Lifecycle of a Pod

Kubernetes Pune Meetup #7

08-July-2017

About Me

Contents

Kubernetes Architecture recap

Introduction to Pod

Interaction with Pod

Birth of a Pod

Stages of a Pod’s lifecycle

Probes

Handlers for Lifecycle Events

Recap : Kubernetes Architecture

Image Courtesy: https://blog.heptio.com/core-kubernetes-jazz-improv-over-orchestration-a7903ea92ca

kubeproxy

kubeproxy

Introduction to Pod

Pod is the smallest schedulable unit in Kubernetes

Containers within a Pod share:

IP address space

Port space

Mounted Volumes

Containers within a Pod can communicate via

Localhost

IPC mechanism such as Semaphores or Shared Memory

Pods are never created directly. Higher level constructs such as RC’s,

Interaction with Pods

Kubectl create -f <POD Definition YAML>

Kubectl get pods

Kubectl describe pod <POD NAME>

Kubectl logs <POD NAME>

Kubectl exec <POD NAME> -- <COMMAND>

Kubectl top <POD NAME>

Birth of a Pod

Image Courtesy: Joe Beda

https://blog.heptio.com/core-kubernetes-

jazz-improv-over-orchestration-

a7903ea92ca

Phases of a Pod

Pending: Accepted by Kubernetes but container not created yet.

Running: Pod bound to a node, all containers created and at least one

container is running/starting/restarting

Succeeded: Container(s) exited with status 0

Failed: All containers exit and at least one exited with non-zero status.

Unknown: State of Pod could not be determined due to communication issues

with its node

CrashLoopBackOff: Container fails to start and is retried again and again.

Probes

Probe is a diagnostic performed periodically by kubelet on a container. There are

three types of handlers which can be used to probe containers:

ExecAction: Executes specified command inside container.

TCPSocketAction: Performs check against TCP socket. Returns success if port

is open

HTTPGetAction: Performs HTTP GET against specified port and path.

Successful if 200 > status code > 400.

Liveness and Readiness probes

livenessProbe: Indicates whether the Container is running. If the liveness probe

fails, the kubelet kills the Container, and the Container is subjected to its restart

policy.

readinessProbe: Indicates whether the Container is ready to service requests. If

the readiness probe fails, the endpoints controller removes the Pod’s IP address

from the endpoints of all Services that match the Pod.

Termination of a Pod

Handlers to Lifecycle events

apiVersion: v1

kind: Pod

metadata:

name: lifecycle-demo

spec:

containers:

- name: lifecycle-demo-container

image: nginx

lifecycle:

postStart:

exec:

command: ["/bin/sh", "-c", "echo Hello from the postStart handler > /usr/share/message"]

preStop:

exec:

command: ["/usr/sbin/nginx","-s","quit"]

Init Containers

Out of Beta since 1.6

Init containers allow some pre-actions to run before the actual container is started.

Init containers run to completion sequentially.

Example Use Cases:

Copy source code from a repo

Generate config file for application (update pod specific variables etc)

Check readiness of dependent services

Init Containers Example

apiVersion: v1

kind: Pod

metadata:

name: myapp-pod

labels:

app: myapp

spec:

containers:

- name: myapp-container

image: busybox

command: ['sh', '-c', 'echo The app is running! && sleep 3600']

initContainers:

- name: init-myservice

image: busybox

command: ['sh', '-c', 'until nslookup myservice; do echo waiting for myservice; sleep 2; done;']

- name: init-mydb

image: busybox

command: ['sh', '-c', 'until nslookup mydb; do echo waiting for mydb; sleep 2; done;']

Init Containers Example (continued)kind: Service

apiVersion: v1

metadata:

name: myservice

spec:

ports:

- protocol: TCP

port: 80

targetPort: 9376

---

kind: Service

apiVersion: v1

metadata:

name: mydb

spec:

ports:

- protocol: TCP

port: 80

targetPort: 9377