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Communication Strategies on top of message base communication for outages or disasters protection

In the last post, we identified different location inside and outside a micro-service architecture where messaging base communication could be used with success. Beside this we identified that to be able to have a high-performance system, we need to use reliable messaging communication channels.
If we decide to run our micro-service solution inside Kubernetes on top of Azure Kubernetes Services (AKS), we need to take into consideration messaging and event bases communication channels like Azure Service Bus or Azure Event Hub.
Even so, this is not enough to be fully protected of any outages or disasters. Another layer of protection that we should define is at the application level. In this post, we will discuss some communication strategies that can increase our protection against outages and disasters.
The below solutions can be used in combination with any messaging system, but in this case, we will present them in combination with Azure Service Bus.

Passive Replication
This strategy is based on two instances of messaging service instances that run in parallel. In one moment in time, the sender is using only one to send messages – so called the active one. When the sender is not able to communicate with the active instance, it will automatically switch to the second one (called passive), that will be marked as active. Basically, it is a switch between active and passive instance. An active instance remains active until a communication problem occurs.
The consumer needs to listen and consume messages from both messaging service instances – active and passive. Because there is a risk to receive the same message on both channels (active and passive) the consumer needs to have a mechanism to identify messages that were already processed.
There are cases when business is not effective is a message is processed twice. If your business or performance it is not affected by processing the same message twice, then you should not handle this scenario.
If the message order it is important for you, please take into consideration that there are cases when by default the message order might not be respected. For example, if the sender sends message A to the active instance that becomes unavailable for a few seconds and sender send messages B and C over passive instance (that becomes active), the receiver might receive message B and C before message A. Based on what kind of messaging solution you used, you might have a mechanism that protects you for scenarios like this. Azure Service Bus allows us to put a group together multiple messages under the same session, protecting us from this kind of scenarios.
Another case is when a message is pushed to the active instance and the send confirmation is not received anymore because the active channels go down. In this case, the message will be sent on the second channel also. For this situation, the receiver needs to be able to detect duplicate messages and decide how to handle them.

Active Replication
This approach involves using two instances of messaging services (like to different instances of Azure Service Bus namespaces) to send messages between sender and receiver. The sender sends the same message twice on both message instances.
Each message needs to have a unique ID that it is used by the receiver to detect the message duplication and suppress the second one. When you are using Azure Service Bus, you can use custom property tag, the Label properties of the message or even the BrokeredMessage.MessageId to set the ID used to detect the message duplication.
From a cost perspective, Active Replication is more expensive than the passive replication because you send the same message twice, but increase the protection to disasters or outages. Considering your business needs and NFR you should decide what approach suits best your needs.

In the next post you'll be able to find a comparison between messages vs event base communication system.



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