Probes

It helps with detecting and correcting application issues.

The probes will run a miniprocess that check for runtime conditions. For example, an http get request. For other apps open a tcp socket, or stuff similar like that.

The verification method is exec by kubelet.

Readiness Probe

Is the app ready to server requests?

This checks if the app is up and running, the kubelet will check the config of the readiness probe periodically.

Liveness Probe

Does the app still function without errors?

Check for deadlocks, memoryleaks. This will restart the container and try to get the app into a running state

Startup Probe

Legacy app may need longer to start. Hold off on starting the liveness probe.

Health Verification Methods

We can choose from different verification methods, in any type of probe

• custom command: exec.command, execute a command inside the container and check the exit code
• http get request: httpGet, make a request and check if it is between 200-399
• tcp socket connection: tcpSocket, open socket at port
• gRPC: grpc

You can customize each of these a bit more.

• initalDelaySeconds
• periodSeconds
• timeoutSeconds
• successThreshold - number of successful attempts until probe is consider successful after a failure failureThreshold.
• terminationGracePeriodSeconds - grace period before forcing a container stop upon failure.

Probes are not different objects. You need to add the config inside the container:

apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-http
spec:
  containers:
  - name: liveness
    image: registry.k8s.io/e2e-test-images/agnhost:2.40
    args:
    - liveness # here we start
    livenessProbe:
      httpGet:
        path: /
        port: 8080
      initialDelaySeconds: 3
      periodSeconds: 3