关于ADO.Net SqlConnection的性能优化

Connections

Database connections are an expensive and limited resource. Your approach to connection management can significantly affect the overall performance and scalability of your application. Issues to consider include acquiring and releasing connections, pooling, and authentication. To improve database connection performance and scalability, apply the following strategies to your connection management policy:

• Open and close the connection in the method.
• Explicitly close connections.
• When using DataReaders, specify CommandBehavior.CloseConnection.
• Do not explicitly open a connection if you use Fill or Update for a single operation.
• Avoid checking the State property of OleDbConnection.
• Pool connections.

Open and Close the Connection in the Method

Acquire connections late and release them early. Opening connections before they are needed reduces the number of connections that are available and increases resource pressure. Close connections quickly to ensure that they can be reused as soon as possible. Do not hold on to connections. Holding on to connections reduces the connections that are available to other code and increases resource pressure. The general pattern is to open and close connections on a per-method basis.

Explicitly Close Connections

Explicitly call the Close or Dispose methods on SqlConnection objects as soon as you finish using them to release the resources that they use. Do not wait for the connection to fall out of scope. The connection is not returned to the pool until garbage collection occurs. This delays the reuse of the connection and negatively affects performance and scalability. The following are guidelines to consider. These guidelines are specific to SqlConnectionbecause of the way it is implemented. These guidelines are not universal for all classes that have Close and Dispose functionality.

• Using either the Close method or the Dispose method is sufficient. You do not have to call one method after the other. There is no benefit to calling one method after the other.
• Dispose internally calls Close. In addition, Dispose clears the connection string.
• If you do not call Dispose or Close, and if you do not use the using statement, you are reliant upon the finalization of the inner object to free the physical connection.
• Use the using statement, instead of Dispose or Close, when you are working with a single type, and you are coding in Visual C#®. Dispose is automatically called for you when you use the using statement, even when an exception occurs.
• If you do not use the using statement, close connections inside a finally block. Code in the finally block always runs, regardless of whether an exception occurs.
• You do not have to set the SqlConnection reference to null or Nothing because there is no complex object graph. Setting object references to null or to Nothing is usually done to make a graph of objects unreachable.

Note   Closing a connection automatically closes any active DataReader objects that are associated with the connection.

Closing Connections in Visual Basic .NET

The following Visual Basic® .NET code snippet shows how to explicitly close a connection as soon as the connection is no longer needed.

Try
  conn.Open()
  cmd.ExecuteNonQuery()
  customerCount = paramCustCount.Value
Catch ex As Exception
  ' … handle exception
Finally
  ' This is guaranteed to run regardless of whether an exception occurs
  ' in the Try block.
  If Not(conn is Nothing) Then
    conn.Close()
  End If
End Try

Closing Connections in C#

The following example shows how to close connections in C#.

public void DoSomeWork()
{
  SqlConnection conn = new SqlConnection(connectionString);
  …  try
  {
    conn.Open();
    // Do Work
  }
  catch (Exception e)
  {
    // Handle and log error
  }
  finally
  {
    if(null!=conn)
       conn.Close();
  }
}

Closing Connections with the Using Statement in C#

The using statement simplifies code for C# developers by automatically generating a try and finally block when the code is compiled. This ensures that the Dispose method is called even if an exception occurs. The following code fragment shows how to use the using statement.

using (SqlConnection conn = new SqlConnection(connString))
{
  conn.Open();
  . . .
} // Dispose is automatically called on the conn variable here

The C# compiler converts this code into the following equivalent code, which has a try and finally block to ensure that the Dispose method on the SqlConnection object is called, regardless of whether an exception occurs.

SqlConnection conn = new SqlConnection(connString);
try
{
  conn.Open();
}
finally
{
  conn.Dispose();
}

One limitation of the using statement is that you can only put a single type in the parentheses. If you want to ensure that Dispose is called on additional resources, you must nest the using statements as shown in the following example.

using (SqlConnection conn = new SqlConnection(connString))
{
  SqlCommand cmd = new SqlCommand("CustomerRead");

  conn.Open();
  using (SqlDataReader dr = cmd.ExecuteReader())
  {
    while (dr.Read())
      Console.WriteLine(dr.GetString(0));
  }
}

Note   Using a nested using statement on the DataReader object is useful only if you need to perform further operations with the same connection after the inner using block. If you close the connection right away, this approach is of limited value because any active DataReader objects are closed automatically when the connection closes.

