Updating Data Asynchronously Using Message Queuing

Send Update Button.Click Checks if the update message queue exists and creates it if necessary.. A MessageQueue object is created to access the queue.. A message is created containing a

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Recipe 4.13 Updating Data Asynchronously Using Message Queuing

Problem

You need to asynchronously update data on a database on system that is not always connected

Solution

You must use message queuing and an XML DiffGram to:

• Construct and send an MSMQ message containing a DiffGram to the server

• Extract the DiffGram from the message and use it to update the data source at the server

• Construct and send a message containing the latest DataSet to the client

• Retrieve the response at the client and deserialize it into a DataSet

The sample code contains four event handlers:

Form.Load

Sets up the sample by loading a DataSet with the Customers table from the

Northwind database The default view of the table is bound to a data grid on the form

Send Update Button.Click

Checks if the update message queue exists and creates it if necessary A

MessageQueue object is created to access the queue A message is created

containing a DiffGram of the changes made to the DataSet containing the

Customers table This message is sent to the update queue

Process Update Button.Click

Checks if the update message queue exists and creates it if necessary A

MessageQueue object is created to access the queue An attempt is made to

receive a message from the queue, waiting one second before giving up If a message is received, a DataAdapter that uses a CommandBuilder to generate updating logic is created and used to update the database with the changes in the DataSet contained in the body of the message The latest version of the Customers

table is retrieved into a new DataSet A result queue is created if necessary and a

message containing the latest DataSet is sent to the queue

Receive Update Button.Click

Checks if the result message queue exists and creates it if necessary A

MessageQueue object is created to access the queue and the formatter set to deserialize the DataSet in the message bodies An attempt is made to receive a message from the queue, waiting one second before giving up If a message is received, the DataSet in the body is deserialized and the default view of the Categories DataTable is bound to the data grid on the form

The C# code is shown in Example 4-32

Example 4-32 File: MessageQueueUpdateForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Windows.Forms;

using System.Messaging;

using System.Data;

using System.Data.SqlClient;

private const String CUSTOMERS_TABLE = "Customers";

private const String QUEUENAMEUPDATE = @".\Private$\adodotnetcb0413update"; private const String QUEUENAMERESULT = @".\Private$\adodotnetcb0413result"; private DataSet ds;

//

private void MessageQueueUpdateForm_Load(object sender, System.EventArgs e) {

// As a starting point, load the data directly

// Create the DataAdapter to load customers data

SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Customers",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

// Get the schema and data for the customers table

ds = new DataSet( );

da.FillSchema(ds, SchemaType.Source, CUSTOMERS_TABLE);

da.Fill(ds, CUSTOMERS_TABLE);

// Bind the default view of the customers table to the grid

dataGrid.DataSource = ds.Tables[CUSTOMERS_TABLE].DefaultView; }

private void sendUpdateButton_Click(object sender, System.EventArgs e) {

// Create the result queue if it does not exist

if(!MessageQueue.Exists(QUEUENAMEUPDATE))

MessageQueue.Create(QUEUENAMEUPDATE);

// Create an object to access the result queue

MessageQueue mq = new MessageQueue(QUEUENAMEUPDATE);

// Set the formatter for serialization of message bodies

mq.Formatter = new XmlMessageFormatter(new Type[] {typeof(DataSet)});

// Create a message containing the changes

mq.Send(ds.GetChanges( ));

MessageBox.Show("Update message sent.","MessageQueue Update",

MessageBoxButtons.OK, MessageBoxIcon.Information);

}

private void processUpdateButton_Click(object sender, System.EventArgs e) {

// Create the result queue if it does not exist

if(!MessageQueue.Exists(QUEUENAMEUPDATE))

MessageQueue.Create(QUEUENAMEUPDATE);

// Create an object to access the result queue

MessageQueue mq = new MessageQueue(QUEUENAMEUPDATE);

// Set the formatter for deserialization of message bodies

mq.Formatter = new XmlMessageFormatter(new Type[] {typeof(DataSet)});

// Receive a message from the query queue

System.Messaging.Message msg;

try

{

msg = mq.Receive(new TimeSpan(0,0,1));

}

catch(MessageQueueException ex)

{

MessageBox.Show(ex.Message, "MessageQueue Receive Error",

MessageBoxButtons.OK, MessageBoxIcon.Error);

return;

}

// Create the DataAdapter and CommandBuilder to update

SqlDataAdapter da =

new SqlDataAdapter("SELECT * FROM " + CUSTOMERS_TABLE, ConfigurationSettings.AppSettings["Sql_ConnectString"]);

SqlCommandBuilder cb = new SqlCommandBuilder(da);

if (msg.BodyStream.Length > 0)

{

// Get the DataSet of changes from the message body

DataSet dsChanges = (DataSet)msg.Body;

// Process the updates

da.Update(dsChanges, CUSTOMERS_TABLE);

}

// Get the updated DataSet

DataSet dsUpdate = new DataSet( );

da.Fill(dsUpdate, CUSTOMERS_TABLE);

// Create the result queue if it does not exist

if(!MessageQueue.Exists(QUEUENAMERESULT))

MessageQueue.Create(QUEUENAMERESULT);

// Create an object to access the result queue

mq = new MessageQueue(QUEUENAMERESULT);

// Send a message with the update DataSet to the queue

mq.Send(dsUpdate);

MessageBox.Show("Update processed Refreshed DataSet sent.",

"MessageQueue Process", MessageBoxButtons.OK,

MessageBoxIcon.Information);

}

private void receiveUpdateButton_Click(object sender, System.EventArgs e) {

// Create the result queue if it does not exist

if(!MessageQueue.Exists(QUEUENAMERESULT))

MessageQueue.Create(QUEUENAMERESULT);

// Create an object to access the result queue

MessageQueue mq = new MessageQueue(QUEUENAMERESULT);

// Set the formatter for deserialization of message bodies

mq.Formatter = new XmlMessageFormatter(new Type[] {typeof(DataSet)});

// Receive a message from the result queue

System.Messaging.Message msg;

try

{

msg = mq.Receive(new TimeSpan(0,0,1));

}

catch(MessageQueueException ex)

{

MessageBox.Show("ERROR: " + ex.Message, "MessageQueue Receive", MessageBoxButtons.OK, MessageBoxIcon.Error);

return;

}

// Refresh the DataSet underlying the DataGrid

ds = (DataSet)msg.Body;

dataGrid.DataSource = ds.Tables[CUSTOMERS_TABLE].DefaultView;

MessageBox.Show("Retrieved and loaded refreshed data.",

"MessageQueue Receive", MessageBoxButtons.OK,

MessageBoxIcon.Information);

}

Discussion

The discussion in Recipe 2.22 provides an overview of Message Queuing (MSMQ) For more information about DiffGrams, see Recipe 8.8

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