Practical Implementation - spending some cache.

anortham — Tue, 18 Mar 2008 01:35:00 GMT

Take a look at my previous post for the details on a cache helper method that I like to use. In this post, we'll be putting it to work (the word for today is J - O - B). Here's a look at the code for reference.

using System; using System.Reflection; using System.Web; using System.Web.Caching; using log4net; namespace CacheManagement { 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 ); } } }

Enter the ASP.NET AJAX AutoComplete Extender. It's a perfect candidate for a little cache help. I certainly wouldn't want the database being hit every time a user typed the next letter in the autocomplete box. I apologize for wearing you down with all this talk. I think I've said too much. Let's look at the code for autocomplete webservice.

using log4net; using System.Reflection; using System.Web.Services; using System.Collections.Generic; using CacheManagement; [WebService( Namespace = "http://tempuri.org/" )] [WebServiceBinding( ConformsTo = WsiProfiles.BasicProfile1_1 )] [System.Web.Script.Services.ScriptService] public class AutoCompleteWebService : WebService { private static readonly ILog log = LogManager.GetLogger( MethodBase.GetCurrentMethod().DeclaringType ); private static readonly int cacheTTL = 15; private static readonly int minimumPrefixLength = 3; public AutoCompleteWebService() { //Uncomment the following line if using designed components //InitializeComponent(); } [WebMethod] public string[] GetWordList( string prefixText, int count ) { //minimumPrefixLength should never be greater than prefixText.Length //this is a sanity check to insure the minimumPrefixLength field was set corretly string prefixKey = prefixText.Substring( 0, minimumPrefixLength > prefixText.Length ? prefixText.Length : minimumPrefixLength ); //get the working list List<string> searchList = CacheHelper.GetFromCache<List<string>>( prefixKey, cacheTTL, delegate( out List<string> instance ) { return DataAccess.GetWordList( prefixKey, out instance ); } ); log.Debug( "searchList full count: " + searchList.Count ); //narrow the results if the prefixText is longer than prefixKey if ( prefixText.Length > prefixKey.Length ) { searchList = searchList.FindAll( delegate( string s ) { return s.Substring( 0, prefixText.Length > s.Length ? s.Length : prefixText.Length ) == prefixText; } ); } //only return at maximum the number of results requested by count if ( searchList.Count > count ) searchList = searchList.GetRange( 0, count ); return searchList.ToArray(); } }

The webservice takes the first part of the prefixText (minimumPrefixLength should match the value in the autocomplete extender control) and queries the database for all matches. It stores that result set in cache and then filters the list by prefixText if needed. Finally it will take the top X (count is specified by the control) and return it to the control.

If a user was searching for "automobile", after typing a-u-t all possible results would be cache so that each successive letter will not hit the database. Another database hit will be incurred when a user searches for a different 3 letter prefix.

I'm attaching the full sample code along with a Sql Express database that contains some huge word list I found online (to be attached later, technical difficulties). Enjoy and try not to spend it all in one place.

Cold, Hard, Cache!

anortham — Fri, 07 Mar 2008 04:04:00 GMT

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".

Alan Northam : HttpRuntime.Cache

Practical Implementation - spending some cache.

Cold, Hard, Cache!