TransactionScope 助手会毫无失败地耗尽连接池 - 有帮助吗？

Question

不久前，我问了一个关于 TransactionScope 升级到 MSDTC 的问题，当时我并没有预料到它会升级。 （上一个问题）

归结为， SQL2005中，为了使用TransactionScope，您只能在TransactionScope的生命周期内实例化并打开单个SqlConnection。使用 SQL2008，您可以实例化多个 SqlConnection，但在任何给定时间只能打开一个。 SQL2000 将始终升级为 DTC...顺便说一句，我们的应用程序（WinForms 应用程序）不支持 SQL2000。

我们对仅单连接问题的解决方案是创建一个 TransactionScope 帮助程序类，称为 LocalTransactionScope（又名“LTS”）。它包装了一个 TransactionScope，最重要的是，它为我们的应用程序创建并维护了一个 SqlConnection 实例。好消息是，它有效 - 我们可以在不同的代码段中使用 LTS，并且它们都加入环境事务。很不错。问题是，创建的每个根 LTS 实例都会创建并有效地终止连接池中的连接。我所说的“有效终止”是指它将实例化一个 SqlConnetion，它将打开一个新连接（无论出于何种原因，它永远不会重用池中的连接），并且当该根 LTS 被处置时，它关闭并处置 SqlConnection，它应该将连接释放回池中，以便可以重用它，但是，它显然永远不会被重用。池会膨胀直至达到最大，然后在建立 max-pool-size+1 连接时应用程序会失败。

下面我附上了 LTS 代码的精简版本和一个示例控制台应用程序类，它将演示连接池耗尽情况。为了观察连接池膨胀情况，请使用 SQL Server Managment Studio 的“活动监视器”或此查询：

SELECT DB_NAME(dbid) as 'DB Name',
COUNT(dbid) as 'Connections'
FROM sys.sysprocesses WITH (nolock)
WHERE dbid > 0
GROUP BY dbid

我在此处附加 LTS，以及一个示例控制台应用程序，您可以使用它来亲自演示它将消耗池中的连接切勿重复使用或释放​​它们。您需要添加对 System.Transactions.dll 的引用，以便 LTS 进行编译。

需要注意的是：打开和关闭 SqlConnection 的是根级 LTS，它总是在池中打开一个新连接。嵌套 LTS 实例没有什么区别，因为只有根 LTS 实例才能建立 SqlConnection。正如您所看到的，连接字符串始终相同，因此它应该重用连接。

是否存在一些我们未满足的神秘条件导致连接无法重复使用？除了完全关闭池之外，还有其他解决方案吗？

public sealed class LocalTransactionScope : IDisposable
{
      private static SqlConnection _Connection;    

      private TransactionScope _TransactionScope;
      private bool _IsNested;    

      public LocalTransactionScope(string connectionString)
      {
         // stripped out a few cases that need to throw an exception
         _TransactionScope = new TransactionScope();

         // we'll use this later in Dispose(...) to determine whether this LTS instance should close the connection.
         _IsNested = (_Connection != null);

         if (_Connection == null)
         {
            _Connection = new SqlConnection(connectionString);

            // This Has Code-Stink.  You want to open your connections as late as possible and hold them open for as little
            // time as possible.  However, in order to use TransactionScope with SQL2005 you can only have a single 
            // connection, and it can only be opened once within the scope of the entire TransactionScope.  If you have
            // more than one SqlConnection, or you open a SqlConnection, close it, and re-open it, it more than once, 
            // the TransactionScope will escalate to the MSDTC.  SQL2008 allows you to have multiple connections within a 
            // single TransactionScope, however you can only have a single one open at any given time. 
            // Lastly, let's not forget about SQL2000.  Using TransactionScope with SQL2000 will immediately and always escalate to DTC.
            // We've dropped support of SQL2000, so that's not a concern we have.
            _Connection.Open();
         }
      }

      /// <summary>'Completes' the <see cref="TransactionScope"/> this <see cref="LocalTransactionScope"/> encapsulates.</summary>
      public void Complete() { _TransactionScope.Complete(); }

