简单利用Memcached进行缓存层设计

正在考虑web应用缓存层的设计，参考了不少资料，估计还是需要用到相对成熟应用广泛的分布式缓存Memcached。在.net平台上早就有相对成熟的Memcached客户端产品，如BeITMemcached和EnyimMemcached，业余时间看了一下源码，自己分析并调用一下并不困难。这里简单介绍一下利用Memcached的一个简单的缓存层设计，示例代码基于EnyimMemcached，下面以贴代码为主。

一、公共缓存接口

分析asp.net web caching的缓存类，我们大致可以抽象出如下几个接口方法：

Contractnamespace DotNet.Common.EnyimCache
{
    /// <summary>
    /// memcached公共缓存调用方法接口(读)
    /// </summary>
    public interface ICacheReaderService
    {

        /// <summary>
        /// 返回指定key的对象
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        object Get(string key);

        /// <summary>
        /// 返回指定key的对象
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="key"></param>
        /// <returns></returns>
        T Get<T>(string key);

        /// <summary>
        /// 是否存在
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        bool isExists(string key);
    }

    /// <summary>
    /// memcached公共缓存调用方法接口(写)
    /// </summary>
    public interface ICacheWriterService
    {
        /// <summary>
        /// 缓存有效间隔时间 (以分钟为单位)
        /// </summary>
        int TimeOut { set; get; }

        /// <summary>
        /// 添加指定key的对象
        /// </summary>
        /// <param name="key"></param>
        /// <param name="obj"></param>
        void Add(string key, object obj);

        /// <summary>
        /// 添加指定key的对象
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="key"></param>
        /// <param name="obj"></param>
        void Add<T>(string key, T obj);

        /// <summary>
        /// 移除指定key的对象
        /// </summary>
        /// <param name="key"></param>
        bool Remove(string key);

        /// <summary>
        /// 修改指定key的对象
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        bool Modify(string key, object destObj);

        /// <summary>
        /// 清空缓存
        /// </summary>
        /// <returns></returns>
        bool Release();
    }
}

看命名就知道，增删改查是也。根据个人使用缓存的经验，修改操作通常是不需要的，如果确实需要修改缓存数据，直接删除然后添加就是改了。

还有，你可能会问，这里为什么要定义两个接口？原因主要是考虑到读操作（查询）是经常使用的，而写操作（增删改）相对较少，所以也把它们设计成读写分离的方式。

二、缓存服务实现

这里就需要调用Memcached客户端封装好的调用方法，实现增删改查等方法。

Implementusing System;

namespace DotNet.Common.EnyimCache
{
    using Enyim.Caching.Memcached;

    public class CacheReaderService : BaseService, ICacheReaderService
    {

        public int TimeOut
        {
            get;
            set;
        }

        public CacheReaderService()
        {

        }

        public object Get(string key)
        {
            object obj = null;
            Client.TryGet(key, out obj);
            return obj;
        }

        public T Get<T>(string key)
        {
            object obj = Get(key);
            T result = default(T);
            if (obj != null)
            {
                result = (T)obj;
            }
            return result;
        }

        public bool isExists(string key)
        {
            object obj = Get(key);
            return (obj == null) ? false : true;
        }
    }

    public class CacheWriterService : BaseService, ICacheWriterService
    {
        public int TimeOut
        {
            get;
            set;
        }

        public CacheWriterService()
        {

        }

        public CacheWriterService(int timeOut)
        {
            this.TimeOut = timeOut;
        }

        public void Add(string key, object obj)
        {
            if (TimeOut > 0)
            {
                Client.Store(StoreMode.Add, key, obj, DateTime.Now.AddMinutes(TimeOut));
            }
            else
            {
                Client.Store(StoreMode.Add, key, obj);
            }
        }

        public void Add<T>(string key, T obj)
        {
            if (TimeOut > 0)
            {
                Client.Store(StoreMode.Add, key, obj, DateTime.Now.AddMinutes(TimeOut));
            }
            else
            {
                Client.Store(StoreMode.Add, key, obj);
            }
        }

        public bool Remove(string key)
        {
            return Client.Remove(key);
        }

        public bool Modify(string key, object destObj)
        {
            return Client.Store(StoreMode.Set, key, destObj);
        }

        /// <summary>
        /// 清空缓存 TO DO
        /// </summary>
        /// <returns></returns>
        public bool Release()
        {
            throw new NotImplementedException();
        }
    }
}

基类里初始化一个MemcachedClient示例Client，这个Client的方法里封装了较多的函数。查看源码可以知道，它们本质上都是向Memcached服务端发送相关指令（run command），然后解析返回的二进制数据，如果您熟悉memcached所使用的协议，理解起来应该会相当简单。本文示例只使用了客户端提供的几个方法。

