Readiness Probes

Pod setup process, status, and conditions

Before we straight away understand how to use Readiness Probes. Let’s understand the Pod setup process, status, and conditions.

The scheduler will determine which node to place the pod.
Once the pod is scheduled on the right node, it will pull the images, convert it into a container, and the container will start after done pulling the image.
Then, the pod will go into a running state by making sure all the containers are ready (ready conditions).

The ready condition actually indicates that the application running inside the pod is now ready to accept the user traffic, because by default, Kubernetes assumes when the container is created, it is ready to accept the user traffic, therefore it will set the ready condition to true for all the containers.

Usage of Readiness Probes

Reference

The concept of Readiness probes basically is to determine whether a container is ready to accept the traffic by performing some health checks. If the readiness probes are successful, then we know that container is going to run and accept the traffic, else, it (kubelet) will remove the pod and try again.

Examples of Readiness Probes usage;

Test the web application is online
Check whether the Database TCP connection is online or ready
Run the exec command to run a custom script that will determine the application is ready

HTTP

readiness-probes.yaml


apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: web-app
      image: webapp
      readinessProbe:
        httpGet:
          path: /api/v1/health
          port: 8000
        initialDelaySeconds: 5 # Number of seconds after the container has started before startup, that means it should wait for 5 seconds before performing the 1st probe
        periodSeconds: 3 # how often to perform the probe
        failureThreshold: 5 # default 3, if the application is not ready after 3 attempts, then the probe will stop

TCP

readiness-probes.yaml


apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: web-app
      image: webapp
      readinessProbe:
        tcpSocket:
          port: 8080
        initialDelaySeconds: 5 # Number of seconds after the container has started before startup, that means it should wait for 5 seconds before performing the 1st probe
        periodSeconds: 3 # how often to perform the probe
        failureThreshold: 5 # default 3, if the application is not ready after 3 attempts, then the probe will stop

kubelet will try to open a socket to the container on the specified port 8080. If it can establish the connection, then the state is healthy.

exec

readiness-probes.yaml


apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: web-app
      image: webapp
      readinessProbe:
        exec:
          command:
            - cat
            - /app/healthy
        initialDelaySeconds: 5 # Number of seconds after the container has started before startup, that means it should wait for 5 seconds before performing the 1st probe
        periodSeconds: 3 # how often to perform the probe
        failureThreshold: 5 # default 3, if the application is not ready after 3 attempts, then the probe will stop

It will return 0 status code if the command succeed.