When Using DataReaders, Specify CommandBehavior.CloseConnection

When you create a DataReader object, specify the CommandBehavior.CloseConnection enumeration in your call to ExecuteReader. This ensures that when you close the DataReader, the connection is also closed. The following code fragment shows how to use the CommandBehaviorenumeration.

// Create connection and command. Open connection.
. . .
SqlDataReader myReader= myCommand.ExecuteReader(CommandBehavior.CloseConnection);
// read some data
. . .
myReader.Close(); // The connection and reader are closed.

The CommandBehavior.CloseConnection is especially helpful when you return a DataReader from a function, and you do not have control over the calling code. If the caller forgets to close the connection but closes the reader, both are closed when the DataReader is created by using CommandBehavior.CloseConnection. This is shown in the following code fragment.

public SqlDataReader CustomerRead(int CustomerID)
{
  //... create connection and command, open connection
  return myCommand.ExecuteReader(CommandBehavior.CloseConnection);
}

//... client code
SqlDataReader myReader = CustomerRead(10248);
//... read some data
myReader.Close(); // reader and connection are closed

Do Not Explicitly Open a Connection if You Use Fill or Update for a Single Operation

If you perform a single Fill or Update operation, do not open the connection before you call the Fill method, because the DataAdapter automatically opens and closes the connection for you. The following code fragment shows how to call Fill.

DataSet dSet = new DataSet("test");
SqlConnection conn = new SqlConnection(connString);
SqlCommand cmd = new SqlCommand(sqlQuery,conn);
SqlDataAdapter dAdapter = new SqlDataAdapter(cmd);
dAdapter.Fill(dSet); // The connection was not explicitly opened.
// The connection is opened and closed by the DataAdapter automatically.

The SqlDataAdapter automatically opens the connection, runs the selected command, and then closes the connection when it is finished. This enables the connection to be open for the shortest period of time.

Note that if you need to perform multiple file or update operations, you need to open the connection before the first Fill or Update method and close it after the last one. Alternatively, you could wrap multiple Fill or Update operations inside a C# using block to ensure that the connection is closed after the last use.

Avoid Checking the State Property of OleDbConnection

If you need to monitor or check connection status and you are using an OleDbConnection, consider handling the StateChange event, and avoid checking the State property. This approach helps to minimize round trips.

Using the State property increases application overhead, because each call results in a call to the OLE DB DBPROP_CONNECTIONSTATUS property (if the connection is an OleDbConnection) for an open connection.

Note   The .NET Framework 2.0 (code named "Whidbey"), at the time of writing, provides an updated OLE DB .NET Data Provider that resolves this problem.

The following code fragment shows how to implement the StateChange event. This event is raised when the state of the connection changes from open to closed or from closed to open.

OleDbConnection conn = new OleDbConnection(connStr);

// Set up a connection state change handler.
conn.StateChange  += new StateChangeEventHandler(OnStateChange);
. . .
// StateChange event handler.
protected static void OnStateChange(object sender, StateChangeEventArgs args)
{
  Console.WriteLine("The current Connection state has changed from {0} to {1}.",
                                  args.OriginalState, args.CurrentState);
}

Note   The ODBC provider also incurs similar overhead when using the State property.

Pool Connections

Creating database connections is expensive. You reduce overhead by pooling your database connections. Make sure you call Close or Dispose on a connection as soon as possible. When pooling is enabled, calling Close or Dispose returns the connection to the pool instead of closing the underlying database connection.

You must account for the following issues when pooling is part of your design:

• Share connections. Use a per-application or per-group service account to connect to the database. This creates a single pool or a small number of pools, and it enables many client requests to share the same connections.
• Avoid per-user logons to the database. Each logon creates a separate pooled connection. This means that you end up with a large number of small pools. If you need a different user for each connection, disable pooling or set a small maximum size for the pool.
• Do not vary connection strings. Different connection strings generate different connection pools. For example, using different capitalization, extra spaces, or different ordering of attributes causes connections to go to different pools. The SQL Server .NET Data Provider performs a byte-by-byte comparison to determine whether connection strings match.
• Release connections. Do not cache connections. For example, do not put them in session or application variables. Close connections as soon as you are finished with them. Busy connections are not pooled.
• Passing connections. Do not pass connections between logical or physical application layers.
• Consider tuning your pool size if needed. For example, in the case of the .NET Framework Data Provider for SQL Server, the default minimum pool size is zero and the maximum is 100. You might need to increase the minimum size to reduce warm-up time. You might need to increase the maximum size if your application needs more than 100 connections.
• Connection pools are managed by the specific database provider. SqlClient, OleDB client, and third-party clients may provide different configuration and monitoring options.

