分散式快取是由多個應用伺服器共享的快取，通常作為外部服務儲存在單個應用伺服器上，常用的有SqlServer，Redis，NCache。

分散式快取可以提高ASP.NET Core應用程式的效能和可伸縮性，尤其是應用程式由雲服務或伺服器場託管時。

分散式快取的特點：

• 跨多個伺服器請求，保證一致性。
• 應用程式的伺服器重啟或部署時，快取資料不丟失。
• 不使用本地快取（如果是多個應用伺服器會出現不一致及資料丟失的風險）

Sql Server Distrubuted Cahce configure and application

1、Nuget下載安裝包

 2、使用sql-cache 工具建立快取列表

     Win+r 開啟cmd命令，輸入如下指令，建立快取表，

dotnet sql-cache create "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=DistCache;Integrated Security=True;" dbo TestCache

如遇到以下error，需要安裝dotnet-sql-cache，命令如下。（Note: NetCore 3.1 對應的dotnet-sql-chche version 3.1.13）

dotnet tool install --global dotnet-sql-cache

如果看到如下提示，證明快取表建立成功：

快取表結構：

3、在請求管道中新增DistributedCache（Note：這裡建議ConnectionString，SchemaName，TableName最好寫在配置檔案裡，在本章最後簡單介紹下如果在NetCore console 裡配置使用appsettings.json）

public static IServiceCollection ConfigureServices(this IServiceCollection services)
{
            var configuration = BuildConfiguration();
            services.AddSingleton<IConfiguration>(configuration);
            services.AddDistributedSqlServerCache(options=> 
            {
                options.ConnectionString = configuration["SqlServerDistributedCache:ConnectionString"];
                options.SchemaName =configuration["SqlServerDistributedCache:SchemaName"];
                options.TableName = configuration["SqlServerDistributedCache:TableName"];
            });
            services.AddTransient<ISqlServerService, SqlServerService>();
            return services;
}

4、通過建構函式依賴注入IDistributedCache

private readonly IDistributedCache _cacheService;

public SqlServerService(IDistributedCache cacheService)
{
      this._cacheService = cacheService;
}

最簡單的方式是直接使用IDistributedCache Extension方法，提供了String型別的cache value還是比較常用的。

以下是簡單封裝實現，根據需求更改即可。

 public class SqlServerService: ISqlServerService
    {
        private readonly IDistributedCache _cacheService;

        public SqlServerService(IDistributedCache cacheService)
        {
            this._cacheService = cacheService;
        }

        public async Task SetAsync(string key, byte[] value, object expiration = null, bool isAbsoluteExpiration = false)
        {
            var options = this.BuildDistributedCacheEntryOptions(expiration, isAbsoluteExpiration);
            await _cacheService.SetAsync(key, value, options);
        }

        public async Task SetAsync(string key, string value, object expiration = null, bool isAbsoluteExpiration = false)
        {
            var options = this.BuildDistributedCacheEntryOptions(expiration, isAbsoluteExpiration);
            await _cacheService.SetStringAsync(key, value, options);
        }

        public async Task<byte[]> GetAsync(string key)
        {
            return await _cacheService.GetAsync(key);
        }

        public async Task<string> GetStringAsync(string key)
        {
            return await _cacheService.GetStringAsync(key);
        }

        public async Task RemoveAsync(string key)
        {
            await _cacheService.RemoveAsync(key);
        }

        public async Task RefreshAsync(string key)
        {
            await _cacheService.RefreshAsync(key);
        }

        private DistributedCacheEntryOptions BuildDistributedCacheEntryOptions(object expiration = null, bool isAbsoluteExpiration = false)
        {
            var options = new DistributedCacheEntryOptions();
            if (expiration != null)
            {
                if (expiration is TimeSpan)
                {
                    if (isAbsoluteExpiration)
                        options.SetAbsoluteExpiration((TimeSpan)expiration);
                    else
                        options.SetSlidingExpiration((TimeSpan)expiration);
                }
                else if (expiration is DateTimeOffset)
                {
                    options.SetAbsoluteExpiration((DateTimeOffset)expiration);
                }
                else
                {
                    throw new NotSupportedException("Not support current expiration object settings.");
                }
            }
            return options;
        }
    }

這裡主要說下DistributedCacheEntryOptions這個類，作用是設定快取項的過期時間

public class DistributedCacheEntryOptions
{
    public DistributedCacheEntryOptions()
    public DateTimeOffset? AbsoluteExpiration { get; set; }//絕對過期時間
    public TimeSpan? AbsoluteExpirationRelativeToNow { get; set; }//絕對過期時間
    public TimeSpan? SlidingExpiration { get; set; } //滑動過期時間（如果在滑動過期時間內重新整理，將重新設定滑動過去時間，對應IDistributedCache中的Refresh方法）
}

OK，Sql Server IDistributeCache 就介紹到這裡。

附加：NetCore Console 中使用appsettings.json檔案

1、Nuget下載安裝packages

 2、新增appsettings.json檔案，修改property->copy always

"SqlServerDistributedCache": {
    "ConnectionString": "",
    "SchemaName": "",
    "TableName": ""
  }

3、構建Configuration物件並新增到請求管道

public static IServiceCollection ConfigureServices(this IServiceCollection services)
{
            var configuration = BuildConfiguration();
            services.AddSingleton<IConfiguration>(configuration);
            services.AddDistributedSqlServerCache(options=> 
            {
                options.ConnectionString = configuration["SqlServerDistributedCache:ConnectionString"];
                options.SchemaName = configuration["SqlServerDistributedCache:SchemaName"];
                options.TableName = configuration["SqlServerDistributedCache:TableName"];
            });
            services.AddTransient<ISqlServerService, SqlServerService>();
            return services;
}

        private static IConfigurationRoot BuildConfiguration()
        {
            var env = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");

            var builder = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile($"appsettings.json", true, true)
                .AddJsonFile($"appsettings.{env}.json", true, true)
                .AddEnvironmentVariables();
            return builder.Build();
        }

這裡是根據環境變數“ASPNETCORE_ENVIRONMENT”讀取不同的appsettings檔案，比如Development，Staging，Product

4、通過建構函式注入使用IConfiguration物件即可。

完整程式碼地址： Github。

&n