K8s APIs and How to use Them

• K8s has workloads and workloads help in managing and run the applications in k8s cluster
• K8s workloads will have
  • Pods
  • Controllers

Pods

• Pod is basic unit of execution in K8s (vms in hypervisor and container in docker)
• Pod has application container, storage resources and network identity (ip address)
• Pod has docker container
• Pods can have single or multiple containers in it.
• One-container-per-pod is the most common k8s usecase
• pods might encapsulate an application of multiple containers that are tightly coupled.

Lets use API Reference to create a Sample Pod

• Navigate to k8s API Reference From here (we will be using API version v1.18)
• Now navigate to pod
• Now fill the yaml file by using apireference

---
apiVersion: v1
kind: Pod
metadata:
  name: hello-pod
spec:
  containers:
    - name: myjenkins
      image: jenkins

• Now login into k8s master and execute the following commands

kubectl get nodes
kubectl apply -f hello-pod.yaml
kubectl get pods
kubectl get pods -o wide
kubectl describe pod hello-pod
kubectl get pods -oyaml 
kubectl get pods -oyaml | grep uid
kubectl delete -f hello-pod.yaml

• Dont try to remember commands refer here for cheatsheet

How kubectl helps

• kubectl takes commandline args or yaml file as input and it creates json and speaks with k8s apis with the created json

Imperative vs Declarative style in kubectl

• Imperative style is building commandlines for creation of k8s objects.
• Declarative style is building a yaml file with state information
• If we build commandlines (imperative) changes will not be version controlled and difficult to handle
• Repeating the activity would mean rebuilding the command

Durability of Pods

• Pods aren’t created as durable entities
• They wont survive failures (node/pod/other)
• so to ensure pods state we use controllers

Pod lifecycle States

• Pending
• Running
• Failed
• succeded

Container Probes inside POd

• A probe is a diagnostic/check performed by a kublet on a container.
• Kubelet will call Handler (which is implemented by a container)
  • ExecAction
  • TCPSocketAction
  • HTTPGetAction
• Probe can have one of following results
  • Success
  • Failure
  • Unknown
• Kubelet can react to three kinds of probes on running containers
  • livenessProbe: indicates wheter the container is running or not
  • readinessProbe: indicates whether the container is ready to service requests
  • startupProbe: indicates whether application in container is started

Init Containers

• Specialize container inside the pod that run before the application containers in a Pod
• Init containers can contain utilities or setup scripts

Challenges in the Pod

1. Pods cannot maintain state (Pod cannot correct itself)
2. How Networking is applied to Pods
3. How to persis the data from the containers running inside pods

Controllers to the Rescue

• Controllers help in maintaining desired state. Controllers manage pods
• In k8s cluster directly creating pods is not a good idea dealing with controllers and making controllers manage pods is way ahead
• Controllers in K8s
  • ReplicatSet
  • ReplicationController
  • Deployments
  • StatefulSets
  • DaemonSet
  • Jobs
  • CronJob

Replication Controller

• A replication controller ensures specified number of pod replicas are running at any time
• To create a Replication Controller create a template/manifest by using API Reference
• Lets create a replication controller with 3 jenkins pods

---
apiVersion: v1
kind: ReplicationController
metadata:
  name: hello-rc
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: jenkins
    spec:
      containers:
        - name: jenkins
          image: jenkins:2.60.3
  

• Now execute the following commands

kubectl apply -f hello-rc.yaml
kubectl get rc -o wide
kubectl get pods -o wide
kubectl get rc -oyaml| grep uid

• Now lets increase the replications to 5

---
apiVersion: v1
kind: ReplicationController
metadata:
  name: hello-rc
spec:
  replicas: 5
  template:
    metadata:
      labels:
        app: jenkins
    spec:
      containers:
        - name: jenkins
          image: jenkins:2.60.3
  

• Now lets see what happens when one node fails. (I will be stopping 24 node for check)
• Experment with 2 replicas and kill one node