利用订阅模式实现缓存更新

1. 引言

很多Web项目，都需要和数据库打交道，典型的就是CRUD（读，写，更新，删除）操作。无论是哪种数据库，Asp.Net MVC 作为后端框架的项目，都有很多操作数据库的类库。最近在一个Web项目中就用到了EntityFramework来存取Sql Server。相信很多人都懂得，如何利用EntityFramework存取数据，所以这方面不做详细的介绍。 今天给大家介绍一种如何利用订阅模式来实现缓存更新。

实现过程主要参照NopCommerce，它是一个开源的电商平台，里面有不少精妙的设计，值得每一个.Net程序员一看。

 

2. 实现

先来看看未采用Cache的设计，定义一个Service，这个Service主要是利用EntityFramework存取数据。

   //  定义简单的数据模型
    public class TargetSegment
    {
        public int Id { get; set; }
        public string Segment { get; set; }
    }

Service接口和默认实现

    public interface ITargetSegmentService
    {
        TargetSegment GetById(object id);

        void Insert(TargetSegment item);

        void Update(TargetSegment item);

        void Delete(TargetSegment item);

        IList<TargetSegment> GetAll();
    }

    public class TargetSegmentService : ITargetSegmentService
    {
        private IRepository<TargetSegment> _repository;
        public TargetSegmentService(IRepository<TargetSegment> repository)
        {
            this._repository = repository;
        }

        public TargetSegment GetById(object id)
        {
            return this._repository.GetById(id);
        }

        public void Insert(TargetSegment item)
        {
            if (item == null)
                throw new ArgumentNullException("item");

            this._repository.Insert(item);
        }

        public void Update(TargetSegment item)
        {
            if (item == null)
                throw new ArgumentNullException("item");

            this._repository.Update(item);
        }

        public void Delete(TargetSegment item)
        {
            if (item == null)
                throw new ArgumentNullException("item");

            this._repository.Delete(item);
        }

        public IList<TargetSegment> GetAll()
        {
            return this._repository.Table.ToList();
        }
    }

这个实现能够满足基本的需求，但是缺乏优化使得每次当Service调用GetAll 函数的时候，都会从数据库读取所有该类条目。如果没有Update,Create或者Delete每次GetAll返回的数据都是一样的。

因此可以在GetAll这里添加缓存，在Update,Create，Delete更新缓存。

首先我们利用.Net自带的System.Runtime.Caching.ObjectCache 类 定义缓存接口以及简单实现：

    public interface ICache : IDisposable
    {
        T Get<T>(string key);
        void Set(string key, object data, int cacheMinutes);
        bool IsSet(string key);
        void Remove(string key);
        void RemoveByPattern(string pattern);
        void Clear();
    }

    public static class CacheExtension
    {
        public static T GetOrAdd<T>(this ICache cache, string key, int cacheMinutes, Func<T> factory)
        {
            if (cache.IsSet(key))
            {
                return cache.Get<T>(key);
            }
            else
            {
                var data = factory();
                cache.Set(key, data, cacheMinutes);
                return data;
            }
        }

        public static void RemoveByPattern(this ICache cache, string pattern, IEnumerable<string> keys)
        {
            Regex regex = new Regex(pattern, RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.Singleline);
            foreach (var target in keys.Where(k=>regex.IsMatch(k)))
            {
                cache.Remove(target);
            }
        }
    }

    public partial class MemoryCache : ICache
    {
        protected System.Runtime.Caching.ObjectCache Cache
        {
            get { return System.Runtime.Caching.MemoryCache.Default; }
        }

        public MemoryCache()
        {
        }

        public T Get<T>(string key)
        {
            return (T) this.Cache[key];
        }

        public void Set(string key, object data, int cacheMinutes)
        {
            if (data == null)
                return;

            this.Cache.Add(new CacheItem(key, data),
                new CacheItemPolicy() {AbsoluteExpiration = DateTime.Now + TimeSpan.FromMinutes(cacheMinutes)});
        }

        public bool IsSet(string key)
        {
            return this.Cache.Contains(key);
        }

        public void Remove(string key)
        {
            this.Cache.Remove(key);
        }

        public void RemoveByPattern(string pattern)
        {
            this.RemoveByPattern(pattern, Cache.Select(item => item.Key));
        }

        public void Clear()
        {
            foreach (var item  in Cache)
            {
                this.Remove(item.Key);
            }
        }

        public void Dispose()
        {
            this.Clear();
        }
    }

到此为止，已经可以实现缓存跟新了，只要在Insert,Update，Delete 函数里面添加 缓存更新即可。我们进一步利用订阅模式。

订阅模式 一般有三个对象，一个是Subject代表发送给所有订阅者的主题信息。一个是Consumer代表订阅者接受订阅的Subject, 另一个是Publisher代表发送Subject的实现。

