Understanding Liveness, Readiness and Startup Probes in Kubernetes

 This is a small article about understanding the liveness, readiness and startup in kubernetes. 

There's good explanation in the kubernetes documentation: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/

This video also explains well the process: https://www.youtube.com/watch?v=aTlQBofihJQ

But I wanted to understand it in a practical way.

So I have this demo: https://github.com/DiegoTc/guest-book-js-docker/tree/Running-App-Version-1

It's a simple application running on a kubernetes cluster.

https://github.com/DiegoTc/guest-book-js-docker/blob/Running-App-Version-1/argo/deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: chat-ui
spec:
  replicas: 1
  revisionHistoryLimit: 3
  selector:
    matchLabels:
      app: chat-ui
  template:
    metadata:
      labels:
        app: chat-ui
    spec:
      containers:
        - image: diegotc/guestbook:20230803-064434
          imagePullPolicy: Always
          name: chat-ui
          ports:
            - containerPort: 5500

Everything is OK.  You can see how it appears in our ArgoCD

Repo with failing image: https://github.com/DiegoTc/guest-book-js-docker/tree/Crash-App-Version-1

Docker image working: diegotc/guestbook:20230803-064434

But what happens, if we deploy an image that crashes?

Docker images crashes: diegotc/guestbook:20230803-070920

So what happens, is that our application is in a infinite loop, trying to start and it doesn't works.

So now, we're going to see the same example, but with the startup configuration.

Repo with solution: https://github.com/DiegoTc/guest-book-js-docker/blob/Running-App-Version-2/argo/deployment.yaml

Docker image working: diegotc/guestbook:20230803-073811

After we deploy our application that will cause the crash
Repo: https://github.com/DiegoTc/guest-book-js-docker/tree/Crash-App-Version-2
Docker images crashes: diegotc/guestbook:20230803-082315


As you can see, our application is still running, even we deploy a wrong image.

You can read more about it in the kubernete documentation: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/