Throttling and Availability over Windows Azure Service Bus

In today post we will talk about different redundancy mechanism when using Windows Azure Service Bus. Because we are using Windows Azure Service Bus as a service, we need to be prepared when something goes wrong.

This is a very stable service, but when you design a solution that needs to handle millions of messages every day you need to be prepared for word case scenarios. By default, Service Bus is not geo-replicated in different data centers, because of this if something is happening on the data center where your namespace is hosted, than you are in big troubles.
The most important thing that you need to cover is the case when the Service Bus node is down and clients cannot send messages anymore. We will see later on how we can handle this problem.
First of all, let’s see why a service like Service Bus can go down. Well, like other services, this has dependencies to databases, storages, other services and resources. There are cases when we can detect pretty easily the cause of the problem.
For example when we receive ‘ServerBusyException’, then we know that the service don’t have enough resources (CPU, memory, …) and we need to retry later. The default retry period is 10 seconds. It is recommended to not set a value under 10 seconds.
This problem can be resolved pretty easily with partition. When we are using partitioning, a topic or a queue is spitted on different messages brokers. This means that we have less chances to have our service down. Also, if something happen with one of our brokers, we will still be able to use the topic/queue without any kind of problems. Don’t forget that brokers will be on the same data center. Using this feature don’t increase your costs.
Enabling this feature can be done in different ways. One option is from portal, Visual Studio Server Explorer or from code.

NamespaceManager namespaceManager = NamespaceManager.CreateFromConnectionString("...");
TopicDescription topicDesc = new TopicDescription("[topicName]")
{
  EnablePartitioning = true;
}

namespaceManager.CreateTopic(topicDesc);

It is so simple to use it. You should know that in this moment you can have maximum 100 topics/queues per namespace that has this feature activated, but I expect this value to change in the future. You should also know different behaviors that are happing when you are using sessions:

• Partition Key – Messages from the same transaction, that has the same partition key, but don’t has a session id will be send to the same broker.
• Session Id – All messages with a specific session if will be send to the same broker.
• Message Id – Messages that are send to a queue/topic with duplicated detection activated will be send to the same broker. Because of this I recommend to use messages duplication detection only where is necessary.
• None of above – Messages are send to all brokers in a round robin manner – one message to each broker.

Another downtown cause can be Service Bus service is upgraded. In this cases, the service will still work, but we can have 15-20 minutes latency until the message will appear in the queue/topic. The most important thing in this case is “We don’t lose any message”.
Also when the system is not stable (internal causes), brokers will be automatically restarted. The restart can take one or more minutes. In this case the service will throw MessagingException or TimeoutException. This problem are resolved build in by the clients SDK’s (if you are using .NET SDK). They have a retry policy build-in, that will retry to resend the message. If the retry policy is not able to send the message, an exception is throw that can be handled in different ways. Until now, all the issues related to this were handle by retry policy with success.
Custom configuration of retry policy can be made in the factory class of messaging.

MessagingFactory messagingFactory = MessagingFactory.Create();
messagingFactory.RetryPolicy = RetryExponential.Default;

The last main cause of failing is external causes like internet connectivity problem, electrical outage or human errors. This problem is handled with a very different approach. The client needs to detect this problem and handle it. Until now, this required a custom code to be written, that would redirect the messages to a topic/queue that is in another datacenter (namespace).
From now we can use paired namespace to handle this scenario. The paired namespace give us the possibility to specify a second namespace (that can be in a different data center) that will be used to send messages until the primary one will be up and running. When messages are send to the second namespace, messages will be persisted until the primary namespace will be up. In the moment when the primary namespace is running, all messages from the second one will need to be redirected to the first one. We can imagine secondary namespace as a buffer that is used to store messages until our main namespace is in good state.
 When we configure this feature, we can set also the failover interval. This is the time interval when our system will accept failovers before switching to the second namespace. The recommended (and default) value is 10 seconds. Also you will need to specify the number of queues that are used to store the messages in the secondary namespace (default value is 10). This value should be greater or equal to 10.
The last option that you should be aware is syphon (‘enableSyphon’ parameter). When you activate this on a client, you tell to the system that this is the system that will transfer the messages from the second namespace to the first one. Usually this value should be set on the consumers clients (backend), because usually clients only send messages to the topics/queues.

NamespaceManager primaryNM = NamespaceManager.CreateFromConnectionString("...");
MessagingFactory primaryMF = ...
NamespaceManager secondaryNM= NamespaceManager.CreateFromConnectionString("...");
MessagingFactory secondaryMF = ...
SendAvailabilityPairedNamespaceOptions sao=
    new SendAvailabilityPairedNamespaceOptions(secondaryNamespaceManager, secondaryMF);
primaryMF.PairNamespaceAsync(sao).Wait();

There are some small things that we should know related to this feature:

• The state and order is guaranteed only in the first queue. When using session, the order of the messages is not guaranteed when secondary namespace is used
• Messages are consumed only from the primary queue/subscription
• You will pay the extra cost of moving messages from the secondary namespace to the primary one
• The default name of the queues that are created on the secondary namespace is ‘x-servicebus-transfer/i’ (where ‘i’ can have a value from 0 to n)
• The queue from the secondary namespace is randomly chosen
• It is not recommended to change the configuration of the queues from the secondary namespace

We saw that we have different mechanism to handle this special scenarios. We don’t have a mechanism that handle all this use cases. Before starting to think about integrating all this feature ask yourself if you need all of them? There are cases when a 10-15 minutes downtime is acceptable.