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一步步实现一个基本的缓存模块
一步步实现一个基本的缓存模块
1. 前言
2. 请求级别缓存
2.1 多线程
3. 进程级别缓存
3.1 分区与计数
3.2 可空缓存值
3.3 封装与集成
4. 小结
1. 前言
- 面向读者:初、中级用户;
- 涉及知识:HttpContext、HttpRuime.Cache、DictionaryEntry、Unit Test等;
- 文章目的:这里的内容不会涉及 Memcached、Redies 等进程外缓存的使用,只针对包含WEB应用的常见场景,实现一个具有线程安全、分区、过期特性的缓存模块,略微提及DI等内容。
- jusfr 原创,转载请注明来自博客园。
2. 请求级别缓存
如果需要线程安全地存取数据,System.Collections.Concurrent 命名空间下的像 ConcurrentDictionary 等实现是首选;更复杂的特性像过期策略、文件依赖等就需要其他实现了。ASP.NET中的HttpContext.Current.Items 常常被用作自定义数据容器,注入工具像Unity、Autofac 等便借助自定义 HttpModule 将容器挂接在 HttpContext.Current 上以进行生命周期管理。
基本接口 ICacheProvider,请求级别的缓存从它定义,考虑到请求级别缓存的运用场景有限,故只定义有限特性;
1 public interface ICacheProvider {2 Boolean TryGet<T>(String key, out T value);3 T GetOrCreate<T>(String key, Func<T> function);4 T GetOrCreate<T>(String key, Func<String, T> factory);5 void Overwrite<T>(String key, T value);6 void Expire(String key);7 }
HttpContext.Current.Items 从 IDictionary 定义,存储 Object-Object 键值对,出于便利与直观,ICacheProvider 只接受String类型缓存键,故HttpContextCacheProvider内部使用 BuildCacheKey(String key) 方法生成真正缓存键以避免键值重复;
同时 HashTable 可以存储空引用作为缓存值,故 TryGet() 方法先进行 Contains() 判断存在与否,再进行类型判断,避免缓存键重复使用;
1 public class HttpContextCacheProvider : ICacheProvider { 2 protected virtual String BuildCacheKey(String key) { 3 return String.Concat("HttpContextCacheProvider_", key); 4 } 5 6 public Boolean TryGet<T>(String key, out T value) { 7 key = BuildCacheKey(key); 8 Boolean exist = false; 9 if (HttpContext.Current.Items.Contains(key)) {10 exist = true;11 Object entry = HttpContext.Current.Items[key];12 if (entry != null && !(entry is T)) {13 throw new InvalidOperationException(String.Format("缓存项`[{0}]`类型错误, {1} or {2} ?",14 key, entry.GetType().FullName, typeof(T).FullName));15 }16 value =http://www.mamicode.com/ (T)entry;17 }18 else {19 value = http://www.mamicode.com/default(T);20 }21 return exist;22 }23 24 public T GetOrCreate<T>(String key, Func<T> function) {25 T value;26 if (TryGet(key, out value)) {27 return value;28 }29 value =http://www.mamicode.com/ function();30 Overwrite(key, value);31 return value;32 }33 34 public T GetOrCreate<T>(String key, Func<String, T> factory) {35 T value;36 if (TryGet(key, out value)) {37 return value;38 }39 value =http://www.mamicode.com/ factory(key);40 Overwrite(key, value);41 return value;42 }43 44 public void Overwrite<T>(String key, T value) {45 key = BuildCacheKey(key);46 HttpContext.Current.Items[key] = value;47 }48 49 public void Expire(String key) {50 key = BuildCacheKey(key);51 HttpContext.Current.Items.Remove(key);52 }53 }
这里使用了 Func<T> 委托的运用,合并查询、判断和添加缓存项的操作以简化接口调用;如果用户期望不同类型缓存值可以存储到相同的 key 上,则需要重新定义 BuildCacheKey() 方法将缓存值类型作为参数参与生成缓存键,此时 Expire() 方法则同样需要了。测试用例:
