Dead Letter Queues
This sample demonstrates how to handle and process messages that have failed delivery. It is based on the Transacted MSMQ Binding sample. This sample uses the netMsmqBinding binding. The service is a self-hosted console application to enable you to observe the service receiving queued messages.
Note
The WCF samples may already be installed on your machine. Check this (default) directory before continuing: <InstallDrive>:\Samples\WCFWFCardspaceIf this directory doesn’t exist, click the download sample link at the top of this page. Note that this will download and install all of the WF, WCF, and CardSpace samples, you will only have to do this once. The sample is located in the following directory <InstallDrive>:\Samples\WCFWFCardSpace\WCF\Basic\Binding\Net\Msmq\DeadLetter.
Note
The setup procedure and build instructions for this sample are located at the end of this topic.
Note
This sample demonstrates each application dead letter queue that is only available on Windows Vista. The sample can be modified to use the default system-wide queues for MSMQ 3.0 on Windows Server 2003 and Windows XP.
In queued communication, the client communicates to the service using a queue. More precisely, the client sends messages to a queue. The service receives messages from the queue. The service and client therefore, do not have to be running at the same time to communicate using a queue.
Because queued communication can involve a certain amount of dormancy, you may want to associate a time-to-live value on the message to ensure that the message does not get delivered to the application if it has gone past the time. There are also cases, where an application must be informed whether a message failed delivery. In all of these cases, such as when the time-to-live on the message has expired or the message failed delivery, the message is put in a dead letter queue. The sending application can then read the messages in the dead-letter queue and take corrective actions that range from no action to correcting the reasons for failed delivery and resending the message.
The dead-letter queue in the NetMsmqBinding binding is expressed in the following properties:
DeadLetterQueue property to express the kind of dead-letter queue required by the client. This enumeration has the following values:
None: No dead-letter queue is required by the client.
System: The system dead-letter queue is used to store dead messages. The system dead-letter queue is shared by all applications running on the machine.
Custom: A custom dead-letter queue specified using the CustomDeadLetterQueue property is used to store dead messages. This feature is only available on Windows Vista. This is used when the application must use its own dead letter queue instead of sharing it with other applications running on the same machine.
CustomDeadLetterQueue property to express the specific queue to use as a dead-letter queue. This is available only in Windows Vista.
In this sample, the client sends a batch of messages to the service from within the scope of a transaction and specifies an arbitrarily low value for "time-to-live" for these messages (about 2 seconds). The client also specifies a custom dead-letter queue to use to enqueue the messages that have expired.
The client application can read the messages in the dead-letter queue and either retry sending the message or correct the error that caused the original message to be placed in the dead-letter queue and send the message. In the sample, the client displays an error message.
The service contract is IOrderProcessor, as shown in the following sample code.
[ServiceContract(Namespace="http://Microsoft.ServiceModel.Samples")]
public interface IOrderProcessor
{
[OperationContract(IsOneWay = true)]
void SubmitPurchaseOrder(PurchaseOrder po);
}
The service code in the sample is that of the Transacted MSMQ Binding.
Communication with the service takes place within the scope of a transaction. The service reads messages from the queue, performs the operation and then displays the results of the operation. The application also creates a dead-letter queue for dead-letter messages.
//The service contract is defined in generatedClient.cs, generated from the service by the svcutil tool.
//Client implementation code.
class Client
{
static void Main()
{
// Get MSMQ queue name from app settings in configuration
string deadLetterQueueName = ConfigurationManager.AppSettings["deadLetterQueueName"];
// Create the transacted MSMQ queue for storing dead message if necessary.
if (!MessageQueue.Exists(deadLetterQueueName))
MessageQueue.Create(deadLetterQueueName, true);
// Create a proxy with given client endpoint configuration
OrderProcessorClient client = new OrderProcessorClient("OrderProcessorEndpoint");
// Create the purchase order
PurchaseOrder po = new PurchaseOrder();
po.CustomerId = "somecustomer.com";
po.PONumber = Guid.NewGuid().ToString();
PurchaseOrderLineItem lineItem1 = new PurchaseOrderLineItem();
lineItem1.ProductId = "Blue Widget";
lineItem1.Quantity = 54;
lineItem1.UnitCost = 29.99F;
PurchaseOrderLineItem lineItem2 = new PurchaseOrderLineItem();
lineItem2.ProductId = "Red Widget";
lineItem2.Quantity = 890;
lineItem2.UnitCost = 45.89F;
po.orderLineItems = new PurchaseOrderLineItem[2];
po.orderLineItems[0] = lineItem1;
po.orderLineItems[1] = lineItem2;
//Create a transaction scope.
using (TransactionScope scope = new TransactionScope(TransactionScopeOption.Required))
{
// Make a queued call to submit the purchase order
client.SubmitPurchaseOrder(po);
// Complete the transaction.
scope.Complete();
}
client.Close();
Console.WriteLine();
Console.WriteLine("Press <ENTER> to terminate client.");
Console.ReadLine();
}
}
The client's configuration specifies a short duration for the message to reach the service. If the message cannot be transmitted within the duration specified, the message expires and is moved to the dead-letter queue.
