Receiving Results

Response Queues

If more information than an acknowledgement is needed from the receiving application, a response queue can be used. A response queue is like a normal queue, but the original sender uses the queue as a receiver and the original receiver uses the response queue as a sender.

The sender must assign the response queue with the ResponseQueue property of the Message class. The sample code here shows how the receiver uses the response queue to return a response message. With the response message responseMessage the property CorrelationId is set to the id of the original message. This way the client application knows what message the answer belongs to. This is similar to acknowledgement queues. The response message is sent with the Send() method of the MessageQueue object that is returned from the ResponseQueue property:

public void ReceiveMessage(Message message)


{


Message responseMessage = new Message("response");


responseMessage.CorrelationId = message.Id;


message.ReesponseQueue.Send(responseMessage);


}

Transactional Queues

With recoverable messages it is not guaranteed that the message will arrive in order and just once. Failures on the network can cause messages to arrive multiple times; this happens also if both the sender and receiver have multiple network protocols installed that are used by Message Queuing.

Transactional queues can be used where these guarantees are required:

• Messages arrive in the same order they have been sent

• Messages arrive only once

With transactional queues, a single transaction doesn't span the sending and receiving of messages. The nature of Message Queuing is that the time between send and receive can be quite long. In contrast, transactions should be short. With Message Queuing the first transaction is used to send the message into the queue, the second transaction forwards the message on the network, and the third transaction is used to receive the messages.

The next example shows how to create a transactional message queue and how to send messages using a transaction.

A transactional message queue is created by passing true with the second parameter of the MessageQueue.Create() method.

If you would like to write multiple messages to a queue within a single transaction, you have to instantiate a MessageQueueTransaction object and invoke the Begin() method. When you are finished with sending all messages that belong to the transaction, the Commit() method of the MessageQueueTransaction object must be called. To cancel a transaction (and no messages are written to the queue), the Abort() method must be called as you can see within the catch block:

using System; using System.Messaging; namespace Wrox.ProCSharp.Messaging { class Program { static void Main(string[] args) {

if (!MessageQueue.Exists(@".\MyTransactionalQueue"))


{


MessageQueue.Create(@".\MyTransactionalQueue", true);


}


MessageQueue queue = new MessageQueue(@".\MyTransactionalQueue");


MessageQueueTransaction transaction =


new MessageQueueTransaction();


try


{


transaction.Begin();


queue.Send("a", transaction);


queue.Send("b", transaction);


queue.Send("c", transaction);


transaction.Commit();


}


catch


{


transaction.Abort();


}

} } }