cluster is collection of node(s)
each node can have a set of running container(s)
You are correct.
- set of tightly coupled container(s) itself can be grouped together to form a pod (despite of the node in which the container is running).
All containers belonging to a pod run on the same node.
My question, rather confusion is since we have already containers
running in different nodes, why do we need additional node to run a
pod on ?
It's not the pod that actually runs. The only things that actually run on your nodes are containers. Pod is just a logical grouping of containers and is the basic unit in kubernetes to create a container. (Docker container logo is a whale, a group of whales is called a pod if you want a parallel to remember this). So if the containers that belong to the pod are running, the pod is termed as running.
In the following pod specification, nginx-container and debian-container containers belong to the pod named two-containers. When you create this pod object, kube-scheduler will select a node to run this pod (i.e., to run the two containers) and assigns a node to the pod. The kubelet running on that node then gets notified and starts the two containers on the node. Since the two containers belong to same pod, they are run in same network namespace.
apiVersion: v1
kind: Pod
metadata:
name: two-containers
spec:
restartPolicy: Never
volumes:
- name: shared-data
emptyDir: {}
containers:
- name: nginx-container
image: nginx
volumeMounts:
- name: shared-data
mountPath: /usr/share/nginx/html
- name: debian-container
image: debian
volumeMounts:
- name: shared-data
mountPath: /pod-data
command: ["/bin/sh"]
args: ["-c", "echo Hello from the debian container > /pod-data/index.html"]
