Probes

About

A Probe is a diagnostic mechanism that Kubernetes uses to check the health of a container. They run periodically and report the status back to the kubelet. Based on the probe result, Kubernetes can decide to:

• Restart a container

• Stop routing traffic to a container

• Mark the Pod as ready or not ready

Types of Probes

Liveness probe

• Checks if the container is still running properly.

• If it fails, Kubernetes restarts the container.

• Example: an app is running but deadlocked → liveness probe fails → kubelet restarts it.

Ensures apps that get stuck or crash get restarted automatically.

Example

deployment.yaml

...
spec:
  template:
    ...
    spec:
      containers:
        - name: myapp-container
          image: nginx:1.25
          ports:
            - containerPort: 80
          livenessProbe:
            httpGet:
              path: /healthz
              port: 8000
            initialDelaySeconds: 5
            periodSeconds: 10

Readiness probe

• Checks if the container is ready to accept traffic.

• If it fails, the Pod is marked unready → Service will stop sending traffic to it.

• Example: your app takes 30 seconds to load caches. Until ready probe passes, no traffic is sent.

Ensures traffic is only routed to healthy Pods.

deployment.yaml

...
spec:
  template:
    ...
    spec:
      containers:
        - name: myapp-container
          image: nginx:1.25
          ports:
            - containerPort: 80
          readinessProbe:
            httpGet:
              path: /healthz
              port: 8000
            initialDelaySeconds: 5
            periodSeconds: 10

Startup probe

• Checks if the container has started successfully.

• Especially useful for apps with slow startup times.

• Disables liveness and readiness checks until it succeeds.

• Prevents Kubernetes from killing apps that just need more time to boot.

Ensures slow apps don’t get killed prematurely during initialization.

deployment.yaml

...
spec:
  template:
    ...
    spec:
      containers:
        - name: myapp-container
          image: nginx:1.25
          ports:
            - containerPort: 80
          startupProbe:
            httpGet:
              path: /healthz
              port: 8000
            initialDelaySeconds: 5
            periodSeconds: 10

Probe Actions

Probes can use one of these mechanisms to check health:

httpGet

Hits an HTTP endpoint in your container:

httpGet:
  path: /healthz
  port: 8080

tcpSocket

Tries to open a TCP connection:

tcpSocket:
  port: 3306

exec

Runs a command inside the container:

exec:
  command: ["cat", "/tmp/healthy"]

Probe Parameters

• initialDelaySeconds: how long to wait before starting checks.

• periodSeconds: how often to run the probe.

• timeoutSeconds: how long to wait before probe times out.

• successThreshold: how many successes before marking healthy.

• failureThreshold: how many failures before marking unhealthy.

Example

deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  replicas: 2
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
        - name: myapp-container
          image: myapp:1.0
          ports:
            - containerPort: 8080
          livenessProbe:
            httpGet:
              path: /healthz
              port: 8080
            initialDelaySeconds: 5
            periodSeconds: 10
          readinessProbe:
            httpGet:
              path: /ready
              port: 8080
            initialDelaySeconds: 10
            periodSeconds: 5
          startupProbe:
            httpGet:
              path: /startup
              port: 8080
            failureThreshold: 30
            periodSeconds: 10