DoubleCache

A layered distributed cache implementation following cache-aside and decorator pattern. Backed by Redis, combined with an in-memory cache.

##Usage Add a reference to DoubleCache using nuget Install-Package DoubleCache and initialize the DoubleCache using a factory method, providing redis connection and a serializer.

var connection = ConnectionMultiplexer.Connect("localhost");
var serializer = new MsgPackItemSerializer();

_pubSubCache = CacheFactory.CreatePubSubDoubleCache(connection, serializer);

To use the cache call the GetAsync<T> method. This method takes a Func called dataRetriever. This method should call your repository or other service. The dataRetriever method executes if the requested key does not exist in the cache, adding the result to the cache.

var cacheKey = Request.RequestUri.PathAndQuery;

pubSubCache.GetAsync(cacheKey, () => _repo.GetSingleDummyUser()));

##Why Redis is a fast, distributed key value store and then some. Using a remote store such as Redis adds two extra costs, IO and serialization. Having a local in-memory object store eliminates these two factors, at the cost of having a local cache on each client. Besides memory management, having multiple cache instances often result in synchronization issues.

DoubleCache provides synchronized local caches using Redis pub/sub combined with Redis key/value as a fallback if the local cache is empty.

###Azure Managed Cache retirement Having a local cache in front of a centralized cache is nothing new, it's a feature available to the users of Azure Managed Cache. On December 3rd 2015, Microsoft announced the retirement of Azure Managed Cache, the migrate path is to use the Azure Redis cache offering. As Microsoft has not made their own client for Redis, they recommend using the StackExchange.Redis client. This client does not include a local cache feature and the migration document states

Local cache: Client applications would need to implement this functionality using a dictionary or similar data structure.

DoubleCache provides this functionality using System.Runtime.Cache.MemoryCache. By creating your own implementation of the ICacheAside interface, it is easy to replace the local or remote cache with your own.

##TimeToLive Lifetime configuration for cache entries is set when creating the local and remote cache. By default the TimeSpan? defaultTtl parameter is set to null, leaving the items in the cache without time to live.

For specific entries, it is possible to override the default lifetime by passing a timespan for the specific item, or null if it should not expire.

##Implementation The ICacheAside interface is the main part of DoubleCache, all variants relies on implementations of this single interface.

public interface ICacheAside
  {
    void Add<T>(string key, T item);
    void Add<T>(string key, T item, TimeSpan? timeToLive);

    T Get<T>(string key, Func<T> dataRetriever) where T : class;
    T Get<T>(string key, Func<T> dataRetriever, TimeSpan? timeToLive) where T : class;

    object Get(string key, Type type, Func<object> dataRetriever);
    object Get(string key, Type type, Func<object> dataRetriever, TimeSpan? timeToLive);

    Task<T> GetAsync<T>(string key, Func<Task<T>> dataRetriever) where T : class;
    Task<T> GetAsync<T>(string key, Func<Task<T>> dataRetriever, TimeSpan? timeToLive) where T : class;

    Task<object> GetAsync(string key, Type type, Func<Task<object>> dataRetriever);
    Task<object> GetAsync(string key, Type type, Func<Task<object>> dataRetriever, TimeSpan? timeToLive);

    void Remove(string key);
  }

The Add and remove methods are implemented with fire and forget, hence it does not need to be Async as this is handled by the StackExchange.Redis client.

DoubleCache comes with the following implementations of this interface

• LocalCache.MemCache - using System.Runtime.Memory
• Redis.RedisCache - using StackExchange.Redis client
• SubscribingCache - a decorator supporting push notifications of cache updates
• PublishingCache - a decorator publishing cache changes
• DoubleCache - a decorator wrapping a local and a remote cache

Depending on your cache need, you can combine these implementations and decorators as you need. The most complete example would be a DoubleCache which takes a local cache decorated with a SubscribingCache and a RedisCache decorated with a PublishingCache. This will result in a local cache that will be in sync with the other local caches; if a value isn't found the value will be retrieved from Redis before ultimately being resolved using the func provided to the cache.