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Service Bus Queues from Windows Azure - Death letter and poison messages

I will continue the series of posts about Service Bus Queues. In this post we will talk about death letter. First of all, this concept is common to Service Bus and is not specific only to Service Bus Queues.
We should imagine death letter as an internal queue in our queue where all the messages that cannot be processes are added. This kind of messages is called death messages. For example what is happening in the following situation with the message that is abandoned:
BrokeredMessage message = qc.Receive();
try
{
if (messsage !=null)
{
    ...
    message.Complete();
}
catch(Exception ex)
{
    message.Abandon();
}
We will have a message that hangs on the queue and will be consumers over and over again without success. In this case we would need a method to mark the message as invalid. One solution is to invalidate the message and write this to a log. In this case even if the message is invalid we call the Complete() method. Be very careful when you call the Abandon() method. The message will be available in the queue again – it is very easy to create in this way messages that hang on the queue and cannot be processes.
Other option is to mark the message as “Differ”. In this case we will remain in the queue but we will be able to access it only by the message ID. This is not a valid case for our discussion.
Another solution is to mark the message as a poison message. A message marks as poison will be putted as a death letter.
message. DeadLetter()
or
message. DeadLetter(“…”,”…”)
In this way we mark the message as dead letter. The message is moved to death letter queue.
This sub-queue can be consumed by any other system or consumer and check the messages, log them and so on. The only thing that is not possible to do with it is to add the message to the original queue. To be an able to do this we will need to create a new BrokeredMessage with the same content. As a hint, the name of the sub-queue is [queueName]/$ DeadLetterQueue.
To be able to consume messages from this sub-queue we need to get the name of death letter queue. This can be very easy obtained using the FormatDeadLetterPath method of the QueueClient. After this step we obtained a normal QueueClient that can be used as we know it.
QueueClient deadletterQC = factory.CreateQueueClient(deadLetterQueuePath);
…
var message = deadletterQC.Receive();
…
We are not the only one that can put the messages in death letter queue. Windows Azure can mark a message from the queue as death when:
  • Message expired (TTL)
  • The maxim delivery count for a message is exceed
In this post we saw when is good to use death letter messages from Service Bus Queues. This type of messages can help us to decrease the complexity of our project. For example it is very simple to write a consumer that try for 3 times to process a message and if the action is without success to throw the message in the death letter queue.

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