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Pods

Pod's abbreviation is po. (You can type po or pod or pods in your command.)

A pod is a group of co-located containers (they run in one worker node).

Each pod can communicate to one another using the pod's IP.

Relationship of nodes, pods, and containers. Notice that each pod has their own IP (internal to the cluster unless you expose them).
Relationship of nodes, pods, and containers. Notice that each pod has their own IP (internal to the cluster unless you expose them). (Source: Lukša)

Sharing of Linux namespaces

The containers in a pod run in the same Linux namespace(s), i.e.,

  • They share the same network and UTS namespaces — They share the same hostname and network interfaces (i.e., they share the same IP and port space). They can communicate to each other using the loopback network interface (localhost). Also, be careful not to have different processes bind to the same port.
  • They share the same IPC namespace — They can communicate through IPC.
  • BUT they don't share the same PID namespace by default — unless you enable this, each container will only see their own processes when you run ps aux inside them

Pod descriptor

A sample pod descriptor looks like the following:

# uses the v1 version of the Kubernetes API
apiVersion: v1

# it's a pod
kind: Pod

metadata:
  # name of the pod
  name: kubia-manual
  # key-value pair for organization
  # otherwise your cluster will look like a mess
  label:
    appName: kubia-app
    version: 1.0.0]

spec:
  containers:
    - # the image used
      image: luksa/kubia
      # name of the pod
      name: kubia
      # use local image (don't pull from registry)
      imagePullPolicy: Never
      # for documentation only (listing ports are optional)
      ports:
        - containerPort: 8080
          protocol: TCP

Working with pods

Creating a new pod

Send the pod descriptor to the Kubernetes API server:

# If you create a descriptor named kubia-manual.yaml:
kubectl create -f kubia-manual.yaml

Stopping and deleting a pod

You can also use -l <selector> to filter by selectors:

kubectl delete po kubia-manual

Stopping and deleting all pods

Delete all pod in the current namespace as follows:

kubectl delete po --all

If you delete the namespace, all the pods running in there will also be deleted:

# kubectl delete namespace <namespace>
kubectl delete namespace custom-namespace

Get a shell to a pod's container

If the pod only has one container:

# kubectl exec <podName> -- <command>
kubectl exec kubia-7nog1 -- curl -s http://10.111.249.153

If the pod has multiple containers, select it using -c <containerName>:

# kubectl exec <podName> -c -- <command>
# The "-it" flag basically makes it interactive
# (*) --container, -c <containerName>: the container name to use
# (*) --stdin, -i: keep stdin open on the container in the pod
# (*) --tty, -t: allocate a TTY for the container in the pod
kubectl exec -it my-pod -c main-app -- /bin/bash

References