同时要注意，在实现具体缓存服务的时候，CacheWriterService有两个构造函数，其中带参数的是为缓存显式指定过期时间。这个参数在实际应用中通常需要配置，显然是比较灵活一些的。

备注：在接口中有一个函数Release，本来的目标是清空所有的缓存数据，但是客户端没有直接提供对应的函数，如果您有好的方法，请不吝赐教。

三、简单的读写测试

贴一下字符串、时间、单个类和集合的增删改查示例代码：

CRUD            ICacheWriterService writer = CacheBuilder.GetWriterService();//writer 使用memcached默认过期时间
            ICacheReaderService reader = CacheBuilder.GetReaderService();//reader

            #region 字符串

            string strKey = "hello";

            bool isOK = writer.Remove(strKey); //移除
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            writer.Add(strKey, "hello world"); //添加
            Console.WriteLine("Add key {0}, value:hello world", strKey);

            bool isExists = reader.isExists(strKey);//是否存在
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            string result = reader.Get(strKey) as string;//查询
            Console.WriteLine("Get key {0}:{1}", strKey, result);

            bool isModify = writer.Modify(strKey, "Hello Memcached!");//修改
            Console.WriteLine("Modify key {0}, value:Hello Memcached. The result is:{1}", strKey, isModify);

            result = reader.Get<string>(strKey);
            Console.WriteLine("Generic get key {0}:{1}", strKey, result);

            isOK = writer.Remove(strKey);
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            isExists = reader.isExists(strKey);
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            result = reader.Get(strKey) as string;
            Console.WriteLine("Get key {0}:{1}", strKey, result);

            result = reader.Get<string>(strKey);
            Console.WriteLine("Generic get key {0}:{1}", strKey, result);
            Console.WriteLine();
            Console.WriteLine("===========================================");
            Console.Read();

            #endregion

            #region 时间

            DateTime dtNow = DateTime.Now;
            strKey = "datetime";
            isOK = writer.Remove(strKey); //移除
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            writer.Add(strKey, dtNow); //添加
            Console.WriteLine("Add key {0}, value:{1}", strKey, dtNow);

            isExists = reader.isExists(strKey);//是否存在
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            DateTime dt = (DateTime)reader.Get(strKey);//查询
            Console.WriteLine("Get key {0}:{1}", strKey, dt);

            dt = reader.Get<DateTime>(strKey);
            Console.WriteLine("Generic get key {0}:{1}", strKey, dt);

            isOK = writer.Remove(strKey);
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            isExists = reader.isExists(strKey);
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            Console.WriteLine("Get key {0}:{1}", strKey, reader.Get(strKey));

            Console.WriteLine("Generic get key {0}:{1}", strKey, reader.Get<DateTime>(strKey));//default(datetime)
            Console.WriteLine();
            Console.WriteLine("===========================================");

            Console.Read();

            #endregion

            #region 类

            dtNow = DateTime.Now;
            Province province = new Province(13579, "江苏", dtNow, dtNow);

            strKey = string.Format("{0}_{1}", province.GetType().Name, province.Id);//省
            isOK = writer.Remove(strKey); //移除
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            writer.Add(strKey, province); //添加
            Console.WriteLine("Add key {0}, value:{1}", strKey, dtNow);

            isExists = reader.isExists(strKey);//是否存在
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            Province queryProvince = (Province)reader.Get(strKey);//查询
            Console.WriteLine("Get key {0}:{1}", strKey, queryProvince.ProvinceName);

            queryProvince = reader.Get<Province>(strKey);
            Console.WriteLine("Generic get key {0}:{1}", strKey, queryProvince.ProvinceName);

            isOK = writer.Remove(strKey);
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            isExists = reader.isExists(strKey);
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            Console.WriteLine("Get key {0}:{1}", strKey, reader.Get(strKey));

            Console.WriteLine("Generic get key {0}:{1}", strKey, reader.Get<Province>(strKey));
            Console.WriteLine();
            Console.WriteLine("===========================================");

            Console.Read();

