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Patterns in Windows Azure Service Bus - Resequencer Pattern

Today we will talk about another message pattern: Resequencer. In the last post I presented Recipient List Pattern. In comparison with this pattern, Resequencer Pattern is very different. The main scope of this pattern is to help us to put messages back in a specific order.
When we are talking about messages, we can talk about a stream of messages that need to be received in a specific order. It is very crucial for the receiver to retrieve the messages in the same order he receives.
Theoretically, in a simple case we will receive messages in the expected order. This is offer Service Bus by default. But what happen if an error occurs on the receiver and message is putted back in the queue. We will need to retry to consume that message one more time and not the next message.
Another case when the order can be broken is when we have more than one producer. For this case we can have two different situations.
In the first scenario, each producer will produce messages for different stream of messages. In this case we can very easily use the session id of the messages to be able to receive only messages for a specific stream. But we will still need a way to detect if the messages if the message that we expect. First step is to add two properties to each message. The first property will tell us how many messages are in this message stream and the other one will tell us the index of the current message. Base on this information, the receiver will know the index of the next message and will be able to validate it. If we will receive messages that don’t have a valid index id, we can throw them in the defer queue, from where we will be able to retrieve them anytime.
Producer:
QueueClient  queueClient = …
BrokekedMessage message = new BrokeredMessage();
message.Properties[“index”] = 1;
message.Properties[“count”] = 10;
message.SessionId = 123;
queueClient.Send(message);
Consumer:
MessageSession messageSession = queueClient.AcceptMessageSession(123);
int currentIndex = 1;
while(true)
{
    BrokeredMessage message = messageSession.Receive();
    if(int.Parse(message.Properties[“index”]) != currentIndex)
    {
        message.DeadLetter();
        continue;
    }
    …
    message.Complete();
    if(int.Parse(messsage[“count”]) == currentIndex)
    {
        break;
    }
    currentIndex++;
}
Next we need to take message that were marked as dead letters and moved automatically to the dead letter queue.
QueueClient deadLetterQueue = QueueClient.CreateConnectionString(
    connectionString,
    QueueClient.FormatDeadLetterPath(“FooQueue”));
while (true)
{
    BrokeredMessage message = deadLetterQueue.Receive();
    // same logic as in the normal queue.
    // we need to abandon the message and not to mark him as dead letter.
    // we already process the dead letter messages
}
The second scenario is a little more complicated. In the same queue, we have more than one producer that produce message for a specific stream. The chances to have messages in the expected order are very low. The solution is similar to the first one. We can add the index of each message and total number of messages as properties to the message that is added to the queue. The consumer can check this values and when the message index is wrong, the message will be added to the defer queue.
In the both solutions, the most time consuming is retrieving the message from the defer queue. The good part that usually the messages are in a kind of order, even if is not perfect (eq. 1 3 5 2 6 7 10 8 9). Because of this we will not need to “iterate” through the defer queue to many times.
This pattern can be used for cases when it is critical to process messages in a specific order. In a system that sells tickets for a baseball game this is not so critical. But for a system that receives commands using messages it is very important to execute the commands in the same order – for example a nuclear power station.
This is a pattern that is not very common. When you reach a case where you need this pattern, try to double check again if you need it, because this pattern can be very expensive – from the perspective of processing time and resources.
Last edit: A list of all patterns that can be used with Windows Azure Service Bus, that were described by me LINK.  

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