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01-scheduling-internals.md

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Scheduling

Back to overview

Scheduling Internals

In Kubernetes, scheduling refers to making sure that Pods are matched to Nodes so that Kubelet can run them. (see Kubernetes Documentation)

Scheduling is done within the control plane (master-node) of kubernetes.

If you have access to the control plane, there are several extensions points for extending the scheduling behaviour: Scheduling Framework Extensions

Pod Lifecycle

Pod Lifecycle

Scheduling

Show the labels of your nodes

kubectl get nodes --show-labels

Node Selector

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    env: test
spec:
  containers:
    - name: nginx
      image: nginx
      imagePullPolicy: IfNotPresent
  nodeSelector:
    disktype: ssd

Pod Affinity/Anti-Affinity

Attract pods to certain nodes

The affinity feature consists of two types of affinity, "node affinity" and "inter-pod affinity/anti-affinity". Node affinity is like the existing nodeSelector, while inter-pod affinity/anti-affinity constrains against pod labels rather than node labels.

Node Affinity

apiVersion: v1
kind: Pod
metadata:
  name: with-node-affinity
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
              - key: kubernetes.io/e2e-az-name
                operator: In
                values:
                  - e2e-az1
                  - e2e-az2
      preferredDuringSchedulingIgnoredDuringExecution:
        - weight: 1
          preference:
            matchExpressions:
              - key: another-node-label-key
                operator: In
                values:
                  - another-node-label-value
  containers:
    - name: with-node-affinity
      image: k8s.gcr.io/pause:2.0

Inter-pod affinity and anti-affinity

apiVersion: v1
kind: Pod
metadata:
  name: with-pod-affinity
spec:
  affinity:
    podAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        - labelSelector:
            matchExpressions:
              - key: security
                operator: In
                values:
                  - S1
          topologyKey: failure-domain.beta.kubernetes.io/zone
    podAntiAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
        - weight: 100
          podAffinityTerm:
            labelSelector:
              matchExpressions:
                - key: security
                  operator: In
                  values:
                    - S2
            topologyKey: failure-domain.beta.kubernetes.io/zone
  containers:
    - name: with-pod-affinity
      image: k8s.gcr.io/pause:2.0

Taints & Tolerations

Prevent scheduling of pods on certain nodes

tolerations:
  - key: "key1"
    operator: "Equal"
    value: "value1"
    effect: "NoSchedule"
  - key: "key1"
    operator: "Equal"
    value: "value1"
    effect: "NoExecute"

or

tolerations:
  - key: "key1"
    operator: "Equal"
    value: "value1"
    effect: "NoExecute"
    tolerationSeconds: 3600

References

https://medium.com/@dominik.tornow/the-kubernetes-scheduler-cd429abac02f