EF Core讀寫分離進一步完善
如評論前輩們指出在EF 6.x中我們知道有IDbCommandInterceptor接口，我們可對執行的SQL語句進行攔截，從而可自定義實現我們想要的需求，雖然在EF Core中卻沒有-特性的實現，但是針對此特性的實現DiagnosticSource記錄跟蹤類來實現等效于-的實現，當前DiagnosticSource使用文檔尚未完善，估計還得等待一段時間，接下來我們來看看如何實現。在DiagnosticSource包中有DiagnosticListener類，我們通過此類來跟蹤記錄，如果執行的EF Core包，那么我們將利用DiagnosticListener進行訂閱，訂閱到之后我們拿到跟蹤命令，從而實現無感知更換數據庫，代碼如下：
public?class?DbCommandInterceptor?:?IObserver> ????{????????private?const?string?masterConnectionString?=?"data?source=WANGPENG;User?Id=sa;Pwd=sa123;initial?catalog=Demo1;integrated?security=True;";????????private?const?string?slaveConnectionString?=?"data?source=WANGPENG;User?Id=sa;Pwd=sa123;initial?catalog=Demo2;integrated?security=True;";????????public?void?OnCompleted() ????????{ ????????}????????public?void?replaceString(Exception?error) ????????{ ????????}????????public?void?OnNext(KeyValuePair?value) ????????{????????????if?(value.Key?==?RelationalEventId.CommandExecuting.Name) ????????????{????????????????var?command?=?((CommandEventData)value.Value).Command;????????????????var?executeMethod?=?((CommandEventData)value.Value).ExecuteMethod;????????????????if?(executeMethod?==?DbCommandMethod.ExecuteNonQuery) ????????????????{ ????????????????????ResetConnection(command,?masterConnectionString); ????????????????}????????????????else?if?(executeMethod?==?DbCommandMethod.ExecuteScalar) ????????????????{ ????????????????????ResetConnection(command,?slaveConnectionString); ????????????????}????????????????else?if?(executeMethod?==?DbCommandMethod.ExecuteReader) ????????????????{ ????????????????????ResetConnection(command,?slaveConnectionString); ????????????????} ????????????} ????????}????????void?ResetConnection(DbCommand?command,?string?connectionString) ????????{????????????if?(command.Connection.State?==?ConnectionState.Open) ????????????{????????????????if?(!command.CommandText.Contains("@@ROWCOUNT")) ????????????????{ ????????????????????command.Connection.Close(); ????????????????????command.Connection.ConnectionString?=?connectionString; ????????????????} ????????????}????????????if?(command.Connection.State?==?ConnectionState.Closed) ????????????{ ????????????????command.Connection.Open(); ????????????} ????????} ????}
這里存在一個問題，上述?command.CommandText.Contains("@@ROWCOUNT")?代碼，因為在進行添加操作時，會返回主鍵，那么此時會進行查詢，所以暫時沒有更好的方式是確認主-從數據庫。
public?class?CommandListener?:?IObserver ????{????????private?readonly?DbCommandInterceptor?_dbCommandInterceptor?=?new?DbCommandInterceptor();????????public?void?OnCompleted() ????????{ ????????}????????public?void?replaceString(Exception?error) ????????{ ????????}????????public?void?OnNext(DiagnosticListener?listener) ????????{? ????????????if?(listener.Name?==?DbLoggerCategory.Name) ????????????{ ????????????????listener.Subscribe(_dbCommandInterceptor); ????????????} ????????} ????}
上述?DbLoggerCategory.Name?也就是?Microsoft.EntityFrameworkCore?，通過監控的包是Microsoft.EntityFrameworkCore，則進行訂閱，最后我們在startup中進行注冊該監聽類。
DiagnosticListener.AllListeners.Subscribe(new?CommandListener());
接下來我們通過動態圖來看看最終實際效果，主-從復制依然是通過SQL Server發布-訂閱的方式來同步數據。在控制器中，我們只利用demo1上下文來添加和查詢數據，當查詢時更換到從數據庫，此時已是無感知（請見上一篇），如下：
[HttpGet("index")]????????public?IActionResult?Index() ????????{????????????var?blogs?=?_demo1DbContext.Blogs.ToList();????????????return?View(blogs); ????????} ????????[HttpGet("create")]????????public?IActionResult?CreateDemo1Blog() ????????{????????????var?blog?=?new?Blog() ????????????{ ????????????????Name?=?"demoBlog1", ????????????????Url?=?"http://www.cnblogs.com/createmyslef" ????????????}; ????????????_demo1DbContext.Blogs.Add(blog); ????????????_demo1DbContext.SaveChanges();????????????return?RedirectToAction(nameof(Index)); ????????}
總結
本文只是在上一篇的基礎上進一步改善了讀寫分離的操作，得益于github上有提出可通過跟蹤記錄來解決，通過跟蹤記錄從底層出發來完善讀寫分離操作，我們可拿到底層實現的命令以及其他和EF 6.x中利用-等效，至于更加復雜的邏輯可自行實現。
數據庫
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