system is under heavy load. Threads will get blocked, and the OS will be spending more time synchronizing processors and thread-switching, which can be a major bottleneck.

Deep Copy Our Objects

A alternative, and my preference, is to deep copy objects going in and objects coming out. This way, every request reading the cache is getting its own copy of the objects.

namespace ProtectedCache {

    // Simple interface to indicate intent. I was thinking about using the
    // existing ICloneable interface and documenting it for developers to ensure
    // deep copies are performed. I decided not to and opted for a new self-
    // documenting interface.

    public interface IDeepCloneable {

        object DeepClone();
    }
}

using System;
using System.Web.Caching;

namespace ProtectedCache {

    // This is a purely a data object without behavior. It is holding caching
    // expiration data that will be specified when adding to the cache.

    // This may seem pointless or overkill but it does play a huge part in a 
    // RowDataGateway framework (it's a code generated DAL) I have written.
    // I'm not introducing the framework at this stage because I don't want
    // to lose focus and over-complicate this code.

    public sealed class CacheArgs {

        private bool _isCacheable;
        private DateTime _absoluteExpiration = Cache.NoAbsoluteExpiration;
        private TimeSpan _slidingExpiration = Cache.NoSlidingExpiration;        

        public CacheDependency Dependency {
            get {
                return null; // Implement when required
            }
        }

        public DateTime AbsoluteExpiration {
            get { return _absoluteExpiration; }
            set { _absoluteExpiration = value; }
        }

        public TimeSpan SlidingExpiration {
            get { return _slidingExpiration; }
            set { _slidingExpiration = value; }
        }
    }
}

using System;
using System.Web;
using System.Collections.Generic;

namespace ProtectedCache {

    // This is the Cacher; a static helper class that is responsible for
    // performing the deep cloning/copying.

    public static class Cacher {

        public static T Load<t>(string key) where T : IDeepCloneable {
            
            // Perform a thread-safe read of the cache

            object item = HttpRuntime.Cache[key];

            // If the item isn't null and it's type is equal to the
            // type of generic T then cast the item, deep clone it, then
            // return it casted of type generic T, otherwise, return the
            // default of generic T

            if (item != null && (item.GetType() == typeof(T))) {
                return (T)((IDeepCloneable)item).DeepClone();            
            } else {
                return default(T);
            }
        }

        public static void Save<t>(T item, string key, CacheArgs args) where T : 
                                     IDeepCloneable {

            // Insert a deep cloned item in to the cache

            HttpRuntime.Cache.Insert(key, item.DeepClone(), args.Dependency,
                args.AbsoluteExpiration, args.SlidingExpiration);
        }
    }
}

Resources

I try to write when I can, but sometimes it's difficult finding time to complete lengthy articles. However, I do write snippy-snappy posts, providing useful tips and code samples at http://designcodetest.blogspot.com/. Come along, have a read, and please leave feedback.

History

This is the first version of my article.