1 [TestClass] 2 public class HttpContextCacheProviderTest { 3 [TestInitialize] 4 public void Initialize() { 5 HttpContext.Current = new HttpContext(new HttpRequest(null, "http://localhost", null), new HttpResponse(null)); 6 } 7 8 [TestMethod] 9 public void NullValue() {10 var key = "key-null";11 HttpContext.Current.Items.Add(key, null);12 Assert.IsTrue(HttpContext.Current.Items.Contains(key));13 Assert.IsNull(HttpContext.Current.Items[key]);14 }15 16 [TestMethod]17 public void ValueType() {18 var key = "key-guid";19 ICacheProvider cache = new HttpContextCacheProvider();20 var id1 = Guid.NewGuid();21 var id2 = cache.GetOrCreate(key, () => id1);22 Assert.AreEqual(id1, id2);23 24 cache.Expire(key);25 Guid id3;26 var exist = cache.TryGet(key, out id3);27 Assert.IsFalse(exist);28 Assert.AreNotEqual(id1, id3);29 Assert.AreEqual(id3, Guid.Empty);30 }31 }
引用类型测试用例忽略。
2.1 多线程
异步等情况下,HttpContext.Current并非无处不在,故异步等情况下 HttpContextCacheProvider 的使用可能抛出空引用异常,需要被处理,对此园友有过思考 ,这里贴上A大的方案 ,有需求的读者请按图索骥。
3. 进程级别缓存
HttpRuntime.Cache 定义在 System.Web.dll 中,System.Web 命名空间下,实际上是可以使用在非 Asp.Net 应用里的;另外 HttpContext 对象包含一个 Cache 属性,它们的关系可以阅读 HttpContext.Cache 和 HttpRuntime.Cache;
HttpRuntime.Cache 为 System.Web.Caching.Cache 类型,支持滑动/绝对时间过期策略、支持缓存优先级、缓存更新/过期回调、基于文件的缓存依赖项等,功能十分强大,这里借用少数特性来实现进程级别缓存,更多文档请自行检索。
从 ICacheProvider 定义 IHttpRuntimeCacheProvider,添加相对过期与绝对过期、添加批量的缓存过期接口 ExpireAll();
1 public interface IHttpRuntimeCacheProvider : ICacheProvider {2 T GetOrCreate<T>(String key, Func<T> function, TimeSpan slidingExpiration);3 T GetOrCreate<T>(String key, Func<T> function, DateTime absoluteExpiration);4 void Overwrite<T>(String key, T value, TimeSpan slidingExpiration);5 void Overwrite<T>(String key, T value, DateTime absoluteExpiration);6 void ExpireAll();7 }
System.Web.Caching.Cache 只继承 IEnumerable,内部使用 DictionaryEntry 存储Object-Object 键值对,但 HttpRuntime.Cache 只授受字符串类型缓存键及非空缓存值,关于空引用缓存值的问题,我们在3.2中讨论;
故 TryGet() 与 HttpContextCacheProvider.TryGet() 具有显著差异,前者需要拿出值来进行非空判断,后者则是使用 IDictionary.Contains() 方法;
除了 TryGet() 方法与过期过期参数外的差异外,接口实现与 HttpContextCacheProvider 类似;
1 public class HttpRuntimeCacheProvider : IHttpRuntimeCacheProvider { 2 private static readonly Object _sync = new Object(); 3 4 protected virtual String BuildCacheKey(String key) { 5 return String.Concat("HttpRuntimeCacheProvider_", key); 6 } 7 8 public Boolean TryGet<T>(String key, out T value) { 9 key = BuildCacheKey(key);10 Boolean exist = false;11 Object entry = HttpRuntime.Cache.Get(key);12 if (entry != null) {13 exist = true;14 if (!(entry is T)) {15 throw new InvalidOperationException(String.Format("缓存项[{0}]类型错误, {1} or {2} ?",16 key, entry.GetType().FullName, typeof(T).FullName));17 }18 value =http://www.mamicode.com/ (T)entry;19 }20 else {21 value = http://www.mamicode.com/default(T);22 }23 return exist;24 }25 26 public T GetOrCreate<T>(String key, Func<String, T> factory) {27 T result;28 if (TryGet<T>(key, out result)) {29 return result;30 }31 result = factory(key);32 Overwrite(key, result);33 return result;34 }35 36 public T GetOrCreate<T>(String key, Func<T> function) {37 T result;38 if (TryGet<T>(key, out result)) {39 return result;40 }41 result = function();42 Overwrite(key, result);43 return result;44 }45 46 47 public T GetOrCreate<T>(String key, Func<T> function, TimeSpan