Kubernetes Scheduling: Taints, Tolerations, and Node Affinity

Control where pods run in your cluster using scheduling constraints.

Node Selectors

Simple way to constrain pods to nodes with specific labels:

spec:
  nodeSelector:
    disktype: ssd
    zone: us-west-1a

# Label a node
kubectl label node worker-1 disktype=ssd

Taints and Tolerations

Taints are applied to nodes to repel pods. Tolerations are applied to pods to allow scheduling on tainted nodes.

Taint Effects

Applying Taints

# Add taint
kubectl taint nodes worker-1 key=value:NoSchedule

# Remove taint
kubectl taint nodes worker-1 key=value:NoSchedule-

Adding Tolerations

spec:
  tolerations:
  - key: "key"
    operator: "Equal"
    value: "value"
    effect: "NoSchedule"

Node Affinity

More expressive than nodeSelector:

Required (Hard) Affinity

spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: zone
            operator: In
            values:
            - us-west-1a
            - us-west-1b

Preferred (Soft) Affinity

spec:
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 100
        preference:
          matchExpressions:
          - key: disktype
            operator: In
            values:
            - ssd

Pod Affinity/Anti-Affinity

Schedule pods relative to other pods:

Pod Affinity

spec:
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchLabels:
            app: cache
        topologyKey: kubernetes.io/hostname

Pod Anti-Affinity

spec:
  affinity:
    podAntiAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 100
        podAffinityTerm:
          labelSelector:
            matchLabels:
              app: web
          topologyKey: kubernetes.io/hostname

Common Scenarios

1. GPU nodes: Taint GPU nodes, tolerate for ML workloads
2. Zone spreading: Anti-affinity for HA
3. Dedicated nodes: Taint for specific teams
4. SSD preference: Node affinity for database pods

Practice Scheduling

Scheduling questions appear in CKA exams. Practice at Sailor.sh.

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