            #endregion

            #region 集合(列表)

            dtNow = DateTime.Now;
            IList<City> listCities = new List<City>();
            City city = new City(135, province.Id, "南京", "210000", dtNow, dtNow);
            listCities.Add(city);
            city = new City(246, province.Id, "苏州", "215000", dtNow, dtNow);
            listCities.Add(city);

            strKey = string.Format("List_{0}_{1}_{2}", province.GetType().Name, province.Id, city.GetType().Name);//省份对应城市
            isOK = writer.Remove(strKey); //移除
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            writer.Add(strKey, listCities); //添加
            Console.WriteLine("Add key {0}, value:", strKey);
            foreach (var item in listCities)
            {
                Console.WriteLine("CityId:{0} CityName:{1}", item.Id, item.CityName);
            }

            isExists = reader.isExists(strKey);//是否存在
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            IList<City> queryCities = reader.Get(strKey) as IList<City>;//查询
            Console.WriteLine("Get key {0}:", strKey);
            foreach (var item in queryCities)
            {
                Console.WriteLine("CityId:{0} CityName:{1}", item.Id, item.CityName);
            }

            queryCities = reader.Get<IList<City>>(strKey);
            Console.WriteLine("Generic get key {0}:", strKey);
            foreach (var item in queryCities)
            {
                Console.WriteLine("CityId:{0} CityName:{1}", item.Id, item.CityName);
            }

            isOK = writer.Remove(strKey);
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            isExists = reader.isExists(strKey);
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            Console.WriteLine("Get key {0}:{1}", strKey, reader.Get(strKey));

            Console.WriteLine("Generic get key {0}:{1}", strKey, reader.Get<IList<City>>(strKey));
            Console.WriteLine();
            Console.WriteLine("===========================================");

            Console.Read();

            #endregion

            #region 集合(字典)

            dtNow = DateTime.Now;
            IDictionary<int, City> dictCities = new Dictionary<int, City>();
            city = new City(123, province.Id, "镇江", "212000", dtNow, dtNow);
            dictCities.Add(city.Id, city);
            city = new City(321, province.Id, "扬州", "225000", dtNow, dtNow);
            dictCities.Add(city.Id, city);

            strKey = string.Format("Dictionary_{0}_{1}_{2}", province.GetType().Name, province.Id, city.GetType().Name);//省份对应城市
            isOK = writer.Remove(strKey); //移除
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            writer.Add(strKey, dictCities); //添加
            Console.WriteLine("Add key {0}, value:", strKey);
            foreach (var item in dictCities)
            {
                Console.WriteLine("CityId:{0} CityName:{1}", item.Key, item.Value.CityName);
            }

            isExists = reader.isExists(strKey);//是否存在
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            IDictionary<int, City> queryDictCities = reader.Get(strKey) as IDictionary<int, City>;//查询
            Console.WriteLine("Get key {0}:", strKey);
            foreach (var item in queryDictCities)
            {
                Console.WriteLine("CityId:{0} CityName:{1}", item.Key, item.Value.CityName);
            }

            queryDictCities = reader.Get<IDictionary<int, City>>(strKey);
            Console.WriteLine("Generic get key {0}:", strKey);
            foreach (var item in queryDictCities)
            {
                Console.WriteLine("CityId:{0} CityName:{1}", item.Key, item.Value.CityName);
            }

            isOK = writer.Remove(strKey);
            Console.WriteLine("Removed key {0}:{1}", strKey, isOK);

            isExists = reader.isExists(strKey);
            Console.WriteLine("Key {0} exists:{1}", strKey, isExists);

            Console.WriteLine("Get key {0}:{1}", strKey, reader.Get(strKey));

            Console.WriteLine("Generic get key {0}:{1}", strKey, reader.Get<IDictionary<int, City>>(strKey));
            Console.WriteLine();
            Console.WriteLine("===========================================");

            Console.Read();

            #endregion

这里就不贴全部代码了，文章最后有示例可以下载。

在我的简单测试中，对常见基础数据类型如（字符串、数组、数字和时间）、集合（列表和字典）都有良好的表现，对datatable和dataset同样表现不俗，但是不太建议直接缓存这两种重粒度的类型。

在显式指定过期时间的示例中，指定过期时间是一分钟，但是memcached实际过期时间有时候好像会多于一分钟，估计是系统内部的延迟。

在本地计算机上进行10万次循环添加缓存的过程中，发现系统内存果然增加的非常厉害。然后查询性能并没有显著下降，也许和我的单机测试环境有关，所以我认为测试结果并没有说服力，要知道，memcached的优势是它的分布式缓存实现。

有人发现如何保证缓存系统的键唯一也非常令人头疼。同样的缓存框架，不同项目不同开发者如何保证自己程序添加的缓存键唯一呢？有一种简单方法就是通过拼接字符串成为有意义的主键，比如按照项目名、命名空间、类名、数据库中的主键组合构成主键等等。当然了，在查询的时候也要自己封装特定格式的字符串主键。个人感觉确实是一个行之有效的方法。

demo下载：SimpleCacheApp