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Liveness Checks (Kubernetes)

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Geek_Dude

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Liveness Checks (Kubernetes)

Kubernetes can use a function called a livenessProbe to help identify unhealthy pods or pods that are having issues.

The below livenessProbe uses a httpGet to check for response codes between 200 and 400. The probe waits 3 seconds (initialDelaySeconds) before starting. The probe checks for a response every 3 seconds (periodSeconds). The probe flags an unhealthy container after 3 response failures (failureThreshold), e.g. it gets three 500 server errors.

livenessProbe:
          httpGet:
            path: /
            port: 5000
          initialDelaySeconds: 3
          periodSeconds: 3
          failureThreshold: 3

Here is the livenessProbe in the context of my previous deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: dep-flask-hello
spec:
  replicas: 5
  selector:
    matchLabels:
      name: geektechstuff-flask
  template:
    metadata:
      labels:
        name: geektechstuff-flask
    spec:
      containers:
      - name: flask-hello-world
        image: geektechstuff/flask-hello-world:latest
        livenessProbe:
          httpGet:
            path: /
            port: 5000
          initialDelaySeconds: 3
          periodSeconds: 3
          failureThreshold: 3
        ports:
        - containerPort: 5000
        resources:
          requests:
            cpu: 0.2
            memory: 128Mi
          limits:
            cpu: 1.0
            memory: 256Mi

Using the describe command on a pod will show what liveness options the pod is using:

kubectl describe pod PODNAME_HERE
kubectl describe pod, with Liveness showing towards the bottom of the image

Kubernetes also offers other types of liveness probes, for example if the container does not handle HTTP requests. A tcpSocket probe may be more appropriate in these situations:

livenessProbe:
          tcpSocket:
            port: 5000
          initialDelaySeconds: 5
          periodSeconds: 5
          failureThreshold: 3

Replacing the httpGet in my previous example, the full deployment now looks like:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: dep-flask-hello
spec:
  replicas: 5
  selector:
    matchLabels:
      name: geektechstuff-flask
  template:
    metadata:
      labels:
        name: geektechstuff-flask
    spec:
      containers:
      - name: flask-hello-world
        image: geektechstuff/flask-hello-world:latest
        livenessProbe:
          tcpSocket:
            port: 5000
          initialDelaySeconds: 5
          periodSeconds: 5
          failureThreshold: 3
        ports:
        - containerPort: 5000
        resources:
          requests:
            cpu: 0.2
            memory: 128Mi
          limits:
            cpu: 1.0
            memory: 256Mi

A kubectl describe will now show that tcp socket is being used to check the liveness.

kubectl describe now shows liveness using tcp-socket

More information on Liveness can be found at: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/

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