中小型网站的缓存策略

在上一篇文章《大型网站系统架构分析》中，介绍了大型网站的系统架构，其中缓存环境非常重要，大型网站往往使用Squid前端缓存服务器群，memcache分布式缓存，负载均衡，CDN等来提升性能。但中小型网站使用这些大家伙就有杀鸡用牛刀的感觉，但毋庸置疑，缓存是需要的，开发者开发程序的时候若没有并发思维，一味注重业务逻辑，往往导致并发用户可能在差不多相同的时间请求相同的数据，网站再请求数据库，如果使用缓存，对这种相同的请求，或者不变的数据，可以缓存起来，直接从内存读取，可以大大提升并发访问的性能，减少数据库访问次数。因为数据库往往是最容易出现瓶颈的地方，而通过使用恰当的缓存技术可以非常有效地减轻数据库的负载。

根据我的经验，中小型网站可以采用这样的缓存策略：

如果是单台服务器可以直接缓存在内存，怕内存不够就合理利用ASP.NET所提供的缓存机制，有现成的干嘛不用呢？缓存存在XML文件中也是一种备选方案，但首先速度没有内存的来的快，其次是写入并发写入XML的时候会锁住文件。可以选用现成的微软Cache application block来实现。
有多台的服务器的话可以考虑memcached，或者其他类似的东西，或者另加一个数据库作为缓存库也行啊。当然有钱的可以用内存数据库。

但不是所有数据都可以缓存的：

对于恒定不变的数据，系统启动后放入缓存就不过期不更新了。
对于偶尔改变的数据，缓存过期时间可以稍微长一些，比如15分钟。
对于经常改变的数据，但访问量极大的热点数据，可以缓存很短的时间，例如30秒，或60秒。（微软有一种数据库数据改变自动同步缓存的SqlDependency功能，有兴趣的读者可以关注）

如此，便可最大程度的利用缓存，从而可以有效提高系统性能，并明显减轻数据库和网络负载。

你也可以统计并发访问最高的页面和DB访问的频率，把网页静态化，或缓存该些热点，淘汰不热的(缓存替换算法)，即使缓存30秒，或60秒也是很大的性能提升，因为他们的并发访问量很大，导致数据库的压力也很大。

代码参考：使用策略模式和单例模式的单机内存缓存：(可以扩展到其他的缓存策略，例如memcache等)

ClassDiagram

ICacheStrategy.cs

  1: using System;  2: using System.Text;  3:    4: namespace Sample.Caching  5: {  6: /// <summary>  7: /// 缓存策略接口  8: /// </summary>  9: public interface ICacheStrategy  10: {  11: void AddObject(string objId, object o);  12:   13: void AddObjectWithTimeout(string objId, object o, int timeoutSec);  14:   15: void AddObjectWithFileChange(string objId, object o, string[] files);  16:   17: void AddObjectWithDepend(string objId, object o, string[] dependKey);  18:    19: void RemoveObject(string objId);  20:    21: object RetrieveObject(string objId);  22:    23: int TimeOut { set;get;}  24: }  25: } 

