Tài liệu Querying Data Asynchronously with Message Queuing docx

The sample code contains three event handlers: Send Button.Click Checks if the query message queue exists and creates it if necessary.. A MessageQueue object is created to access the que

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Recipe 2.22 Querying Data Asynchronously with Message Queuing

Problem

You want to asynchronously retrieve data from a system that is not always connected

Solution

You must:

• Use message queuing to construct and send a data request from the client

• Access and process the requesting message at the server

• Compose and send a response message containing the result set to the client

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

The sample code contains three event handlers:

Send Button.Click

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

MessageQueue object is created to access the queue A message is sent to the queue containing the CustomerID which the user wants information about

Process Query Button.Click

Checks if the query 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, the CustomerID is extracted from the message and the

message queue is closed A DataSet is created and a DataAdapter is used to return the record for the requested CustomerID into a Customer DataTable in the

DataSet A result queue is created if necessary and a message labeled with the CustomerID and containing the DataSet with the asynchronous query results is sent to the queue

Process Result 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 for one second before giving up If a message is

received, the DataSet in the body is deserialized and the contents are displayed The C# code is shown in Example 2-31

Example 2-31 File: MessageQueueQueryForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.IO;

using System.Text;

using System.Messaging;

using System.Data;

using System.Data.SqlClient;

private const String QUEUENAMEQUERY = @".\Private$\adodotnetcb0222query"; private const String QUEUENAMERESULT = @".\Private$\adodotnetcb0222result"; private System.Messaging.MessageQueue messageQueue;

//

private void sendButton_Click(object sender, System.EventArgs e)

{

// Create the query queue if it does not exist

if(!MessageQueue.Exists(QUEUENAMEQUERY))

MessageQueue.Create(QUEUENAMEQUERY);

// Create an object to access the query queue

MessageQueue mq = new MessageQueue(QUEUENAMEQUERY);

// Send a message containing the user-enetered customer ID

String msg = "CustomerId=" + customerIdTextBox.Text;

mq.Send(msg);

resultTextBox.Text = "Query sent.";

}

private void processQueryButton_Click(object sender, System.EventArgs e)

{

// Create the query queue if it does not exist

if(!MessageQueue.Exists(QUEUENAMEQUERY))

MessageQueue.Create(QUEUENAMEQUERY);

// Create an object to access the query queue

MessageQueue mq = new MessageQueue(QUEUENAMEQUERY);

// Set the formatter for (de)serialization of message bodies

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

// Receive a message from the query queue

System.Messaging.Message msg;

try

{

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

resultTextBox.Text = "Query " + msg.Id + " received." +

Environment.NewLine;

}

catch(MessageQueueException ex)

{

resultTextBox.Text = ex.Message;

return;

}

// Get the customer ID from the message body

String customerId = ((String)msg.Body).Substring(11);

// Close the queue

mq.Close( );

// Create a DataAdapter to retrieve data for the specified customer

String sqlText = "SELECT * FROM Customers WHERE CustomerID='" + customerId + "'";

SqlDataAdapter da = new SqlDataAdapter(sqlText,

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

// Fill the Customer table in the DataSet with customer data

DataSet ds = new DataSet( );

da.Fill(ds, "Customers");

// 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 customer DataSet to the queue

mq.Send(ds, customerId);

resultTextBox.Text = "Response sent.";

}

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

StringBuilder result = new StringBuilder( );

// 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 (de)serialization 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)

{

resultTextBox.Text = ex.Message;

return;

}

// Create the customer DataSet from the message body

DataSet ds = (DataSet)msg.Body;

// Display the results of the query

result.Append("QUERY RESULTS:" + Environment.NewLine);

if (ds.Tables["Customers"].Rows.Count == 0)

result.Append("Customer not found for ID = '" +

msg.Label + "'.");

else

for(int i = 0; i < ds.Tables[0].Columns.Count; i++)

{

result.Append(ds.Tables[0].Columns[i].ColumnName +

" = " + ds.Tables[0].Rows[0][i] +

Environment.NewLine);

}

resultTextBox.Text = result.ToString( );

}

Discussion

Message Queuing (MSMQ) provides an inter-application messaging infrastructure that

allows messages to be sent between disconnected applications MSMQ provides for

message transport, queuing, transactional message support, error handling and auditing,

and makes available a variety of Application Programming Interfaces to interact with

MSMQ programmatically The System.Messaging namespace contains the NET classes

that support MSMQ

To send a message using MSMQ, perform the following actions:

• Create a connection to the message queue to which you want to send the message.

• Specify a formatter—an object that controls the type of data that can be sent in the

message body and how it is persisted—for the data that you want to send Table

2-19 describes the different formatters available

Table 2-19 .NET predefined formatters Formatter Description

ActiveXMessageFormatter

Serializes or deserializes primitive data types and other objects using a format compatible with MSMQ ActiveX Component to allow interoperability with previous versions

of MSMQ It is fast and produces a compact serialization

BinaryMessageFormatter

Serializes or deserializes an object or an object graph using

a binary format It is fast and produces a compact serialization

XMLMessageFormatter

Serializes or deserializes objects and primitive data types into XML based on an XSD schema This is the default formatter for MessageQueue components

• Call the Send( ) method of the MessageQueue to write the Message to the queue

The object to be sent is passed as an argument to the method

When the Send( ) method of the MessageQueue is called, the body of the message is

serialized using the XMLMessageFormatter if the Formatter property is not specified

To read a message and recreate the serialized body, formatter properties must be set

before reading the message The properties that must be set are specific to the formatter: ActiveXMessageFormatter

No properties must be set

BinaryMessageFormatter

Specify the format of the root object and the type descriptions either in the

constructor or by explicitly setting the TopObjectFormat and TypeFormat

properties

XmlMessageFormatter

Specify the target types or target type names either in the constructor or by

explicitly setting the TargetTypes or TargetTypeNames property

The message can now be read by using the Receive( ) method of the MessageQueue You can retrieve the serialized object from the Body property of the Message returned by the Receive( ) method

For more information about Microsoft Message Queue (MSMQ), see the MSDN Library [ Team LiB ]