Probes/Health Checks

• It can be configured to check the health of the containers running in a pod.

• used to determine whether a container is running or ready to receive requests

A probe/health check may return the following results:

Success: The container passed the health check.

Failure: The container failed the health check.

Unknown: The health check failed for unknown reasons.

Types of Probes

1. Liveness Probe

• used to determine whether a particular container is running or not.

• If a container fails the liveness probe, the controller will try to restart the pod on the same node according to the restart policy configured for the pod.

Restart Policy

We can define the restart Policy in the pod to instruct the controller about the conditions required to restart the Pod

Default value is always

Values for the Restart Policy as follows

• Always: Always restart the pod when it terminates.

• OnFailure: Restart the pod only when it terminates with failure.

• Never: Never restart the pod after it terminates.

Pod with a Liveness Probe and No Restart Policy

In this yaml file we will define the Liveness Probe and no restart Policy.

If we don’t specify the restart Policy then by default it is always

kind: Pod
apiVersion: v1
metadata:
  name: liveness-probe
spec:
  containers:
  - name: ubuntu-container
    image: ubuntu
    command:
        - /bin/bash
        - -ec
        - touch /tmp/live; sleep 30; rm /tmp/live; sleep 600
    livenessProbe:
       exec:
         command:
            - cat
            - /tmp/live
       initialDelaySeconds: 5
       periodSeconds: 5

Pod configuration

• Creating a container with ubuntu image

• When container starts it will create a file /tmp/live then sleep for 30 seconds and at last remove the file /tmp/live

• This means the file will be available only for 30 seconds and after that it is no longer available in the container

• In liveness configuration — It will try to find the file every 5 seconds with an initial delay of 5 seconds

initalDelaySeonds : Number of seconds controller will wait before launching the probe

periodSeconds : Number of seconds after which the probe will be repeated periodically

Create a pod

kubectl create -f liveness-probe.yaml
kubectl describe pod liveness-probe

You will see the liveness-probe is succeed because the command is executed successfully

Now wait for 30 seconds and the run the below command

kubectl describe pod liveness-probe

You will see that liveness-probe has failed

Now you can see that container is restarting again and again because of the default Restart policy

Creating a Pod Running a Container with a Liveness Probe and a Restart Policy

Now we will use the same pod configuration but with Restart policy as Never

kind: Pod
apiVersion: v1
metadata:
  name: liveness-probe-never-restart
spec:
  restartPolicy: Never
  containers:
  - name: ubuntu-container
    image: ubuntu
    command:
        - /bin/bash
        - -ec
        - touch /tmp/live; sleep 30; rm /tmp/live; sleep 100
    livenessProbe:
       exec:
         command:
            - cat
            - /tmp/live
       initialDelaySeconds: 5
       periodSeconds: 5

Create a pod

kubectl create -f liveness-probe-with-restart-policy.yaml

Wait for a minute and then run the below command

kubectl describe pod liveness-probe-never-restart

From the output you can see that controller killed the container and never attempted to restart the Pod.

TCPSocketAction handler example

In this example, the liveness probe uses the TCP handler to check port 8080 is open and responding. With this configuration, the kubelet will attempt to open a socket to your container on the specified port. If the liveness probe fails, the container will be restarted.

apiVersion: v1
kind: Pod
metadata:
  name: liveness
  labels:
    app: liveness-tcp
spec:
  containers:
  - name: liveness
    image: registry.k8s.io/liveness:0.1
    ports:
    - containerPort: 8080
    livenessProbe:
      tcpSocket:
        port: 8080
      initialDelaySeconds: 5
      periodSeconds: 5

HTTPGetAction handler example

This example shows the HTTP handler, which will send an HTTP GET request on port 8080 to the /health path. If a code between 200–400 is returned, the probe is considered successful. If a code outside of this range is returned, the probe is unsuccessful, and the container is restarted. The httpHeaders option is used to define any custom headers you want to send.

apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-http
spec:
  containers:
  - name: liveness
    image: registry.k8s.io/liveness:0.1
    livenessProbe:
      httpGet:
        path: /health
        port: 8080
        httpHeaders:
        - name: Custom-Header
          value: ItsAlive
      initialDelaySeconds: 5
      periodSeconds: 5

gRPC handler example

This example shows the use of the gRPC health checking protocol to check port 2379 is responding. To use a gRPC probe, port must be configured. If the health endpoint is configured on a non-default service, you must also specify the service. All errors are considered as probe failures as there are no error codes for gRPC built-in probes.

apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-gRPC
spec:
  containers:
  - name: liveness
    image: registry.k8s.io/liveness:0.1
    ports:
    - containerPort: 2379
    livenessProbe:
      grpc:
        port: 2379
      initialDelaySeconds: 5
      periodSeconds: 5

Readiness Probe

• used to determine whether a particular container is ready to receive requests or not.

• For ex- A container which serving a web-application, readiness mean that container has loaded all the static assets, database connection, started the webserver and opened a port to start serving request

What happen if container fails its readiness probe ?

Kubernetes controller will ensure that the pod doesn’t receive any requests.

Readiness Probe States

Failure — is the default state until the readiness probe succeeds.

Success- The container will start receiving requests only after the readiness probe returns with the Success state.

If no readiness probe is configured, the container will start receiving requests as soon as it starts.

Creating a Pod Running a Container with a Readiness Probe

In this yaml file we will define the Readiness Probe

kind: Pod
apiVersion: v1
metadata:
  name: readiness-probe
spec:
  containers:
  - name: ubuntu-container
    image: ubuntu
    command:
        - /bin/bash
        - -ec
        - sleep 30; touch /tmp/ready; sleep 600
    readinessProbe:
       exec:
         command:
            - cat
            - /tmp/ready
       initialDelaySeconds: 10
       periodSeconds: 5

Pod configuration

• Creating a container with ubuntu image

• When container starts it will sleep for 30 seconds and then create file /tmp/ready

• This means the file will be available only after 30 seconds

• In readiness configuration — It will try to find the file every 5 seconds with an initial delay of 10 seconds

Create a pod

kubectl create -f readiness-probe.yaml
kubectl get pod readiness-probe
kubectl describe pod readiness-probe

Wait for a minute and then run the below command

kubectl describe pod readiness-probe

Best Practices while using Probes

Liveness Probes

initialDelaySeconds : should be more then the application start up time so that container doesn’t get stuck in a restart Loop

Readiness Probes

initialDelaySeconds : could be small because we want to enable the traffic to the pod as soon as container is ready