Asynchronous Query Execution
In This Topic
dotConnect for Salesforce 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 Salesforce 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 |
SalesforceConnection |
Open |
BeginOpen |
EndOpen |
SalesforceCommand |
ExecuteReader |
BeginExecuteReader |
EndExecuteReader |
SalesforceCommand |
ExecuteNonQuery |
BeginExecuteNonQuery |
EndExecuteNonQuery |
SalesforceDataTable |
Fill |
BeginFill |
EndFill |
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 Salesforce 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.
- Callback: pass a delegate as callback parameter, along with (optionally) a user-defined
state object stateObject. When this approach is used, dotConnect for Salesforce will call that passed-in delegate,
and make the state object available through the IAsyncResult object (passed as the second parameter to the delegate). To avoid calling a delegate, pass null (Nothing in Visual Basic) to the callback parameter.
- Synchronization objects: the IAsyncResult objects returned by the begin methods have a WaitHandle property that contains an event object. The event object can be used in synchronization primitives such as WaitHandle.WaitAny and WaitHandle.WaitAll. This allows the calling code to wait for multiple pending operations, and be notified either when one or all of them finish.
- Polling: the IAsyncResult object also has an IsCompleted boolean property. This property will change to true when the operation completes, so it can be used by code that needs to perform some continuous activity; that code can periodically check the property and, if it changes, process the results.
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.
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.Salesforce;
namespace MyNamespace
{
class Class1
{
static SalesforceConnection myConnection = new SalesforceConnection(
"Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;");
static SalesforceCommand myCommand = new SalesforceCommand(
"UPDATE Contact SET OtherCity = 'Las Vegas' WHERE City = 'New York'");
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.Salesforce
Imports System
Module Module1
Dim myConnection As New SalesforceConnection( _
"Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;")
Dim myCommand As SalesforceCommand = New SalesforceCommand( _
"UPDATE Contact SET OtherCity = 'Las Vegas' WHERE City = 'New York'")
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.Salesforce;
namespace MyNamespace
{
public class Class1
{
static void Main(string[] args)
{
//Set up objects required to execute the queries
SalesforceConnection[] myConnections = new SalesforceConnection[2];
SalesforceCommand[] myCommands = new SalesforceCommand[2];
IAsyncResult[] aResults = new IAsyncResult[2];
string conStr = "Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;";
myConnections[0] = new SalesforceConnection(conStr);
myCommands[0] = new SalesforceCommand(
"DELETE FROM Contact WHERE City='Boston'", myConnections[0]);
myConnections[1] = new SalesforceConnection(conStr);
myCommands[1] = new SalesforceCommand(
"UPDATE Contact SET OtherCity = 'Las Vegas' WHERE City = 'New York'", 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.Salesforce
Module Module1
Sub Main()
'Set up objects required to execute the queries
Dim myConnections(1) As SalesforceConnection
Dim myCommands(1) As SalesforceCommand
Dim aResults(1) As IAsyncResult
Dim conStr As String = "Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;"
myConnections(0) = New SalesforceConnection(conStr)
myCommands(0) = New SalesforceCommand( _
"DELETE FROM Contact WHERE City='Boston'", myConnections(0))
myConnections(1) = New SalesforceConnection(conStr)
myCommands(1) = New SalesforceCommand( _
"UPDATE Contact SET OtherCity = 'Las Vegas' WHERE City = 'New York'", 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 user accounts. A connection itself is passed as
stateObject parameter. This allows a delegate to know what connection has finished
negotiation with user accounts 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.Salesforce;
namespace MyNamespace
{
class Class1
{
static SalesforceConnection myConnection1 = new SalesforceConnection(
"Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;");
static SalesforceConnection myConnection2 = new SalesforceConnection(
"Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;");
static void OperationComplete(IAsyncResult ar)
{
try
{
SalesforceConnection currentConnection = (SalesforceConnection)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.Salesforce
Module Module1
Dim myConnection1 As New SalesforceConnection( _
"Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;")
Dim myConnection2 As New SalesforceConnection( _
"Server=login.salesforce.com;User Id= [email protected];Password=mypassword;Security Token=qweASDzcx1234567890rtyui;")
Public Sub OperationComplete(ByVal ar As IAsyncResult)
Try
Dim currentConnection As SalesforceConnection = CType(ar.AsyncState, SalesforceConnection)
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
SalesforceCommand Class
| SalesforceConnection Class