Multi-Container Pods
Use Cases for Multi-Container Pods
Why does Kubernetes allow more than one container in a Pod
- Containers in a Pod runs on a “logical host”: they use the same network namespace (same IP address and port space), they can use shared volumes
- using several containers for an application is simpler to use, more transparent, and allows decoupling software dependencies
Use Cases for Multi-Container Pods
The primary purpose of a multi-container Pod is to support co-located, co-managed helper processes for a main program
Sidecar containers: “help” the main container. For example, log or data change watchers, monitoring adapters, and so on. A log watcher, for example, can be built once by a different team and reused across different applications Another example of a sidecar container is a file or data loader that generates data for the main container.
Communication Between Containers in a Pod
Shared volumes: you can use a shared Kubernetes Volume as a simple and efficient way to share data between containers in a Pod. Volumes enables data to survive container restarts. It has the same lifetime as a Pod. it is sufficient to use a directory on the host that is shared with all containers within a Pod
- A standard use case for a multi-container Pod with shared Volume is when one container writes to the shared directory (logs or other files), and the other container reads from the shared directory
- The second container uses Debian image and has the shared volume mounted to the directory /html. The second container every second adds current date and time and into index.html that is located in the shared volume.
- Nginx servers reads this file and transfers it to the user for each HTTP request to the web server.
apiVersion: v1
kind: Pod
metadata:
name: mc1
spec:
volumes:
- name: html
emptyDir: {}
containers:
- name: 1st
image: nginx
volumeMounts:
- name: html
mountPath: /usr/share/nginx/html
- name: 2nd
image: debian
volumeMounts:
- name: html
mountPath: /html
command: ["/bin/sh", "-c"]
args:
- while true; do
date >> /html/index.html;
sleep 1;
done
kubectl apply
➜ k8s101 git:(main) ✗ kubectl apply -f mc1.yaml
pod/mc1 created
exec into mc1 pod 1st container
➜ k8s101 git:(main) ✗ kubectl exec mc1 -c 1st -- /bin/cat /usr/share/nginx/html/index.html
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exec into mc1 pod 3nd container
k8s101 git:(main) ✗ kubectl exec mc1 -c 2nd -i -t -- bash -il
root@mc1:/# ls
bin boot dev etc home html lib media mnt opt proc root run sbin srv sys tmp usr var
root@mc1:/# cd html
root@mc1:/html# ls
index.html
root@mc1:/html#
Kubernetes has three Object Types you should know about:
Pods - runs one or more closely related containers
Services - sets up networking in a Kubernetes cluster
Deployment - Maintains a set of identical pods, ensuring that they have the correct config and that the right number of them exist.
Pods:
- Runs a single set of containers
- Good for one-off dev purposes
- Rarely used directly in production
Deployment:
- Runs a set of identical pods
- Monitors the state of each pod, updating as necessary
- Good for dev
- Good for production
Pod templates :
Controllers for workload resources create Pods from a pod template and manage those Pods on your behalf.
-PodTemplates are specifications for creating Pods, and are included in workload resources such as Deployments, Jobs, and DaemonSets.
k8sworkshop git:(main) ✗ kubectl get all
NAME READY STATUS RESTARTS AGE
pod/command3 0/1 CrashLoopBackOff 65 (4m59s ago) 20h
pod/mc1 2/2 Running 0 77m
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
service/kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 115d
Last updated 29 Apr 2024, 11:44 +0530 .