slidingExpiration) {48 T result;49 if (TryGet<T>(key, out result)) {50 return result;51 }52 result = function();53 Overwrite(key, result, slidingExpiration);54 return result;55 }56 57 public T GetOrCreate<T>(String key, Func<T> function, DateTime absoluteExpiration) {58 T result;59 if (TryGet<T>(key, out result)) {60 return result;61 }62 result = function();63 Overwrite(key, result, absoluteExpiration);64 return result;65 }66 67 public void Overwrite<T>(String key, T value) {68 HttpRuntime.Cache.Insert(BuildCacheKey(key), value);69 }70 71 //slidingExpiration 时间内无访问则过期72 public void Overwrite<T>(String key, T value, TimeSpan slidingExpiration) {73 HttpRuntime.Cache.Insert(BuildCacheKey(key), value, null,74 Cache.NoAbsoluteExpiration, slidingExpiration);75 }76 77 //absoluteExpiration 绝对时间过期78 public void Overwrite<T>(String key, T value, DateTime absoluteExpiration) {79 HttpRuntime.Cache.Insert(BuildCacheKey(key), value, null,80 absoluteExpiration, Cache.NoSlidingExpiration);81 }82 83 public void Expire(String key) {84 HttpRuntime.Cache.Remove(BuildCacheKey(key));85 }86 87 public void ExpireAll() {88 lock (_sync) {89 var entries = HttpRuntime.Cache.OfType<DictionaryEntry>()90 .Where(entry => (entry.Key is String) && ((String)entry.Key).StartsWith("HttpRuntimeCacheProvider_"));91 foreach (var entry in entries) {92 HttpRuntime.Cache.Remove((String)entry.Key);93 }94 }95 }96 }
测试用例与 HttpContextCacheProviderTest 类似,这里贴出缓存过期的测试:
1 public class HttpRuntimeCacheProviderTest { 2 [TestMethod] 3 public void GetOrCreateWithAbsoluteExpirationTest() { 4 var key = Guid.NewGuid().ToString(); 5 var val = Guid.NewGuid(); 6 7 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(); 8 var result = cacheProvider.GetOrCreate<Guid>(key, () => val, DateTime.UtcNow.AddSeconds(2D)); 9 Assert.AreEqual(result, val);10 11 var exist = cacheProvider.TryGet<Guid>(key, out val);12 Assert.IsTrue(exist);13 Assert.AreEqual(result, val);14 15 Thread.Sleep(2000);16 exist = cacheProvider.TryGet<Guid>(key, out val);17 Assert.IsFalse(exist);18 Assert.AreEqual(val, Guid.Empty);19 }20 21 [TestMethod]22 public void ExpireAllTest() {23 var key = Guid.NewGuid().ToString();24 var val = Guid.NewGuid();25 26 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider();27 var result = cacheProvider.GetOrCreate<Guid>(key, () => val);28 Assert.AreEqual(result, val);29 30 cacheProvider.ExpireAll();31 Guid val2;32 var exist = cacheProvider.TryGet<Guid>(key, out val2);33 Assert.IsFalse(exist);34 Assert.AreEqual(val2, Guid.Empty);35 }36 }
3.1 分区与计数
缓存分区是常见需求,缓存用户A、用户B的认证信息可以拿用户标识作为缓存键,但每个用户分别有一整套包含授权的其他数据时,为创建以用户分区的缓存应该是更好的选择;
常规的想法是为缓存添加类似 `Region` 或 `Partition`的参数,个人觉得这不是很好的实践,因为接口被修改,同时过多的参数非常让人困惑;
读者可能对前文中 BuildCacheKey() 方法被 protected virtual 修饰觉得很奇怪,是的,个人觉得定义新的接口,配合从缓存Key的生成算法作文章来分区貌似比较巧妙,也迎合依赖注册被被广泛使用的现状;
分区的进程级别缓存定义,只需多出一个属性:
1 public interface IHttpRuntimeRegionCacheProvider : IHttpRuntimeCacheProvider {2 String Region { get; }3 }
