Skip Navigation LinksAzure Developers Guide > Azure Service Bus > Brokered Messaging > Brokered Messaging Features & Patterns > Deadlettering > Deadlettering Messages in Queues and Subscriptions

Training Courses

All course material is in English, and courses are delivered in English. Feel free to contact me for further information. cloudcasts.net@gmail.com

Deadlettering Messages in Queues and Subscriptions

In the previous section it was stated that there are three scenarios where messages will messages will be deadlettered by the messaging system.

·        When a message expires and deadlettering for expired messages is set to true in a queue or subscription.

·        When the max delivery count for a message is exceeded on a queue or subscription.

·        When a filter evaluation exception occurs in a subscription and deadlettering is enabled on filter evaluation exceptions.

This section will focus on the second of these three scenarios and discuss how the use of dead-lettering and delivery counts can be used to handle service errors and “poison messages” when using Service Bus queues and topics.

MaxDeliveryCount Property

Every message in a Service Bus queue or subscription maintains a count of how many times that message has been received. The DeliveryCount property of the message starts at zero, and then increments by one each time a receiving application receives the message.

When a message is received from a queue by an application using the peek-lock receive mode the receiving application can perform one of the following operations.

·        Complete – The message is marked as complete and removed from the queue.

·        Abandon – Message processing is abandoned and is immediately available on the queue again.

·        Deadletter – The message is moved to the deadletter queue.

·        Differ – The message remains on the queue in the deferred state and can be received using the message sequence ID.

·        No action – The message will remain on the queue and become available on the queue again after the time interval specified in the LockDuration property of the queue.

The two operations that will allow the message to be received again in the normal way are Abandon and No action. Abandon is typically called by a receiving application if it cannot process a message and message processing is to be retried at a later point in time. If a receiving application terminates without calling one of the above operations on a message, then no action is taken.

Handling “Poison Messages”

In message oriented systems a “poison message” is a message that contains data or information that cannot be processed by the receiving application.

Suppose the following code is used to receive and process messages from a queue.

 

while (true)

{

    BrokeredMessage msg = queueClient.Receive();

    if (msg != null)

    {

        try

        {

            // Process message           

            msg.Complete();

        }

        catch (Exception ex)

        {

            // Log error

            msg.Abandon();

        }

    }

}

 

 

If the message is processed successfully it will be marked as complete. If an exception occurs during message processing then the exception will be handled, an error will be logged, and the message will be abandoned and returned to the queue and will be processed again.

If the message processing exception was related to a service call, or database operation, and there was a connection failure, the receiving application will process the message again and may well succeed on the second try. If, however, the exception was caused by the content of the message, the application will receive the message again, attempt to process it, catch an exception and abandon it, receive the message again, attempt to process it, catch an exception and abandon it, receive the message again, attempt to process it, catch an exception and abandon it. If there is no mechanism to detect that this is a “poison message” the cycle will continue indefinitely.

Speaking Engagements