Active-passive disaster recovery solution overview for Azure Kubernetes Service (AKS)

When you create an application in Azure Kubernetes Service (AKS) and choose an Azure region during resource creation, it's a single-region app. When the region becomes unavailable during a disaster, your application also becomes unavailable. If you create an identical deployment in a secondary Azure region, your application becomes less susceptible to a single-region disaster, which guarantees business continuity, and any data replication across the regions lets you recover your last application state.

This guide outlines an active-passive disaster recovery solution for AKS. Within this solution, we deploy two independent and identical AKS clusters into two paired Azure regions with only one cluster actively serving traffic.

Active-passive solution overview

In this disaster recovery approach, we have two independent AKS clusters being deployed in two Azure regions. However, only one of the clusters is actively serving traffic at any one time. The secondary cluster (not actively serving traffic) contains the same configuration and application data as the primary cluster but doesn’t accept any traffic unless directed by Azure Front Door traffic manager.

Scenarios and configurations

This solution is best implemented when hosting applications reliant on resources, such as databases, that actively serve traffic in one region. In scenarios where you need to host stateless applications deployed across both regions, such as horizontal scaling, we recommend considering an active-active solution, as active-passive involves added latency.

Components

The active-passive disaster recovery solution uses many Azure services. This example architecture involves the following components:

Multiple clusters and regions: You deploy multiple AKS clusters, each in a separate Azure region. During normal operations, network traffic is routed to the primary AKS cluster set in the Azure Front Door configuration.

Configured cluster prioritization: You set a prioritization level between 1-5 for each cluster (with 1 being the highest priority and 5 being the lowest priority). You can set multiple clusters to the same priority level and specify the weight for each cluster. If the primary cluster becomes unavailable, traffic automatically routes to the next region selected in Azure Front Door. All traffic must go through Azure Front Door for this system to work.

Azure Front Door: Azure Front Door load balances and routes traffic to the Azure Application Gateway instance in the primary region (cluster must be marked with priority 1). In the event of a region failure, the service redirects traffic to the next cluster in the priority list.

Hub-spoke pair: A hub-spoke pair is deployed for each regional AKS instance. Azure Firewall Manager policies manage the firewall rules across each region.

Key Vault: You provision an Azure Key Vault in each region to store secrets and keys.

Log Analytics: Regional Log Analytics instances store regional networking metrics and diagnostic logs. A shared instance stores metrics and diagnostic logs for all AKS instances.

Container Registry: The container images for the workload are stored in a managed container registry. With this solution, a single Azure Container Registry instance is used for all Kubernetes instances in the cluster. Geo-replication for Azure Container Registry enables you to replicate images to the selected Azure regions and provides continued access to images even if a region experiences an outage.

Failover process

If a service or service component becomes unavailable in one region, traffic should be routed to a region where that service is available. A multi-region architecture includes many different failure points. In this section, we cover the potential failure points.

Application Pods (Regional)

A Kubernetes deployment object creates multiple replicas of a pod (ReplicaSet). If one is unavailable, traffic is routed between the remaining replicas. The Kubernetes ReplicaSet attempts to keep the specified number of replicas up and running. If one instance goes down, a new instance should be recreated. Liveness probes can check the state of the application or process running in the pod. If the pod is unresponsive, the liveness probe removes the pod, which forces the ReplicaSet to create a new instance.

For more information, see Kubernetes ReplicaSet.

Application Pods (Global)

When an entire region becomes unavailable, the pods in the cluster are no longer available to serve requests. In this case, the Azure Front Door instance routes all traffic to the remaining health regions. The Kubernetes clusters and pods in these regions continue to serve requests. To compensate for increased traffic and requests to the remaining cluster, keep in mind the following guidance:

• Make sure network and compute resources are right sized to absorb any sudden increase in traffic due to region failover. For example, when using Azure Container Network Interface (CNI), make sure you have a subnet that can support all pod IPs with a spiked traffic load.
• Use the Horizontal Pod Autoscaler to increase the pod replica count to compensate for the increased regional demand.
• Use the AKS Cluster Autoscaler to increase the Kubernetes instance node counts to compensate for the increased regional demand.

Kubernetes node pools (Regional)

Occasionally, localized failure can occur to compute resources, such as power becoming unavailable in a single rack of Azure servers. To protect your AKS nodes from becoming a single point regional failure, use Azure Availability Zones. Availability zones ensure that AKS nodes in each availability zone are physically separated from those defined in another availability zones.

Kubernetes node pools (Global)

In a complete regional failure, Azure Front Door routes traffic to the remaining healthy regions. Again, make sure to compensate for increased traffic and requests to the remaining cluster.

Next steps

If you're considering a different solution, see the following articles:

• Active active high availability solution overview for Azure Kubernetes Service (AKS)
• Passive cold solution overview for Azure Kubernetes Service (AKS)