      /// <summary>Creates a new <see cref="SqlCommand"/> from the current <see cref="SqlConnection"/> this <see cref="LocalTransactionScope"/> is managing.</summary>
      public SqlCommand CreateCommand() { return _Connection.CreateCommand(); }

      void IDisposable.Dispose() { this.Dispose(); }

      public void Dispose()
      {
          Dispose(true); GC.SuppressFinalize(this);
      }

      private void Dispose(bool disposing)
      {
         if (disposing)
         {
            _TransactionScope.Dispose();
            _TransactionScope = null;    

            if (!_IsNested)
            {
               // last one out closes the door, this would be the root LTS, the first one to be instanced.
               LocalTransactionScope._Connection.Close();
               LocalTransactionScope._Connection.Dispose();    

               LocalTransactionScope._Connection = null;
            }
         }
      }
   }

这是一个将显示连接池耗尽的 Program.cs：

class Program
{
      static void Main(string[] args)
      {
         // fill in your connection string, but don't monkey with any pooling settings, like
         // "Pooling=false;" or the "Max Pool Size" stuff.  Doesn't matter if you use 
         // Doesn't matter if you use Windows or SQL auth, just make sure you set a Data Soure and an Initial Catalog
         string connectionString = "your connection string here";

         List<string> randomTables = new List<string>();
         using (var nonLTSConnection = new SqlConnection(connectionString))
         using (var command = nonLTSConnection.CreateCommand())
         {
             command.CommandType = CommandType.Text;
             command.CommandText = @"SELECT [TABLE_NAME], NEWID() AS [ID]
                                    FROM [INFORMATION_SCHEMA].TABLES]
                                    WHERE [TABLE_SCHEMA] = 'dbo' and [TABLE_TYPE] = 'BASE TABLE'
                                    ORDER BY [ID]";

             nonLTSConnection.Open();
             using (var reader = command.ExecuteReader())
             {
                 while (reader.Read())
                 {
                     string table = (string)reader["TABLE_NAME"];
                     randomTables.Add(table);

                     if (randomTables.Count > 200) { break; } // got more than enough to test.
                 }
             }
             nonLTSConnection.Close();
         }    

         // we're going to assume your database had some tables.
         for (int j = 0; j < 200; j++)
         {
             // At j = 100 you'll see it pause, and you'll shortly get an InvalidOperationException with the text of:
             // "Timeout expired.  The timeout period elapsed prior to obtaining a connection from the pool.  
             // This may have occurred because all pooled connections were in use and max pool size was reached."

             string tableName = randomTables[j % randomTables.Count];

             Console.Write("Creating root-level LTS " + j.ToString() + " selecting from " + tableName);
             using (var scope = new LocalTransactionScope(connectionString))
             using (var command = scope.CreateCommand())
             {
                 command.CommandType = CommandType.Text;
                 command.CommandText = "SELECT TOP 20 * FROM [" + tableName + "]";
                 using (var reader = command.ExecuteReader())
                 {
                     while (reader.Read())
                     {
                         Console.Write(".");
                     }
                     Console.Write(Environment.NewLine);
                 }
             }

             Thread.Sleep(50);
             scope.Complete();
         }

         Console.ReadKey();
     }
 }

Answer 1

根据 MSDN：

using(TransactionScope scope = ...) {
  using (SqlConnection conn = ...) {
    conn.Open();
    SqlCommand.Execute(...);
    SqlCommand.Execute(...);
  }
  scope.Complete();
}

因此，在 MSDN 示例中，连接是在范围内、范围完成之前处理的。尽管您的代码有所不同，但它会在范围完成后处理连接。我不是 TransactionScope 及其与 SqlConnection 交互方面的专家（我知道一些事情，但你的问题非常深入），我找不到任何规范什么是正确的模式。但我建议您重新访问代码并在最外层作用域完成之前处理单例连接，类似于 MSDN 示例。

另外，我希望您确实意识到，当第二个线程进入您的应用程序时，您的代码就会崩溃。

Answer 2

这段代码合法吗？

using(TransactionScope scope = ..)
{
    using (SqlConnection conn = ..)
    using (SqlCommand command = ..)
    {
        conn.Open();

        SqlCommand.Execute(..);
    }

    using (SqlConnection conn = ..) // the same connection string
    using (SqlCommand command = ..)
    {
        conn.Open();

        SqlCommand.Execute(..);
    }

    scope.Complete();
}