Background
This blog is a part of a mini-series for preparing for the CKA exam. This blog would focus on the concepts related to Containers.
How to read through the various blogs in this mini-series
Please feel free to go through all blogs (or) a particular blog, for a concept that You want to have a quick review. However, I would recommend going through the exam tips, as it may help You better organize Your time.
- Part-1 : Tips
- Part-2: Storage
- Part-3: Containers - You are reading this
- Part-4 to Part-8: would update the links here as I publish them
Environment
Examples are executed using Docker Desktop on Mac, with Kubernetes.
% kubectl get nodes
NAME STATUS ROLES AGE VERSION
docker-desktop Ready control-plane 10d v1.24.0
Containers and Workloads Usecases
A Pod is the smallest deployable unit of computing that could be created in Kubernetes - it contains one or more containers along with a shared network and storage.
- Assigning CPU and memory resources to containers
- Pod with an init container
- Pod with multiple containers
Assigning CPU and memory resources to containers
- Following is an example for requesting resources and specifying the CPU and memory limits. CPU is throttled as per limits and if the container tries to exceed memory limits, it becomes a candidate for termination
apiVersion: v1
kind: Pod
metadata:
name: pod-resources-demo
spec:
containers:
- name: cpu-mem-demo
image: nginx
resources:
limits:
cpu: "1"
memory: "200Mi"
requests:
cpu: "0.5"
memory: "100Mi"
- get the pod details and check
% kubectl get pod pod-resources-demo -o yaml
apiVersion: v1
kind: Pod
metadata:
annotations:
kubectl.kubernetes.io/last-applied-configuration: |
{"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"name":"pod-resources-demo","namespace":"default"},"spec":{"containers":[{"image":"nginx","name":"cpu-mem-demo","resources":{"limits":{"cpu":"1","memory":"200Mi"},"requests":{"cpu":"0.5","memory":"100Mi"}}}]}}
creationTimestamp: "2022-05-28T06:16:40Z"
name: pod-resources-demo
...
Pod with an init container
- A pod can have multiple containers running apps
- It could also have one or more init containers that are executed before apps are started
- In the example below the init container just sleeps for 60 seconds and the app container is only started after init container is finished
apiVersion: v1
kind: Pod
metadata:
name: init-container-pod
spec:
containers:
- name: app
image: busybox
command: ['sh', '-c', 'echo The app is running! && sleep 3600']
initContainers:
- name: init
image: busybox
command: ['sh', '-c', 'echo Initializing! && sleep 60']
- check the output that app container is not started before 60 seconds
% kubectl get pods
NAME READY STATUS RESTARTS AGE
init-container-pod 0/1 Init:0/1 0 46s
% kubectl logs init-container-pod
Defaulted container "app" out of: app, init (init)
Error from server (BadRequest): container "app" in pod "init-container-pod" is waiting to start: PodInitializing
- check that app container is started after 60 seconds
% kubectl get pods
NAME READY STATUS RESTARTS AGE
init-container-pod 1/1 Running 0 71s
% kubectl logs init-container-pod -c init
Initializing!
% kubectl logs init-container-pod -c app
The app is running!
Pod with an multiple containers
- A pod can have multiple containers running apps
- It could also have one or more init containers that are executed before apps are started
- In the example below, one container is writing files locally and the other container exposes them to stdout
apiVersion: v1
kind: Pod
metadata:
name: multi-container-pod
spec:
containers:
- name: app
image: busybox
command: ['sh', '-c', 'while true; do echo App is Logging >> /outputlogs/log.log; sleep 10; done']
volumeMounts:
- name: logs
mountPath: /outputlogs
- name: logoutput
image: busybox
command: ['sh', '-c', 'tail -f /inputlogs/log.log']
volumeMounts:
- name: logs
mountPath: /inputlogs
volumes:
- name: logs
emptyDir: {}
- Check that pod contains multiple containers and that logs from app container are sent to stdout using logs container
% kubectl get pod multi-container-pod
NAME READY STATUS RESTARTS AGE
multi-container-pod 2/2 Running 0 15s
% kubectl logs -f multi-container-pod -c logoutput
App is Logging
App is Logging
App is Logging
....
References