DefaultCacheStrategy.cs

  1: using System;  2: using System.Web;  3: using System.Collections;  4: using System.Web.Caching;  5:    6: namespace Sample.Caching  7: {  8: /// <summary>  9: /// 默认的缓存策略，实现了缓存策略接口  10: /// </summary>  11: public class DefaultCacheStrategy : ICacheStrategy  12: {  13: private static readonly DefaultCacheStrategy instance = new DefaultCacheStrategy();  14:    15: protected static volatile System.Web.Caching.Cache webCache = System.Web.HttpRuntime.Cache;  16:    17: protected int _timeOut = 1; //默认缓存一分钟，也可以单独设置对象的超时时间  18:    19: private static object syncObj = new object();  20:    21: /// <summary>  22: /// Initializes the <see cref="DefaultCacheStrategy"/> class.  23: /// </summary>  24: static DefaultCacheStrategy()  25: {  26: //lock (syncObj)  27: //{  28: // //System.Web.HttpContext context = System.Web.HttpContext.Current;  29: // //if(context != null)  30: // // webCache = context.Cache;  31: // //else  32: // webCache = System.Web.HttpRuntime.Cache;  33: //}   34: }  35:    36:    37: public int TimeOut  38: {  39: set { _timeOut = value > 0 ? value : 6000; }  40: get { return _timeOut > 0 ? _timeOut : 6000; }  41: }  42:    43:   44: public static System.Web.Caching.Cache GetWebCacheObj  45: {  46: get { return webCache; }  47: }  48:    49: public void AddObject(string objId, object o)  50: {  51:    52: if (objId == null || objId.Length == 0 || o == null)  53: {  54: return;  55: }  56:    57: CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(onRemove);  58:    59: if (TimeOut == 6000)  60: {  61: webCache.Insert(objId, o, null, DateTime.MaxValue, TimeSpan.Zero, System.Web.Caching.CacheItemPriority.High, callBack);  62: }  63: else  64: {  65: webCache.Insert(objId, o, null, DateTime.Now.AddMinutes(TimeOut), System.Web.Caching.Cache.NoSlidingExpiration, System.Web.Caching.CacheItemPriority.High, callBack);  66: }  67: }  68:    69: public void AddObjectWith(string objId, object o)  70: {  71: if (objId == null || objId.Length == 0 || o == null)  72: {  73: return;  74: }  75:    76: CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(onRemove);  77:    78: webCache.Insert(objId, o, null, System.DateTime.Now.AddHours(TimeOut), System.Web.Caching.Cache.NoSlidingExpiration, System.Web.Caching.CacheItemPriority.High, callBack);  79: }  80:    81:   82: public void AddObjectWithTimeout(string objId, object o, int timeoutSec)  83: {  84: if (objId == null || objId.Length == 0 || o == null || timeoutSec <= 0)  85: {  86: return;  87: }  88:    89: CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(onRemove);  90:    91: webCache.Insert(objId, o, null, System.DateTime.Now.AddSeconds(timeoutSec), System.Web.Caching.Cache.NoSlidingExpiration, System.Web.Caching.CacheItemPriority.High, callBack);  92: }  93:    94: public void AddObjectWithFileChange(string objId, object o, string[] files)  95: {  96: if (objId == null || objId.Length == 0 || o == null)  97: {  98: return;  99: }  100:    101: CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(onRemove);  102:    103: CacheDependency dep = new CacheDependency(files, DateTime.Now);  104:    105: webCache.Insert(objId, o, dep, System.DateTime.Now.AddHours(TimeOut), System.Web.Caching.Cache.NoSlidingExpiration, System.Web.Caching.CacheItemPriority.High, callBack);  106: }  107:    108:   109: public void AddObjectWithDepend(string objId, object o, string[] dependKey)  110: {  111: if (objId == null || objId.Length == 0 || o == null)  112: {  113: return;  114: }  115:    116: CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(onRemove);  117:    118: CacheDependency dep = new CacheDependency(null, dependKey, DateTime.Now);  119:    120: webCache.Insert(objId, o, dep, System.DateTime.Now.AddMinutes(TimeOut), System.Web.Caching.Cache.NoSlidingExpiration, System.Web.Caching.CacheItemPriority.High, callBack);  121: }  122:    123: public void onRemove(string key, object val, CacheItemRemovedReason reason)  124: {  125: switch (reason)  126: {  127: case CacheItemRemovedReason.DependencyChanged:  128: break;  129: case CacheItemRemovedReason.Expired:  130: {  131: //CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(this.onRemove);  132:    133: //webCache.Insert(key, val, null, System.DateTime.Now.AddMinutes(TimeOut),  134: // System.Web.Caching.Cache.NoSlidingExpiration,  135: // System.Web.Caching.CacheItemPriority.High,  136: // callBack);  137: break;  138: }  139: case CacheItemRemovedReason.Removed:  140: {  141: break;  142: }  143: case CacheItemRemovedReason.Underused:  144: {  145: break;  146: }  147: default: break;  148: }  149:    150: //TODO: write log here  151: }  152:    153:   154: public void RemoveObject(string objId)  155: {  156: //objectTable.Remove(objId);  157: if (objId == null || objId.Length == 0)  158: {  159: return;  160: }  161: webCache.Remove(objId);  162: }  163:    164:    165: public object RetrieveObject(string objId)  166: {  167: //return objectTable[objId];  168:    169: if (objId == null || objId.Length == 0)  170: {  171: return null;  172: }  173:    174: return webCache.Get(objId);  175: }  176:    177: }  178: } 

