实现一个读写分离的数据库访问中间件

主库（master）：增删改，以及所有事务操作。

从库（slaver）：读操作。

根据数据库连接字符串约定数据库类型。

/// <summary>
/// 数据库方言集合
/// </summary>
private readonly Dictionary<string, DatabaseDialectEnum> DialectDictionary
= new Dictionary<string, DatabaseDialectEnum>
{
["sqlconnection"] = DatabaseDialectEnum.MSSQL,
["sqlceconnection"] = DatabaseDialectEnum.SQLCE,
["npgsqlconnection"] = DatabaseDialectEnum.POSTGRES,
["sqliteconnection"] = DatabaseDialectEnum.SQLLITE,
["mysqlconnection"] = DatabaseDialectEnum.MYSQL,
["fbconnection"] = DatabaseDialectEnum.FIREBASE
};

以mssql为例，配置数据库连接字符串

  "ConnectionString": {
    "sqlconnection": "Data Source=.;Initial Catalog=Db;User ID=sa;Password=**;Enlist=false;Max Pool SIZE=500;Min Pool SIZE=50;MultipleActiveResultSets=True",
    "sqlconnection_slaver_1": "Data Source=.;Initial Catalog=Db;User ID=sa;Password=**;Enlist=false;Max Pool SIZE=500;Min Pool SIZE=50;MultipleActiveResultSets=True",
    "sqlconnection_slaver_2": "Data Source=.;Initial Catalog=Db;User ID=sa;Password=**;Enlist=false;Max Pool SIZE=500;Min Pool SIZE=50;MultipleActiveResultSets=True"
  }

Key: sqlconnection为主库（master）连接字符串，Key: sqlconnection_slaver_1和sqlconnection_slaver_2为两个从库（slaver）连接字符串。多个从库(slaver)可以实现随机访问。也可以采用其他算法来负载均衡。

下面建立一个ConnectionFactory。根据配置得到数据库连接字符串：

        /// <summary>
        /// 主数据库连接串
        /// </summary>
        private string MasterConnectionString { get; set; }
        /// <summary>
        /// 从数据库连接串集合
        /// </summary>
        private List<string> SlaverConnectionStrings { get; set; } = new List<string>();
        public ConnectionFactory(IConfiguration configuration, ILoggerFactory loggerFactory)
        {
            _logger = loggerFactory.CreateLogger<ConnectionFactory>();
            var connectionKeys = configuration.GetSection("ConnectionString").GetChildren().Select(s => s.Key).ToArray();
            foreach (var connKey in connectionKeys)
            {
                var connSplit = connKey.Split('_');
                if (connSplit.Length == 1)
                {
                    MasterConnectionString = configuration[$"ConnectionString:{connKey}"];
　　　　　　　　　　　　//根据连接字符串约定，确定数据库类型
                    DatabaseDialect = DialectDictionary[connKey];
                }
                else
                {
                    SlaverConnectionStrings.Add(configuration[$"ConnectionString:{connKey}"]);
                }

            }
        }

        /// <summary>
        /// 数据库类型
        /// </summary>
        public DatabaseDialectEnum DatabaseDialect { get; private set; }

获取主库连接

        private IDbConnection GetMasterConnection()
        {
            return GetConnection(MasterConnectionString);
        }

获取从库连接，这里采用随机算法，如果没有配置从库，这里会返回主库连接。

        private IDbConnection GetSlaverConnection()
        {
            int sc = SlaverConnectionStrings.Count();
            if (sc > 0)
            {
                Random random = new Random();
                int index = random.Next(0, sc);
                return GetConnection(SlaverConnectionStrings[index]);
            }
            else
            {
                _logger.LogInformation("没有设置从库，将建立主库连接");
                return GetMasterConnection();
            }
        }

        private IDbConnection GetConnection(string connectionString) => DatabaseDialect switch
        {
            DatabaseDialectEnum.MSSQL =>new ProfiledDbConnection(new SqlConnection(connectionString),MiniProfiler.Current),
            DatabaseDialectEnum.MYSQL => new ProfiledDbConnection(new MySqlConnection(connectionString), MiniProfiler.Current),
            _ => throw new NotImplementedException()
        };

注：这里采用MiniProfiler来监控数据库连接性能，所以 返回的connection用ProfiledDbConnection进行了包装。

主从数据源类型如下：

    public enum DataSourceEnum
    {
        MASTER,
        SLAVE
    }

本ConnectionFactory为单例模式，存在多线程访问的情况，所以数据源设置为ThreadLocal<DataSourceEnum>,线程内共享。

private static ThreadLocal<DataSourceEnum> threadLocal = new ThreadLocal<DataSourceEnum>();
        /// <summary>
        /// 当前线程数据源 
        /// </summary>
        /// <param name="sourceEnum"></param>     
        public DataSourceEnum DataSource
        {
            set { threadLocal.Value = value; }
            get { return threadLocal.Value; }
        }