分区的缓存实现,先为 IHttpRuntimeCacheProvider 添加计数,然后重构HttpRuntimeCacheProvider,提取出过滤算法,接着重写 BuildCacheKey() 方法的实现,使不同分区的生成不同的缓存键,缓存项操作方法无须修改;
1 public interface IHttpRuntimeCacheProvider : ICacheProvider { 2 ... 3 Int32 Count { get; } 4 } 5 6 public class HttpRuntimeCacheProvider : IHttpRuntimeCacheProvider { 7 ... 8 protected virtual Boolean Hit(DictionaryEntry entry) { 9 return (entry.Key is String) && ((String)entry.Key).StartsWith("HttpRuntimeCacheProvider_");10 }11 12 public void ExpireAll() {13 lock (_sync) {14 var entries = HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit);15 foreach (var entry in entries) {16 HttpRuntime.Cache.Remove((String)entry.Key);17 }18 }19 }20 21 public Int32 Count {22 get {23 lock (_sync) {24 return HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit).Count();25 }26 }27 }28 }29 30 public class HttpRuntimeRegionCacheProvider : HttpRuntimeCacheProvider, IHttpRuntimeRegionCacheProvider {31 private String _prefix;32 public virtual String Region { get; private set; }33 34 private String GetPrifix() {35 if (_prefix == null) {36 _prefix = String.Concat("HttpRuntimeRegionCacheProvider_", Region, "_");37 }38 return _prefix;39 }40 41 public HttpRuntimeRegionCacheProvider(String region) {42 Region = region;43 }44 45 protected override String BuildCacheKey(String key) {46 //Region 为空将被当作 String.Empty 处理47 return String.Concat(GetPrifix(), base.BuildCacheKey(key));48 }49 50 protected override Boolean Hit(DictionaryEntry entry) {51 return (entry.Key is String) && ((String)entry.Key).StartsWith(GetPrifix());52 }53 }
测试用例示例了两个分区缓存对相同 key 的操作:
1 [TestClass] 2 public class HttpRuntimeRegionCacheProviderTest { 3 [TestMethod] 4 public void ValueType() { 5 var key = "key-guid"; 6 IHttpRuntimeCacheProvider cache1 = new HttpRuntimeRegionCacheProvider("Region1"); 7 var id1 = cache1.GetOrCreate(key, Guid.NewGuid); 8 9 IHttpRuntimeCacheProvider cache2 = new HttpRuntimeRegionCacheProvider("Region2");10 var id2 = cache2.GetOrCreate(key, Guid.NewGuid);11 Assert.AreNotEqual(id1, id2);12 13 cache1.ExpireAll();14 Assert.AreEqual(cache1.Count, 0);15 Assert.AreEqual(cache2.Count, 1);16 }17 }
至此一个基本的缓存模块已经完成;
3.2 可空缓存值
前文提及过,HttpRuntime.Cache 不授受空引用作为缓存值,与 HttpContext.Current.Items表现不同,另一方面实际需求中,空值作为字典的值仍然是有意义,此处给出一个支持空缓存值的实现;
HttpRuntime.Cache 断然是不能把 null 存入的,查看 HttpRuntimeCacheProvider.TryGet() 方法,可知 HttpRuntime.Cache.Get() 获取的总是 Object 类型,思路可以这样展开:
1) 添加缓存时进行判断,如果非空,常规处理,否则把用一个特定的自定义对象存入;
2) 取出缓存时进行判断,如果为特定的自定义对象,返回 null;
为 HttpRuntimeCacheProvider 的构造函数添加可选参数,TryGet() 加入 null 判断逻辑;添加方法 BuildCacheEntry(),替换空的缓存值为 _nullEntry,其他方法不变;
1 public class HttpRuntimeCacheProvider : IHttpRuntimeCacheProvider { 2 private static readonly Object _sync = new Object(); 3 private static readonly Object _nullEntry = new Object(); 4 private Boolean _supportNull; 5 6 public HttpRuntimeCacheProvider(Boolean supportNull = false) { 7 _supportNull = supportNull; 8 } 9 10 protected virtual String BuildCacheKey(String key) { 11 return String.Concat("HttpRuntimeCacheProvider_", key); 12 } 13 14 protected virtual Object