The following list details the pooling mechanisms that are available, and it summarizes pooling behavior for the .NET Framework data providers:

• The .NET Framework Data Provider for SQL Server pools connections by using a pooling mechanism implemented in managed code. You control pooling behaviors such as lifetime and pool size through connection string arguments.
• The .NET Framework Data Provider for Oracle also pools connections by using a managed code solution.
• The .NET Framework Data Provider for OLE DB automatically uses OLE DB session pooling to pool connections. You control pooling behavior through connection string arguments.
• The .NET Framework Data Provider for ODBC uses ODBC connection pooling.

Monitoring Pooling

You can monitor connection pooling to determine that it is working as expected and to help you identify the best minimum and maximum pool sizes.

Monitoring Pooling on a Computer that is Running SQL Server

You can monitor the number of open connections to SQL Server by using the SQL Server SQLServer:General Statistics performance counter object. This object is available only on a computer that is running SQL Server.

The connections are not specific to one particular application. If there are multiple applications accessing the server, this object reflects the total number of open connections for every application. Figure 12.2 shows the SQLServer:General Statistics object in the Performance Monitor tool.

Figure 12.2: Performance monitor showing the SQLServer:General Statistics counter

When monitoring SQLServer:General Statistics, you should observe the following:

• The number of logins per second increases during application startup when the connection pool is established. The number of logins per second should then drop to zero and stay there. Repeated logins and logouts per second indicate that the connection pool is not being used because a different security context is being used to establish the connection.
• The User Connections value should stabilize and remain constant. If this value increases and you see a jagged pattern in the number of logins per second, you may be experiencing a connection leak in the connection pool.

Monitoring Pooling Using the .NET Framework

The .NET Framework Data Provider for SQL Server provides several counters. The following counters are of particular significance:

• SqlClient: Current # connection pools
• SqlClient: Current # pooled and nonpooled connections
• SqlClient: Current # pooled connections
• SqlClient: Peak # pooled connections

The SqlClient: Current # connection pools counter indicates the number of connection pools that are currently in use. A large number of pools indicates that a pool is not being shared across clients. Using different connection strings creates new pools.

The SqlClient: Peak # pooled connections counter indicates the maximum number of connections that are currently in use. If this value remains at its peak, consider measuring the performance impact of increasing the Max Pool Size attribute in your connection string. The default value is 100. If you see this value at its peak in conjunction with a high number of failed connections in the SqlClient: Total # failed connects counter, consider changing the value and monitoring performance.

Note   These SqlClient counters may not be reset in .NET Framework version 1.1 when you stop and then restart an application. To reset the counters, stop the application and exit System Monitor, and then start the application and System Monitor again.

More Information

For more information about pooling connections, see the following resources on MSDN:

• "Connection Pooling for the .NET Framework Data Provider for SQL Server" in .NET Framework Developer's Guide at http://msdn.microsoft.com/en-us/library/8xx3tyca(VS.71).aspx

For more information about pooling connections, see the following Knowledge Base articles:

• 164221, "INFO: How to Enable Connection Pooling in an ODBC Application" at http://support.microsoft.com/default.aspx?scid=kb;en-us;164221
• 166083, "INFO: How to Enable Connection Pooling in an OLE DB Application" at http://support.microsoft.com/default.aspx?scid=kb;en-us;166083
• 169470, "INFO: Frequently Asked Questions About ODBC Connection Pooling" at http://support.microsoft.com/default.aspx?scid=kb;en-us;169470
• 216950, "How to Enable ODBC Connection Pooling Performance Counters" at http://support.microsoft.com/default.aspx?scid=kb;en-us;216950
• 237977, "INFO: OLE DB Session Pooling Timeout Configuration" at http://support.microsoft.com/default.aspx?scid=kb;en-us;237977
• 316757, "BUG: SqlClient Connection Pooling That Uses Integrated Security Is Slower Than OleDb" at http://support.microsoft.com/default.aspx?scid=kb;en-us;316757

For more information about how to reset the .NET counters, see Knowledge Base article 314429, "BUG: Performance Counters for SQL Server .NET Data Provider Are Not Reset," at http://support.microsoft.com/default.aspx?scid=kb;en-us;314429.