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Probes: Health Checks

Liveness Probe

Checks: "Is the app dead?"

Result: Restart Pod

Readiness Probe

Checks: "Can it handle traffic?"

Result: Remove from LoadBalancer
User Traffic
✅Healthy

Kubernetes needs to know the state of your application to manage it effectively. It does this through Probes.

By default, Kubernetes only checks if the container's process is running. If your app is deadlocked (but the process is running) or returning 500 errors, Kubernetes thinks it's fine. Probes solve this.

1. Liveness Probe​

  • Question: "Is the container still alive?"
  • Action: If this fails, Kubernetes restarts the container.
  • Use Case: Deadlocks, crashed application loops, memory leaks making the app unresponsive.
  • Warning: Do not fail Liveness probes on external dependencies (like a database is down). If the DB is down, restarting the Pod won't fix it.

2. Readiness Probe​

  • Question: "Is the container ready to accept traffic?"
  • Action: If this fails, Kubernetes stops sending traffic (removes IP from Service Endpoints). It does NOT restart the pod.
  • Use Case: App is starting up, loading large cache, waiting for database connection, or is temporarily overloaded.

3. Startup Probe​

  • Question: "Has the container started yet?"
  • Action: Disables other probes until this passes. If it fails after failureThreshold, the pod is restarted.
  • Use Case: Legacy apps that take minutes to boot. Without this, the Liveness probe might kill the app before it finishes starting.

Example Configuration​

apiVersion: v1
kind: Pod
metadata:
  name: goproxy
spec:
  containers:
  - name: goproxy
    image: k8s.gcr.io/goproxy:0.1
    ports:
    - containerPort: 8080
    
    # Check if app is alive
    livenessProbe:
      httpGet:
        path: /healthz
        port: 8080
      initialDelaySeconds: 3
      periodSeconds: 3
      
    # Check if app is ready for traffic
    readinessProbe:
      httpGet:
        path: /readiness
        port: 8080
      initialDelaySeconds: 5
      periodSeconds: 5

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