Note
It is possible for the client to deliver the message to the service queue within the specified time. To ensure you see the dead-letter service in action, you should run the client before starting the service. The message times out and is delivered to the dead-letter service.
The application must define which queue to use as its dead-letter queue. If no queue is specified, the default system-wide transactional dead-letter queue is used to queue dead messages. In this example, the client application specifies its own application dead-letter queue.
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<!-- use appSetting to configure MSMQ Dead Letter queue name -->
<add key="deadLetterQueueName" value=".\private$\ServiceModelSamplesOrdersAppDLQ"/>
</appSettings>
<system.serviceModel>
<client>
<!-- Define NetMsmqEndpoint -->
<endpoint name="OrderProcessorEndpoint"
address="net.msmq://localhost/private/ServiceModelSamplesDeadLetter"
binding="netMsmqBinding"
bindingConfiguration="PerAppDLQBinding"
contract="IOrderProcessor" />
</client>
<bindings>
<netMsmqBinding>
<binding name="PerAppDLQBinding"
deadLetterQueue="Custom"
customDeadLetterQueue="net.msmq://localhost/private/ServiceModelSamplesOrdersAppDLQ"
timeToLive="00:00:02"/>
</netMsmqBinding>
</bindings>
</system.serviceModel>
</configuration>
The dead-letter message service reads messages from the dead-letter queue. The dead-letter message service implements the IOrderProcessor contract. Its implementation however is not to process orders. The dead-letter message service is a client service and does not have the facility to process orders.
Note
The dead-letter queue is a client queue and is local to the client queue manager.
The dead-letter message service implementation checks for the reason a message failed delivery and takes corrective measures. The reason for a message failure is captured in two enumerations, DeliveryFailure and DeliveryStatus. You can retrieve the MsmqMessageProperty from the OperationContext as shown in the following sample code:
public void SubmitPurchaseOrder(PurchaseOrder po)
{
Console.WriteLine("Submitting purchase order did not succed ", po);
MsmqMessageProperty mqProp =
OperationContext.Current.IncomingMessageProperties[
MsmqMessageProperty.Name] as MsmqMessageProperty;
Console.WriteLine("Message Delivery Status: {0} ",
mqProp.DeliveryStatus);
Console.WriteLine("Message Delivery Failure: {0}",
mqProp.DeliveryFailure);
Console.WriteLine();
….
}
Messages in the dead-letter queue are messages that are addressed to the service that is processing the message. Therefore, when the dead-letter message service reads messages from the queue, the Windows Communication Foundation (WCF) channel layer finds the mismatch in endpoints and does not dispatch the message. In this case, the message is addressed to the order processing service but is received by the dead-letter message service. To receive a message that is addressed to a different endpoint, an address filter to match any address is specified in the ServiceBehavior. This is required to successfully process messages that are read from the dead-letter queue.
In this sample, the dead-letter message service resends the message if the reason for failure is that the message timed out. For all other reasons, it displays the delivery failure, as shown in the following sample code:
// Service class that implements the service contract.
// Added code to write output to the console window.
[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single, ConcurrencyMode=ConcurrencyMode.Single, AddressFilterMode=AddressFilterMode.Any)]
public class PurchaseOrderDLQService : IOrderProcessor
{
OrderProcessorClient orderProcessorService;
public PurchaseOrderDLQService()
{
orderProcessorService = new OrderProcessorClient("OrderProcessorEndpoint");
}
[OperationBehavior(TransactionScopeRequired = true, TransactionAutoComplete = true)]
public void SubmitPurchaseOrder(PurchaseOrder po)
{
Console.WriteLine("Submitting purchase order did not succeed ", po);
MsmqMessageProperty mqProp = OperationContext.Current.IncomingMessageProperties[MsmqMessageProperty.Name] as MsmqMessageProperty;
Console.WriteLine("Message Delivery Status: {0} ", mqProp.DeliveryStatus);
Console.WriteLine("Message Delivery Failure: {0}", mqProp.DeliveryFailure);
Console.WriteLine();
// resend the message if timed out
if (mqProp.DeliveryFailure == DeliveryFailure.ReachQueueTimeout ||
mqProp.DeliveryFailure == DeliveryFailure.ReceiveTimeout)
{
// re-send
Console.WriteLine("Purchase order Time To Live expired");
Console.WriteLine("Trying to resend the message");
// reuse the same transaction used to read the message from dlq to enqueue the message to app. queue
orderProcessorService.SubmitPurchaseOrder(po);
Console.WriteLine("Purchase order resent");
}
}
// Host the service within this EXE console application.
public static void Main()
{
// Create a ServiceHost for the PurchaseOrderDLQService type.
using (ServiceHost serviceHost = new ServiceHost(typeof(PurchaseOrderDLQService)))
{
// Open the ServiceHostBase to create listeners and start listening for messages.
serviceHost.Open();
// The service can now be accessed.