这里我们定义几个Subject

   // 代表数据删除的Subject
   public class EntityDeleted<T> where T: class 
    {
        public T Entity { get; private set; }

        public EntityDeleted(T entity)
        {
            this.Entity = entity;
        }
    }

    public class EntityInserted<T> where T : class
    {
        public T Entity { get; private set; }

        public EntityInserted(T entity)
        {
            this.Entity = entity;
        }
    }

    public class EntityUpdated<T> where T : class 
    {
        public T Entity { get; private set; }

        public EntityUpdated(T entity)
        {
            this.Entity = entity;
        }
    }

定义Consumer接口和Cache相关的Consumer实现

    public interface IConsumer<T>
    {
        void HandleEvent(T eventMessage);
    }

    public class CacheEventConsumer : 
        IConsumer<EntityInserted<TargetSegment>>,
        IConsumer<EntityUpdated<TargetSegment>>, 
        IConsumer<EntityDeleted<TargetSegment>>

    {
        private readonly ICache _cache;
        public CacheEventConsumer()
        {
            this._cache = DependencyResolver.Current.GetService(typeof(ICache)) as ICache;
        }

        public const string AllTargetSegmentPattern = "TargetSegment.All";

        public void HandleEvent(EntityInserted<TargetSegment> eventMessage)
        {
            this._cache.RemoveByPattern(AllTargetSegmentPattern);
        }

        public void HandleEvent(EntityUpdated<TargetSegment> eventMessage)
        {
            this._cache.RemoveByPattern(AllTargetSegmentPattern);
        }

        public void HandleEvent(EntityDeleted<TargetSegment> eventMessage)
        {
            this._cache.RemoveByPattern(AllTargetSegmentPattern);
        }
    }

定义publish接口和实现

    public interface IEventPublisher
    {
        void Publish<T>(T eventMessage);
    }

    public static class EventPublisherExtension
    {
        public static void EntityInserted<T>(this IEventPublisher eventPublisher, T entity) where T : class
        {
            eventPublisher.Publish(new EntityInserted<T>(entity));
        }

        public static void EntityUpdated<T>(this IEventPublisher eventPublisher, T entity) where T : class
        {
            eventPublisher.Publish(new EntityUpdated<T>(entity));
        }

        public static void EntityDeleted<T>(this IEventPublisher eventPublisher, T entity) where T : class
        {
            eventPublisher.Publish(new EntityDeleted<T>(entity));
        }
    }

    public class EventPublisher : IEventPublisher
    {
        public EventPublisher()
        {
        }

        public void Publish<T>(T eventMessage)
        {
            var consumers = DependencyResolver.Current.GetServices<IConsumer<T>>();
            foreach (var consumer in consumers)
            {
                this.PublishToConsumer(consumer, eventMessage);
            }
        }

        protected virtual void PublishToConsumer<T>(IConsumer<T> consumer, T eventMessage)
        {
            try
            {
                consumer.HandleEvent(eventMessage);
            }
            catch (Exception exception)
            {
                   throw;
            }
        }
    }

最终的Service多了ICache 和 IEventPublish 两个对象：

    public class TargetSegmentService : ITargetSegmentService
    {
        private IRepository<TargetSegment> _repository;
        private ICache _cache;
        private IEventPublisher _eventPublisher;

        public TargetSegmentService(IRepository<TargetSegment> repository, ICache cache, IEventPublisher eventPublisher)
        {
            this._repository = repository;
            this._cache = cache;
            this._eventPublisher = eventPublisher;
        }

        public TargetSegment GetById(object id)
        {
            return this._repository.GetById(id);
        }

        public void Insert(TargetSegment item)
        {
            if (item == null)
                throw new ArgumentNullException("item");

            this._repository.Insert(item);
            this._eventPublisher.EntityInserted(item);
        }

        public void Update(TargetSegment item)
        {
            if (item == null)
                throw new ArgumentNullException("item");

            this._repository.Update(item);
            this._eventPublisher.EntityUpdated(item);
        }

        public void Delete(TargetSegment item)
        {
            if (item == null)
                throw new ArgumentNullException("item");

            this._repository.Delete(item);
            this._eventPublisher.EntityDeleted(item);
        }

        public IList<TargetSegment> GetAll()
        {
            return this._cache.GetOrAdd(SnappsCacheEventConsumer.AllTargetSegmentPattern, 60, () =>
            {
                return this._repository.Table.ToList();
            });
        }
    }

最后通过Unity实现依赖注入

            Container.RegisterInstance<ICache>(new MemoryCache(), new ContainerControlledLifetimeManager());
            Container.RegisterInstance<IEventPublisher>(new EventPublisher(), new ContainerControlledLifetimeManager());
            Container.RegisterType(typeof(IConsumer<>), typeof(CacheEventConsumer), new ContainerControlledLifetimeManager());

 

3. 总结

这是订阅模式的一种运用，在NopCommerce里面有很多设计模式都运用的非常巧妙，对于EntityFramework的优化远不止这些，以后再给大家分享。

 

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