下面正式获取IDbConnection

        public IDbConnection GetDbConnection()
        {
            if (DataSource == DataSourceEnum.MASTER)
            {
                return GetMasterConnection();
            }
            else
            {
                return GetSlaverConnection();
            }
        }

使用：

根据文章开头所描述的实际操作来进行主从库访问。

        private IDbConnection GetDbConnection(DataSourceEnum dataSource)
        {
            ConnectionFactory.DataSource = dataSource;
            return ConnectionFactory.GetDbConnection();
        }

using var connection = GetDbConnection(DataSourceEnum.MASTER);
 connection.Execute(sql, param, CurrentTransaction, null, commandType)

 using var connection = GetDbConnection(DataSourceEnum.SLAVE);
 connection.Get<T>(id, CurrentTransaction, CommandTimeout)

奉上全部代码

    public class ConnectionFactory : IConnectionFactory
    {
        private readonly ILogger _logger;
        private static ThreadLocal<DataSourceEnum> threadLocal = new ThreadLocal<DataSourceEnum>();
        static ConnectionFactory()
        {
            //设置dapper的tableName取值
            SqlMapperExtensions.TableNameMapper = (type) => type.Name;
        } 

        /// <summary>
        /// 当前线程数据源 
        /// </summary>
        /// <param name="sourceEnum"></param>     
        public DataSourceEnum DataSource
        {
            set { threadLocal.Value = value; }
            get { return threadLocal.Value; }
        }

        /// <summary>
        /// 主数据库连接串
        /// </summary>
        private string MasterConnectionString { get; set; }
        /// <summary>
        /// 从数据库连接串集合
        /// </summary>
        private List<string> SlaverConnectionStrings { get; set; } = new List<string>();
        public ConnectionFactory(IConfiguration configuration, ILoggerFactory loggerFactory)
        {
            _logger = loggerFactory.CreateLogger<ConnectionFactory>();
            var connectionKeys = configuration.GetSection("ConnectionString").GetChildren().Select(s => s.Key).ToArray();
            foreach (var connKey in connectionKeys)
            {
                var connSplit = connKey.Split('_');
                if (connSplit.Length == 1)
                {
                    MasterConnectionString = configuration[$"ConnectionString:{connKey}"];
                    DatabaseDialect = DialectDictionary[connKey];
                }
                else
                {
                    SlaverConnectionStrings.Add(configuration[$"ConnectionString:{connKey}"]);
                }

            }
        }
        /// <summary>
        /// 数据库方言集合
        /// </summary>
        private readonly Dictionary<string, DatabaseDialectEnum> DialectDictionary
          = new Dictionary<string, DatabaseDialectEnum>
          {
              ["sqlconnection"] = DatabaseDialectEnum.MSSQL,
              ["sqlceconnection"] = DatabaseDialectEnum.SQLCE,
              ["npgsqlconnection"] = DatabaseDialectEnum.POSTGRES,
              ["sqliteconnection"] = DatabaseDialectEnum.SQLLITE,
              ["mysqlconnection"] = DatabaseDialectEnum.MYSQL,
              ["fbconnection"] = DatabaseDialectEnum.FIREBASE
          };
        /// <summary>
        /// 数据库方言
        /// </summary>
        public DatabaseDialectEnum DatabaseDialect { get; private set; }

        private IDbConnection GetConnection(string connectionString) => DatabaseDialect switch
        {
            DatabaseDialectEnum.MSSQL =>new ProfiledDbConnection(new SqlConnection(connectionString),MiniProfiler.Current),
            DatabaseDialectEnum.MYSQL => new ProfiledDbConnection(new MySqlConnection(connectionString), MiniProfiler.Current),
            _ => throw new NotImplementedException()
        };
        public IDbConnection GetDbConnection()
        {
            if (DataSource == DataSourceEnum.MASTER)
            {
                return GetMasterConnection();
            }
            else
            {
                return GetSlaverConnection();
            }
        }
        private IDbConnection GetMasterConnection()
        {
            return GetConnection(MasterConnectionString);
        }
        private IDbConnection GetSlaverConnection()
        {
            int sc = SlaverConnectionStrings.Count();
            if (sc > 0)
            {
                Random random = new Random();
                int index = random.Next(0, sc);
                return GetConnection(SlaverConnectionStrings[index]);
            }
            else
            {
                _logger.LogInformation("没有设置从库，将从建立主库连接");
                return GetMasterConnection();
            }
        }    
    }

    public enum DataSourceEnum
    {
        MASTER,
        SLAVE
    }

原文地址：https://www.cnblogs.com/wangyfb/p/12049686.html