前置條件

　　MySQL 主從複製：https://www.cnblogs.com/fallTakeMan/p/14038888.html

環境說明

　　本機開發環境：Win10，VS2019，.NetCoreSDK 3.1

　　MySQL 伺服器：192.168.187.66:3306（寫），192.168.187.66:3307（讀）

專案結構

　　一個 webapi 專案，兩個 dotnetcore 類庫專案，ftm.EFcore 引用 ftm.Entity，ftm.api 引用 ftm.EFcore。

專案說明

　　ftm.EFcore 專案引用 nuget 包Microsoft.EntityFrameworkCore（3.1.1），MySql.Data.EntityFrameworkCore（8.0.22）。

　　建立三個檔案 AppDbContext，IDbContextFactory，DbContextFactory。

 1 using ftm.Entity;
 2 using Microsoft.EntityFrameworkCore;
 3 
 4 
 5 namespace ftm.EFcore
 6 {
 7     public class AppDbContext : DbContext
 8     {
 9         private string connString = string.Empty;
10         /// <summary>
11         ///
 
 傳入資料庫連結字串構造 AppDbContext
12         /// </summary>
13         /// <param name="conn">資料庫連結字串</param>
14         public AppDbContext(string conn)
15         {
16             connString = conn;
17         }
18 
19         public DbSet<User> User { get; set; }
20 
21         protected override
 
 void OnConfiguring(DbContextOptionsBuilder options)
22         {
23             options.UseMySQL(connString);
24         }
25 
26         protected override void OnModelCreating(ModelBuilder builder)
27         {
28             builder.Entity<User>().HasKey(x => x.Id);
29         }
30     }
31 }

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Text;
 4 
 5 namespace ftm.EFcore
 6 {
 7     /// <summary>
 8     /// DbContext 工廠
 9     /// </summary>
10     public interface IDbContextFactory
11     {
12         public AppDbContext CreateDbContext(DbContextType contextType);
13     }
14     /// <summary>
15     /// 讀寫型別
16     /// </summary>
17     public enum DbContextType
18     {
19         Write,
20         Read
21     }
22 }

 1 using Microsoft.Extensions.Configuration;
 2 using System;
 3 using System.Collections.Generic;
 4 using System.Text;
 5 
 6 namespace ftm.EFcore
 7 {
 8     /// <summary>
 9     /// DbContext 工廠
10     /// </summary>
11     public class DbContextFactory : IDbContextFactory
12     {
13         private IConfiguration _configuration;
14         private string[] _readConn;
15         /// <summary>
16         /// 傳入配置介面構造 DbContextFactory
17         /// </summary>
18         /// <param name="configuration">配置項介面</param>
19         public DbContextFactory(IConfiguration configuration)
20         {
21             _configuration = configuration;
22             _readConn = _configuration["ConnectionString:ReadConnStr"].Split(",");
23         }
24         public AppDbContext CreateDbContext(DbContextType contextType)
25         {
26             string connStr = string.Empty;
27             switch (contextType)
28             {
29                 case DbContextType.Write:
30                     connStr = _configuration["ConnectionString:WriteConnStr"];
31                     break;
32                 case DbContextType.Read:
33                     connStr = GetReadConn();
34                     break;
35 
36             }
37             return new AppDbContext(connStr);
38         }
39         /// <summary>
40         /// 獲取讀庫連結字串
41         /// </summary>
42         /// <remarks>
43         /// TODO:根據不同策略獲取連結字串
44         /// </remarks>
45         /// <returns>讀庫連結字串</returns>
46         private string GetReadConn()
47         {
48             // 讀庫策略
49 
50             int index = new Random().Next(0, _readConn.Length - 1);
51             return _readConn[index];
52         }
53     }
54 }

　　ftm.api 專案配置資料庫連結字串。

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*",
  "ConnectionString": {
    "WriteConnStr": "Server=192.168.187.66;port=3306;database=test_db;uid=root;pwd=root",
    "ReadConnStr": "Server=192.168.187.66;port=3307;database=test_db;uid=root;pwd=root"
  }
}

　　Startup 中注入 DbContextFactory。

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
   services.AddScoped<IDbContextFactory, DbContextFactory>();

   services.AddControllers();
}

　　在 ftm.api 專案中應用讀寫分離。

　　檢視 MySQL 主庫和從庫，User 表和資料同步。