dotConnect for PostgreSQL Documentation
In This Topic
    Asynchronous Query Execution
    In This Topic

    dotConnect for PostgreSQL supports a very useful feature - asynchronous execution of methods that take most of time in database applications. Traditionally, the main thread of the application went blocked during opening a connection, transferring large blocks of data, and other operations alike. Now you can entrust execution of a query to a background thread and concentrate on performing other operations, for example, an update of user interface elements, or on a preparation of another query for execution.

    The article consists of following sections:

    Understanding Asynchronous Methods

    dotConnect for PostgreSQL provides you with asynchronous methods for most of the time-consuming operations: connecting, data retrieving, and DML statement execution. For every synchronous method two asynchronous methods are implemented, with prefixes "Begin" and "End" respectively. The table below summarizes the new methods in the command object:

    Table 1. Asynchronous methods

    Class

    Synchronous method

    Asynchronous methods

    Begin part End part
    PgSqlConnection Open BeginOpen EndOpen
    PgSqlCommand ExecuteReader BeginExecuteReader EndExecuteReader
    PgSqlCommand ExecuteNonQuery BeginExecuteNonQuery EndExecuteNonQuery
    PgSqlDataTable Fill BeginFill EndFill
    PgSqlDump Backup BeginBackup EndBackup
    PgSqlDump BackupQuery BeginBackupQuery EndBackupQuery
    PgSqlDataTable Restore BeginRestore EndRestore

    A typical usage of asynchronous methods looks like the following code.

    IAsyncResult myResult = myCommand.BeginExecuteNonQuery(null, null);
    ...
    rowsAffected = myCommand.EndExecuteNonQuery(myResult);
    
    
    Dim myResult As IAsyncResult = myCommand.BeginExecuteNonQuery(Nothing, Nothing)
    ...
    Dim rowsAffected As Int32 = myCommand.EndExecuteNonQuery(myResult)
    
    
    

    First, application calls the "Begin" method and passes it all the parameters needed to accomplish the task. This method returns IAsyncResult object that represents the operation invoked and is used for later processing. After the process has been invoked, application can proceed to do its job.
    Later, when the query has finished its execution, application calls the "End" method, passing it IAsyncResult object returned by the "Begin" method that has been invoked earlier.
    If something goes wrong with the query, the exception is thrown only when application calls "End" method.

    Completion Signalling

    The common synchronous method calls do not return until the operation is finished. In the asynchronous cases, the begin call returns immediately, so there is a need to determine when the operation is actually complete. dotConnect for PostgreSQL provides you with three ways to catch the query termination.

    All of the methods listed in the table 1 accept at least two parameters. For example, DbCommandBase.BeginExecuteReader method is declared in the following way:

    public IAsyncResult BeginExecuteReader(
       AsyncCallback callback,
       object stateObject,
       CommandBehavior behavior
    );
    
    
    Public Function BeginExecuteReader( _
       ByVal callback As AsyncCallback, _
       ByVal stateObject As Object, _
       ByVal behavior As CommandBehavior _
    ) As IAsyncResult
    
    

    The way you get signalled about query termination depends on what values you pass to the "Begin" method.

    In either case you must call corresponding "End" method to continue using the connection that serves the operation. If you try to start another asynchronous operation on connection that already serves a "Begin" method, an exception will be thrown. If you do not call the corresponding "End" method, some system resources may leak. Note that calling the "End" method without waiting for the operation to complete is allowed. In this case the method will block the current thread until the operation finishes.

    Every asynchronous method that invokes an operation has stateObject parameter. You can pass it any object you find useful thus providing the callback routine any information it may require. The value of stateObject is accessible through IAsyncResult.AsyncState property.

    Transparent Asynchronous Fetch

    The PgSqlDataTable component provides even more features in the area of asynchronous execution. In addition to BeginFill and EndFill methods, it allows you to pause (SuspendFill), continue (BeginFill again), or stop (CancelFetch) the fetch process. Moreover, the component has NonBlocking property, which turns common fill into transparent asynchronous operation. To capture the end moment of the operation in a non-blocking fetch, watch for the SyncRoot property or handle the FetchFinished event.

    For more information on the PgSqlDataTable component refer to topic PgSqlDataTable Advanced Features.

    Samples

    A sample is provided below for each of the signalling methods.

