What is Pod?
- POD is the smallest Deployable Unit
- Containers are not directly deployed in Kubernetes on Worker node but deployed as Pod
- Single Pod can contain one or more Containers.
- Characteristics of containers deployed in one POD:
- Address to each other via localhost
- Share IP Address
- Share environment
- Share Volumes
- Share same fate (live together, die together)
- Scheduled on the same worker node
So to scale up do we add another container within POD?
No. You have add another Pod.
How to run a POD?
Command:
kubectl run <pod-name> --image <docker-image-name>
Example:
kubectl run my-nginx-pod --image nginx
I did some cheating, this command not only does create POD but also create deployment.
How to find all running pods?
Command:
kubectl get pods
Output:
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
my-nginx-pod 1/1 Running 0 34sWhat if I want to check IP address and node on which pod is running?
Command:
kubectl get pods -o wide
Output:
$ kubectl get pods -o wide NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES my-nginx-pod 1/1 Running 0 82s 10.32.0.2 node01 <none> <none>
Ok if you think you are that smart, what if I want to check Pod image being run under Pod?
Well enough, you will keep asking, here is the super command.
kubectl describe pod <pod-name>
This command will give:
- On which node Pod is running
- Name of image being used
- Events related to pods
Output:
$ kubectl describe pod my-nginx-pod
Name: my-nginx-pod
Namespace: default
Node: kubernetes-node-wul5/10.240.0.9
Start Time: Thu, 24 Mar 2016 01:39:49 +0000
Labels: app=nginx,pod-template-hash=1006230814
Annotations: kubernetes.io/created-by={"kind":"SerializedReference","apiVersion":"v1","reference":{"kind":"ReplicaSet","namespace":"default","name":"nginx-deployment-1956810328","uid":"14e607e7-8ba1-11e7-b5cb-fa16" ...
Status: Running
IP: 10.244.0.6
Controllers: ReplicaSet/nginx-deployment-1006230814
Containers:
nginx:
Container ID: docker://90315cc9f513c724e9957a4788d3e625a078de84750f244a40f97ae355eb1149
Image: nginx
Image ID: docker://6f62f48c4e55d700cf3eb1b5e33fa051802986b77b874cc351cce539e5163707
Port: 80/TCP
QoS Tier:
cpu: Guaranteed
memory: Guaranteed
Limits:
cpu: 500m
memory: 128Mi
Requests:
memory: 128Mi
cpu: 500m
State: Running
Started: Thu, 24 Mar 2016 01:39:51 +0000
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-5kdvl (ro)
Conditions:
Type Status
Initialized True
Ready True
PodScheduled True
Volumes:
default-token-4bcbi:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-4bcbi
Optional: false
QoS Class: Guaranteed
Node-Selectors: <none>
Tolerations: <none>
Events:
FirstSeen LastSeen Count From SubobjectPath Type Reason Message
--------- -------- ----- ---- ------------- -------- ------ -------
54s 54s 1 {default-scheduler } Normal Scheduled Successfully assigned nginx-deployment-1006230814-6winp to kubernetes-node-wul5
54s 54s 1 {kubelet kubernetes-node-wul5} spec.containers{nginx} Normal Pulling pulling image "nginx"
53s 53s 1 {kubelet kubernetes-node-wul5} spec.containers{nginx} Normal Pulled Successfully pulled image "nginx"
53s 53s 1 {kubelet kubernetes-node-wul5} spec.containers{nginx} Normal Created Created container with docker id 90315cc9f513
53s 53s 1 {kubelet kubernetes-node-wul5} spec.containers{nginx} Normal Started Started container with docker id 90315cc9f513
How to create a pod using yaml based configuration file?
Here is an example: Lets create a pod-def.yaml file containing pod definition:
apiVersion: v1 kind: Pod metadata: name: my-nginx-pod spec: containers: - name: my-nginx-container image: nginx
Note: All apiVersion, kind, metadata and spec are required fields.