BuildCacheEntry<T>(T value) { 15 Object entry = value; 16 if (value =http://www.mamicode.com/= null) { 17 if (_supportNull) { 18 entry = _nullEntry; 19 } 20 else { 21 throw new InvalidOperationException(String.Format("Null cache item not supported, try ctor with paramter ‘supportNull = true‘ ")); 22 } 23 } 24 return entry; 25 } 26 27 public Boolean TryGet<T>(String key, out T value) { 28 Object entry = HttpRuntime.Cache.Get(BuildCacheKey(key)); 29 Boolean exist = false; 30 if (entry != null) { 31 exist = true; 32 if (!(entry is T)) { 33 if (_supportNull && !(entry == _nullEntry)) { 34 throw new InvalidOperationException(String.Format("缓存项`[{0}]`类型错误, {1} or {2} ?", 35 key, entry.GetType().FullName, typeof(T).FullName)); 36 } 37 value = http://www.mamicode.com/(T)((Object)null); 38 } 39 else { 40 value =http://www.mamicode.com/ (T)entry; 41 } 42 } 43 else { 44 value = http://www.mamicode.com/default(T); 45 } 46 return exist; 47 } 48 49 public T GetOrCreate<T>(String key, Func<String, T> factory) { 50 T value; 51 if (TryGet<T>(key, out value)) { 52 return value; 53 } 54 value =http://www.mamicode.com/ factory(key); 55 Overwrite(key, value); 56 return value; 57 } 58 59 public T GetOrCreate<T>(String key, Func<T> function) { 60 T value; 61 if (TryGet<T>(key, out value)) { 62 return value; 63 } 64 value =http://www.mamicode.com/ function(); 65 Overwrite(key, value); 66 return value; 67 } 68 69 public T GetOrCreate<T>(String key, Func<T> function, TimeSpan slidingExpiration) { 70 T value; 71 if (TryGet<T>(key, out value)) { 72 return value; 73 } 74 value =http://www.mamicode.com/ function(); 75 Overwrite(key, value, slidingExpiration); 76 return value; 77 } 78 79 public T GetOrCreate<T>(String key, Func<T> function, DateTime absoluteExpiration) { 80 T value; 81 if (TryGet<T>(key, out value)) { 82 return value; 83 } 84 value =http://www.mamicode.com/ function(); 85 Overwrite(key, value, absoluteExpiration); 86 return value; 87 } 88 89 public void Overwrite<T>(String key, T value) { 90 HttpRuntime.Cache.Insert(BuildCacheKey(key), BuildCacheEntry<T>(value)); 91 } 92 93 //slidingExpiration 时间内无访问则过期 94 public void Overwrite<T>(String key, T value, TimeSpan slidingExpiration) { 95 HttpRuntime.Cache.Insert(BuildCacheKey(key), BuildCacheEntry<T>(value), null, 96 Cache.NoAbsoluteExpiration, slidingExpiration); 97 } 98 99 //absoluteExpiration 时过期100 public void Overwrite<T>(String key, T value, DateTime absoluteExpiration) {101 HttpRuntime.Cache.Insert(BuildCacheKey(key), BuildCacheEntry<T>(value), null,102 absoluteExpiration, Cache.NoSlidingExpiration);103 }104 105 public void Expire(String key) {106 HttpRuntime.Cache.Remove(BuildCacheKey(key));107 }108 109 protected virtual Boolean Hit(DictionaryEntry entry) {110 return (entry.Key is String) && ((String)entry.Key).StartsWith("HttpRuntimeCacheProvider_");111 }112 113 public void ExpireAll() {114 lock (_sync) {115 var entries = HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit);116 foreach (var entry in entries) {117 HttpRuntime.Cache.Remove((String)entry.Key);118 }119 }120 }121 122 public Int32 Count {123 get {124 lock (_sync) {125 return HttpRuntime.Cache.OfType<DictionaryEntry>().Where(Hit).Count();126 }127 }128 }129 }