    The following sample starts an asynchronous execution of UPDATE statement, then periodically checks whether the operation is complete, while performing some operations (writes a dot to console). Once query is done, the routine returns and application is ready to close.

    using System;
    using Devart.Data.PostgreSql;
    
    namespace MyNamespace
    {
      class Class1
      {
        static PgSqlConnection myConnection = new PgSqlConnection(
          "host=server;database=test;user id=postgres;");
        static PgSqlCommand myCommand = new PgSqlCommand(
          "UPDATE Emp SET Sal = Sal + 500 WHERE Sal < 2000");
    
        static void PerformAsync()
        {
          myCommand.Connection = myConnection;
          myConnection.Open();
          int rowsAffected;
          try
          {
            IAsyncResult myResult = myCommand.BeginExecuteNonQuery(null, null);
            Console.Write("In progress...");
            while (!myResult.IsCompleted)
            {
              Console.Write(".");
              //Perform here any operation you need
            }
            rowsAffected = myCommand.EndExecuteNonQuery(myResult);
            Console.WriteLine();
            Console.WriteLine("Operation complete. Rows Affected: " + rowsAffected);
          }
          catch
          {
            Console.WriteLine("Error during execution.");
          }
          finally
          {
            myConnection.Close();
          }
        }
    
        static void Main(string[] args)
        {
          PerformAsync();
          Console.WriteLine("About to exit");
          Console.ReadLine();
        }
      }
    }
    
    
    Imports Devart.Data.PostgreSql
    Imports System
    
    Module Module1
    
      Dim myConnection As New PgSqlConnection( _
          "host=server;database=test;user id=postgres;")
      Dim myCommand As PgSqlCommand = New PgSqlCommand( _
        "UPDATE Emp SET Sal = Sal + 500 WHERE Sal < 2000")
    
      Public Sub PerformAsync()
        myCommand.Connection = myConnection
        myConnection.Open()
        Dim rowsAffected As Int32
        Try
          Dim myResult As IAsyncResult = myCommand.BeginExecuteNonQuery(Nothing, Nothing)
          Console.Write("In progress...")
          While Not myResult.IsCompleted
            Console.Write(".")
            'Perform here any operation you need
          End While
          rowsAffected = myCommand.EndExecuteNonQuery(myResult)
          Console.WriteLine()
          Console.WriteLine("Operation complete. Rows Affected: " & rowsAffected)
        Catch
          Console.WriteLine("Error during execution.")
        Finally
          myConnection.Close()
        End Try
      End Sub
    
      Sub Main()
        PerformAsync()
        Console.WriteLine("About to exit.")
        Console.ReadLine()
      End Sub
    
    End Module
    
    

    This method is very effective when several database operations can be executed simultaneously. The sample code opens two connections, then starts two asynchronous executions and waits for either to finish. Once an operation is signaled complete, the code processes results of the operation. Afterwards the application finishes the second operation. Using this method, we reduce query execution time from sum of two operations to the time of the slowest operation.

    using System;
    using Devart.Data.PostgreSql;
    
    namespace MyNamespace
    {
    
      public class Class1
      {
    
        static void Main(string[] args)
        {
          //Set up objects required to execute the queries
          
          PgSqlConnection[] myConnections = new PgSqlConnection[2];
          PgSqlCommand[] myCommands = new PgSqlCommand[2];
          IAsyncResult[] aResults = new IAsyncResult[2];
          string conStr = "host=server;database=test;user id=postgres;";
          myConnections[0] = new PgSqlConnection(conStr);
          myCommands[0] = new PgSqlCommand(
    	    "UPDATE Dept SET Loc='LA' WHERE DeptNo=10", myConnections[0]);
          myConnections[1] = new PgSqlConnection(conStr);
          myCommands[1] = new PgSqlCommand(
    	    "UPDATE Emp SET Sal = Sal + 500 WHERE Sal < 2000", myConnections[1]);
          try
          {
            //Open the connections
            myConnections[0].Open();
            myConnections[1].Open();
            //Invoke execution of both queries
            aResults[0] = myCommands[0].BeginExecuteNonQuery(null, null);
            aResults[1] = myCommands[1].BeginExecuteNonQuery(null, null);
            Console.WriteLine("Waiting for operations to complete...");
            //Wait for any of the queries to finish
            int ind = System.Threading.WaitHandle.WaitAny(new System.Threading.WaitHandle[] {
                                                                 aResults[0].AsyncWaitHandle,
                                                                 aResults[1].AsyncWaitHandle });
            //Process results of the query that finished first
            Console.WriteLine("Result #{0}: {1}", ind, 
    		  myCommands[ind].EndExecuteNonQuery(aResults[ind]));
            //Process results of another query
            int anotherIndex = 1 - ind;
            Console.WriteLine("Result #{0}: {1}", (anotherIndex),
              myCommands[anotherIndex].EndExecuteNonQuery(aResults[anotherIndex]));
          }
          catch
          {
            Console.WriteLine("Error during a query.");
          }
          finally
          {
            myConnections[0].Close();
            myConnections[1].Close();
          }
          Console.WriteLine("About to exit");
          Console.ReadLine();
        }
      }
    }
    