Console.WriteLine("The dead letter service is ready.");
Console.WriteLine("Press <ENTER> to terminate service.");
Console.WriteLine();
Console.ReadLine();
}
}
}
The following sample shows the configuration for a dead-letter message:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<system.serviceModel>
<services>
<service
name="Microsoft.ServiceModel.Samples.PurchaseOrderDLQService">
<!-- Define NetMsmqEndpoint in this case, DLQ end point to read messages-->
<endpoint address="net.msmq://localhost/private/ServiceModelSamplesOrdersAppDLQ"
binding="netMsmqBinding"
bindingConfiguration="DefaultBinding"
contract="Microsoft.ServiceModel.Samples.IOrderProcessor" />
</service>
</services>
<client>
<!-- Define NetMsmqEndpoint -->
<endpoint name="OrderProcessorEndpoint"
address="net.msmq://localhost/private/ServiceModelSamplesDeadLetter"
binding="netMsmqBinding"
bindingConfiguration="SystemDLQBinding"
contract="IOrderProcessor" />
</client>
<bindings>
<netMsmqBinding>
<binding name="DefaultBinding" />
<binding name="SystemDLQBinding"
deadLetterQueue="System"/>
</netMsmqBinding>
</bindings>
</system.serviceModel>
</configuration>
In running the sample, there are 3 executables to run to see how the dead-letter queue works for each application; the client, service and a dead-letter service that reads from the dead-letter queue for each application and resends the message to the service. All are console applications with output in console windows.
Note
Because queuing is in use, the client and service do not have to be up and running at the same time. You can run the client, shut it down, and then start up the service and it still receives its messages. You should start the service and shut it down so that the queue can be created.
When running the client, the client displays the message:
Press <ENTER> to terminate client.
The client attempted to send the message but with a short timeout, the message expired and is now queued in the dead-letter queue for each application.
You then run the dead-letter service, which reads the message and displays the error code and resends the message back to the service.
The dead letter service is ready.
Press <ENTER> to terminate service.
Submitting purchase order did not succeed
Message Delivery Status: InDoubt
Message Delivery Failure: ReachQueueTimeout
Purchase order Time To Live expired
Trying to resend the message
Purchase order resent
The service starts and then reads the resent message and processes it.
The service is ready.
Press <ENTER> to terminate service.
Processing Purchase Order: 97897eff-f926-4057-a32b-af8fb11b9bf9
Customer: somecustomer.com
OrderDetails
Order LineItem: 54 of Blue Widget @unit price: $29.99
Order LineItem: 890 of Red Widget @unit price: $45.89
Total cost of this order: $42461.56
Order status: Pending
To set up, build, and run the sample
Ensure that you have performed the One-Time Set Up Procedure for the Windows Communication Foundation Samples.
To build the C# or Visual Basic .NET edition of the solution, follow the instructions in Building the Windows Communication Foundation Samples.
To run the sample in a single- or cross-machine configuration change queue names appropriately, replacing localhost with full name of the machine and follow the instructions in Running the Windows Communication Foundation Samples.
To run the sample on a computer joined to a workgroup
If your computer is not part of a domain, turn off transport security by setting the authentication mode and protection level to
Noneas shown in the following sample configuration:<bindings> <netMsmqBinding> <binding name="TransactedBinding"> <security mode="None"/> </binding> </netMsmqBinding> </bindings>Ensure the endpoint is associated with the binding by setting the endpoint's bindingConfiguration attribute.
Ensure that you change the configuration on the DeadLetterService, server and the client before you run the sample.
Note
Setting security mode to None is equivalent to setting MsmqAuthenticationMode, MsmqProtectionLevel and Message security to None.
Comments
By default with the netMsmqBinding binding transport, security is enabled. Two properties, MsmqAuthenticationMode and MsmqProtectionLevel, together determine the type of transport security. By default the authentication mode is set to Windows and the protection level is set to Sign. For MSMQ to provide the authentication and signing feature, it must be part of a domain. If you run this sample on a computer that is not part of a domain, you receive the following error: "User's internal message queuing certificate does not exist".
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