Introduction

High availability (HA) is essential for modern applications to ensure uninterrupted service and minimize downtime. This article explores achieving high availability in Kubernetes clusters by covering fault tolerance and scalability, and avoiding single points of failure. We’ll dive into the key components of a Kubernetes cluster and share best practices for maintaining HA.

Achieving high availability in Kubernetes involves several critical components:

1. 1. Control Plane
2. 2. Worker Nodes
3. 3. Applications

Control Plane High Availability

The control plane manages the Kubernetes cluster, making decisions about scheduling, detecting and responding to cluster events, and maintaining the cluster's overall state.

1. Fault Tolerance and Majority Consensus

If the control plane consists of a single node, it’s a single point of failure. For high availability, multiple control plane nodes are required. Kubernetes typically achieves this with three control plane nodes, ensuring fault tolerance.

Majority Consensus: Changes to the cluster require agreement from a majority of the control plane nodes, calculated as floor(n/2) + 1. Therefore, with three control plane nodes, the cluster can tolerate one node failure.

2. Load Balancing Control Plane Nodes

A highly available load balancer is essential to distribute traffic among the control plane nodes. This ensures that if one node fails, the load balancer can redirect traffic to the remaining nodes.

3. Geographic Distribution

Control plane nodes should be distributed across different data centers (DCs) to ensure high availability. However, they must be close enough to avoid latency issues.

Best Practice: Deploy control plane nodes in three data centers, each hosting one node, to achieve redundancy and minimize the risk of a single point of failure.

Worker Nodes High Availability

Worker nodes run the application workloads. For high availability:

• -  Deploy worker nodes across different zones and data centers to ensure that the failure of one zone does not affect the entire cluster.
• - Use labels to manage zone distribution and scheduling policies to ensure even distribution of workloads.
• - Regularly back up etcd and monitor backup processes to ensure data integrity and recoverability.

Application High Availability

Applications need to be designed for high availability by leveraging Kubernetes’ built-in capabilities. Here are some key areas to focus on:

1. Deployments

• 1.1. Run Pods in Deployment: Configure the number of replicas to ensure redundancy.

  
  
• 1.2. Rolling Updates: Use rolling updates for seamless updates and failure recovery.

kind: Deployment
metadata:
  name: component
spec:
  replicas: 3
  updateStrategy: 
    type: RollingUpdate
    rollingUpdate:  
      maxUnavailable: 1
      maxSurge: 1

• 1.3. Stateful Sets for Stateful Apps: Utilize StatefulSets for applications requiring stable storage and unique network identities per pod.

2. Distribute Pods

• - Add Labels to Nodes: Use labels to manage and distribute pods across different worker nodes.

kind: Deployment metadata:
name: component spec:
affinity: 
  podAntiAffinity:
    requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
              matchLabels:
                app.kubernetes.io/name: app-name
            topologyKey: "kubernetes.io/hostname"

• - Added by Cloud Providers: Many cloud providers offer built-in capabilities to add labels and manage zones.

Application Best Practices

• - Use Deployments for stateless applications with multiple replicas to ensure redundancy.
• - Use StatefulSets for stateful applications to ensure stable storage and unique network identities.

Conclusion

Achieving high availability in Kubernetes clusters is essential for maintaining reliable and continuous services. By following best practices and leveraging Kubernetes’ built-in capabilities, you can ensure that your applications and services remain resilient, even in the face of failures. Implementing high availability not only enhances the performance and reliability of your systems but also provides a competitive advantage in today’s fast-paced digital landscape.