    
    Imports System
    Imports Devart.Data.PostgreSql
    
    Module Module1
    
      Sub Main()
        'Set up objects required to execute the queries
        Dim myConnections(1) As PgSqlConnection
        Dim myCommands(1) As PgSqlCommand
        Dim aResults(1) As IAsyncResult
        Dim conStr As String = "host=server;database=test;user id=postgres;"
        myConnections(0) = New PgSqlConnection(conStr)
        myCommands(0) = New PgSqlCommand( _
    	  "UPDATE Dept SET Loc='LA' WHERE DeptNo=10", myConnections(0))
        myConnections(1) = New PgSqlConnection(conStr)
        myCommands(1) = New PgSqlCommand( _
    	  "UPDATE Emp SET Sal = Sal + 500 WHERE Sal < 2000", myConnections(1))
        Try
          'Open the connections
          myConnections(0).Open()
          myConnections(1).Open()
          'Invoke execution of both queries
          aResults(0) = myCommands(0).BeginExecuteNonQuery(Nothing, Nothing)
          aResults(1) = myCommands(1).BeginExecuteNonQuery(Nothing, Nothing)
          Console.WriteLine("Waiting for operations to complete...")
          Dim waitHandles() As System.Threading.WaitHandle = { _
    	                          aResults(0).AsyncWaitHandle, aResults(1).AsyncWaitHandle}
          'Wait for any of the queries to finish
          Dim ind As Int32 = System.Threading.WaitHandle.WaitAny(waitHandles)
          'Process results of the query that finished first
          Console.WriteLine("Result #{0}: {1}", ind, _
    	    myCommands(ind).EndExecuteNonQuery(aResults(ind)))
          'Process results of another query
          Dim anotherIndex As Int32 = 1 - ind
          Console.WriteLine("Result #{0}: {1}", (anotherIndex), _
            myCommands(anotherIndex).EndExecuteNonQuery(aResults(anotherIndex)))
        Catch
          Console.WriteLine("Error during a query.")
        Finally
          myConnections(0).Close()
          myConnections(1).Close()
        End Try
        Console.WriteLine("About to exit")
        Console.ReadLine()
      End Sub
    
    End Module
    
    

    This method is probably the most effective in many situations. The sample code initiates establishment of two connections with different servers. A connection itself is passed as stateObject parameter. This allows a delegate to know what connection has finished negotiation with servers and to end the asynchronous operation. Using this method, we obtain results of an operation once it is complete, without having to wait for the slower operations that might precede it.

    using System;
    using Devart.Data.PostgreSql;
    
    namespace MyNamespace
    {
      class Class1
      {
        static PgSqlConnection myConnection1 = new PgSqlConnection(
            "User Id=postgres;Host=localhost");
        static PgSqlConnection myConnection2 = new PgSqlConnection(
            "User Id=postgres;Host=server");
    
        static void OperationComplete(IAsyncResult ar)
        {
          try
          {
            PgSqlConnection currentConnection = (PgSqlConnection)ar.AsyncState;
            currentConnection.EndOpen(ar);
            Console.WriteLine("Connection string: "+currentConnection.ConnectionString +
                               ", server version: "+currentConnection.ServerVersion);
          }
          catch
          {
            Console.WriteLine("Error during opening.");
          }
        }
    
        static void PerformAsync()
        {
          myConnection1.BeginOpen(new AsyncCallback(OperationComplete), myConnection1);
          myConnection2.BeginOpen(new AsyncCallback(OperationComplete), myConnection2);
          Console.WriteLine(
    	    "Waiting for operations to complete. 
    		Hit Enter when both connections are established.");
          Console.ReadLine();
          myConnection1.Close();
          myConnection2.Close();
        }
    
        static void Main(string[] args)
        {
          PerformAsync();
        }
      }
    }
    
    
    Imports System
    Imports Devart.Data.PostgreSql
    
    Module Module1
    
      Dim myConnection1 As New PgSqlConnection( _
         "User Id=postgres;Host=localhost")
      Dim myConnection2 As New PgSqlConnection( _
         "User Id=postgres;Host=server")
    
      Public Sub OperationComplete(ByVal ar As IAsyncResult)
        Try
          Dim currentConnection As PgSqlConnection = CType(ar.AsyncState, PgSqlConnection)
          currentConnection.EndOpen(ar)
          Console.WriteLine("Connection string: " & currentConnection.ConnectionString & _
                             ", server version: " & currentConnection.ServerVersion)
        Catch
          Console.WriteLine("Error during opening.")
        End Try
      End Sub
    
      Public Sub PerformAsync()
        myConnection1.BeginOpen(AddressOf OperationComplete, myConnection1)
        myConnection2.BeginOpen(AddressOf OperationComplete, myConnection2)
        Console.WriteLine( _
    	  "Waiting for operations to complete. _
    	    Hit Enter when both connections are established.")
        Console.ReadLine()
        myConnection1.Close()
        myConnection2.Close()
      End Sub
    
      Sub Main()
        PerformAsync()
      End Sub
    
    End Module
    
    

    See Also

    PgSqlCommand Class  | PgSqlConnection Class  | PgSqlDataTable Class