CachingManager.cs

  1: using System;  2: using System.Collections.Generic;  3: using System.Text;  4:    5: namespace Sample.Caching  6: {  7: /// <summary>  8: /// The caching manager  9: /// </summary>  10: public class CachingManager  11: {  12: private static ICacheStrategy cs;  13: private static volatile CachingManager instance = null;  14: private static object lockHelper = new object();  15:   16: //private static System.Timers.Timer cacheConfigTimer = new System.Timers.Timer(15000);//Interval in ms  17:    18:   19: private CachingManager()  20: {  21: cs = new DefaultCacheStrategy();  22:    23: Set timer  24: //cacheConfigTimer.AutoReset = true;  25: //cacheConfigTimer.Enabled = true;  26: //cacheConfigTimer.Elapsed += new System.Timers.ElapsedEventHandler(Timer_Elapsed);  27: //cacheConfigTimer.Start();  28: }  29:    30:   31: private static void Timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)  32: {  33: //TODO:  34: }  35:    36:   37: public static CachingManager GetCachingService()  38: {  39: if (instance == null)  40: {  41: lock (lockHelper)  42: {  43: if (instance == null)  44: {  45: instance = new CachingManager();  46: }  47: }  48: }  49:    50: return instance;  51: }  52:    53:   54: public virtual void AddObject(string key, object o)  55: {  56: if (String.IsNullOrEmpty(key) || o == null) return;  57:    58: lock (lockHelper)  59: {  60: if (cs.TimeOut <= 0) return;  61:    62: cs.AddObject(key, o);  63: }  64: }  65:    66:   67: public virtual void AddObject(string key, object o, int timeout)  68: {  69: if (String.IsNullOrEmpty(key) || o == null) return;  70:    71: lock (lockHelper)  72: {  73: if (cs.TimeOut <= 0) return;  74:    75: cs.AddObjectWithTimeout(key, o, timeout);  76: }  77: }  78:    79:   80: public virtual object RetrieveObject(string objectId)  81: {  82: return cs.RetrieveObject(objectId);  83: }  84:    85:   86: public virtual void RemoveObject(string key)  87: {  88: lock (lockHelper)  89: {  90: cs.RemoveObject(key);  91: }  92: }  93:    94:   95: public void LoadCacheStrategy(ICacheStrategy ics)  96: {  97: lock (lockHelper)  98: {  99: cs = ics;  100: }  101: }  102:    103:   104: public void LoadDefaultCacheStrategy()  105: {  106: lock (lockHelper)  107: {  108: cs = new DefaultCacheStrategy();  109: }  110: }  111: }  112: } 

调用：

  1: CachingManager cm = CachingManager.GetCachingService();  2: cm.LoadDefaultCacheStrategy();  3: //从缓存获取  4: IList list = cm.RetrieveObject("mykey") as IList;  5: if (list == null)  6: {  7: list = DAL.GetData(); //从数据库查询  8: cm.AddObject("mykey", list, 2 * 60); //缓存2分钟  9: }  10: return list; 