Understanding Kubernetes: Part 25 Affinity and Anti-Affinity

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Understanding Kubernetes: Part 25 Affinity and Anti-Affinity

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What is Node Affinity?

Node Affinity is a rule that ensures Pods are scheduled on specific nodes based on defined labels.

What is Pod Affinity and Anti-Affinity?

Pod Affinity ensures that Pods are scheduled close to other Pods based on label selectors.
Pod Anti-Affinity prevents Pods from being scheduled on the same nodes or within a failure domain.

Use Cases

Affinity and Anti-Affinity are useful in various scenarios:

1. Node Affinity

Ensuring that workloads requiring SSD storage run on nodes labeled with storage=ssd.
Running GPU-intensive workloads on nodes labeled gpu=true.

2. Pod Affinity

Ensuring that microservices communicate efficiently by scheduling them on the same node.
Keeping interdependent applications (e.g., frontend and backend) close together to reduce latency.

3. Pod Anti-Affinity

Distributing replicas of an application across multiple nodes for high availability.
Ensuring that critical workloads are not placed on the same node to prevent single points of failure.

Node Affinity Syntax

Node Affinity rules are defined in the Pod specification under affinity.nodeAffinity.

Example: Scheduling a Pod on SSD Nodes

apiVersion: v1
kind: Pod
metadata:
  name: ssd-pod
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
              - key: "storage"
                operator: "In"
                values:
                  - "ssd"
  containers:
    - name: app
      image: my-app

This ensures that the Pod is scheduled only on nodes labeled storage=ssd.

Pod Affinity Syntax

Pod Affinity rules are defined under affinity.podAffinity.

Example: Placing Pods Close Together

apiVersion: v1
kind: Pod
metadata:
  name: web-pod
spec:
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        labelSelector:
          matchLabels:
            app: frontend
        topologyKey: "kubernetes.io/hostname"
  containers:
    - name: web
      image: nginx

This ensures that the Pod is scheduled on the same node as other Pods labeled app=frontend.

Pod Anti-Affinity Syntax

Pod Anti-Affinity is defined under affinity.podAntiAffinity.

Example: Spreading Replicas Across Nodes

apiVersion: v1
kind: Pod
metadata:
  name: backend-pod
spec:
  affinity:
    podAntiAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        labelSelector:
          matchLabels:
            app: backend
        topologyKey: "kubernetes.io/hostname"
  containers:
    - name: backend
      image: my-backend

This ensures that backend Pods are scheduled on different nodes to improve reliability.

In My Previous Role

As a Senior DevOps Engineer, I leveraged Affinity and Anti-Affinity for:

Optimizing Microservice Deployments: Ensured interdependent services were scheduled on the same node to reduce latency.
High Availability Strategies: Used Anti-Affinity to distribute replicas across nodes to prevent single points of failure.
Performance Tuning: Enforced Node Affinity rules to assign workloads to high-memory nodes for better performance.

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