然后是分区缓存需要修改构造函数:
1 public HttpRuntimeRegionCacheProvider(String region) 2 : base(false) { 3 Region = region; 4 } 5 6 public HttpRuntimeRegionCacheProvider(String region, Boolean supportNull) 7 : base(supportNull) { 8 Region = region; 9 }10 ...11 }
测试用例:
1 [TestClass] 2 public class HttpRuntimeCacheProviderTest { 3 [TestMethod] 4 public void NullCacheErrorTest() { 5 var key = "key-null"; 6 Person person = null; 7 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(false); 8 try { 9 cacheProvider.GetOrCreate<Person>(key, () => person); //error10 Assert.Fail();11 }12 catch (Exception ex) {13 Assert.IsTrue(ex is InvalidOperationException);14 }15 16 Person person2;17 var exist = cacheProvider.TryGet(key, out person2);18 Assert.IsFalse(exist);19 Assert.AreEqual(person2, null);20 }21 22 [TestMethod]23 public void NullableCacheTest() {24 var key = "key-nullable";25 Person person = null;26 IHttpRuntimeCacheProvider cacheProvider = new HttpRuntimeCacheProvider(true);27 cacheProvider.GetOrCreate<Person>(key, () => person);28 Person person2;29 var exist = cacheProvider.TryGet(key, out person2);30 Assert.IsTrue(exist);31 Assert.AreEqual(person2, null);32 }33 34 class Person {35 public Int32 Id { get; set; }36 public String Name { get; set; }37 }38 }
3.3 封装与集成
多数情况下我们不需要暴露实现和手动创建上文所提各种 CacheProvider,实践中它们被 internal 修饰,再配合工厂类使用:
1 public static class CacheProviderFacotry { 2 public static ICacheProvider GetHttpContextCache() { 3 return new HttpContextCacheProvider(); 4 } 5 6 public static IHttpRuntimeCacheProvider GetHttpRuntimeCache(Boolean supportNull = false) { 7 return new HttpRuntimeCacheProvider(supportNull); 8 } 9 10 public static IHttpRuntimeRegionCacheProvider GetHttpRuntimeRegionCache(String region, Boolean supportNull = false) {11 return new HttpRuntimeRegionCacheProvider(region, supportNull);12 }13 14 public static IHttpRuntimeRegionCacheProvider Region(this IHttpRuntimeCacheProvider runtimeCacheProvider, String region, Boolean supportNull = false) {15 return GetHttpRuntimeRegionCache(region, supportNull);16 }17 }
然后在依赖注入中的声明如下,这里是 Autofac 下的组件注册:
1 ...2 //请求级别缓存, 使用 HttpContext.Current.Items 作为容器3 builder.Register(ctx => CacheProviderFacotry.GetHttpContextCache()).As<ICacheProvider>().InstancePerLifetimeScope();4 //进程级别缓存, 使用 HttpRuntime.Cache 作为容器5 builder.RegisterInstance(CacheProviderFacotry.GetHttpRuntimeCache()).As<IRuntimeCacheProvider>().ExternallyOwned();6 //进程级别且隔离的缓存, 若出于key算法唯一考虑而希望加入上下文件信息, 则仍然需要 CacheModule 类的实现7 builder.Register(ctx => CacheProviderFacotry.GetHttpRuntimeRegionCache(/*... 分区依据 ...*/))8 .As<IRuntimeRegionCacheProvider>().InstancePerLifetimeScope();9 ...
4. 小结
本文简单探讨了一个具有线程安全、分区、过期特性缓存模块的实现过程,只使用了HttpRuntime.Cache的有限特性,有更多需求的同学可以自行扩展;见解有限,谬误之处还请园友指正。
园友Jusfr 原创,转载请注明来自博客园 。
一步步实现一个基本的缓存模块