Command:
kubectl create -f <pod-definition.yaml>
Output:
$ kubectl create -f pod-def.yaml pod my-nginx-pod created
Now also check if you pod is created:
$ kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-pod 1/1 Running 0 34s
How to delete a pod which was created using yaml file?
Command:
kubectl delete -f <pod-definition.yaml>
How to delete a pod using pod name ?
Command:
kubectl delete pod <pod-name>
How to delete multiple pods in single command ?
Command:
kubectl delete pod <pod1> <pod2> <pod3>
Can I edit a running Pod?
Well yes, but not all parameters. Use the below command:
kubectl edit pod <pod-name>
What you can edit:
- spec.containers[*].image
- spec.initContainers[*].image
- spec.activeDeadlineSeconds
- spec.tolerations
You cannot edit the environment variables, service accounts, resource limits (we will cover these items later) of a running pod.
Hmmm, what if I edit the property you are saying is un-editable?
Well you will get an error 🙂
$ kubectl edit pod my-nginx-pod
[And I am assuming you editing a property like resource limits now and tried saving it]
Now you will see an error on screen:
error: pods “my-ngix-pod” is invalid. A copy of your changes has been stored on “/tmp/kubectl-edit-xxyq.yaml” error: Edit cancelled, no valid changes were saved
What if I still want to edit this property, is there no way?
Well, you can delete the existing pod by running the command:
kubectl delete pod my-nginx-pod
Then create a new pod with your changes using the temporary file, created above.
kubectl create -f /tmp/kubectl-edit-xxyq.yaml
Does it mean I have to get the above error first to get the YAML file from Pod? Is there a way to create a YAML definition file from the running pod?
Yes, you are in luck here buddy, you can run below command to have definition file from pod:
kubectl get pod my-ngixn-pod -o yaml > my-new-pod.yaml
Now you know the drill, what you need to do
Step 1: Edit whatever parameters you want to edit:
vi my-new-pod.yaml
Step 2: Delete the pod
kubectl delete pod webapp
Step 3: Re-create the pod from definition file
kubectl create -f my-new-pod.yaml
What are different Pod life-cycle phases?
The phase of a Pod is a simple, high-level summary of where the Pod is in its lifecycle.
- Pending: The Pod has been accepted by the Kubernetes system, but one or more of the Container images has not been created. This includes time before being scheduled as well as time spent downloading images over the network, which could take a while.
- Running: The Pod has been bound to a node, and all of the Containers have been created. At least one Container is still running, or is in the process of starting or restarting.
- Succeeded: All Containers in the Pod have terminated in success, and will not be restarted.
- Failed: All Containers in the Pod have terminated, and at least one Container has terminated in failure. That is, the Container either exited with non-zero status or was terminated by the system.
- Unknown: For some reason the state of the Pod could not be obtained, typically due to an error in communicating with the host of the Pod.
How to check pod’s logs ?
Command:
kubectl logs <pod-name>
How to get logs of the specific container running inside pod ?
Command:
kubectl logs <pod-name> -c <container-name>
Can I check pod logs even when pod is deleted?
NO.
You can only retrieve container logs of pods that are still alive, but when the pod is deleted, its logs are also deleted.
Can Pod which consists of multiple containers can have containers spawned of two different worker nodes?
NO
Why each container should have one running process only instead of multiple?
Containers provide features like, restarting containers when container crashes which happen when process inside it crashes, and logs are sent to standard output which can be read from there or managed further.
But consider if you are running multiple processes inside a container, then how will you manage that which process is crashed and how to restart that specific process. Also in terms of logs, now you have logs of both the containers going on standard output, and you have find your ways to figure out which log belongs to which process.
Hence easier to have container per process.
Why do we need Pods?
Since in the case of containers you can’t have multiple processes in a single container, hence there is a need for abstraction which can contain multiple containers together which needs to interact with each other, share IP addresses and environment. Basically containers which need to run closely.
Else consider container as the basic unit in Kubernetes, and Kubernetes will start scheduling containers which have to run closely and very much dependent upon each other for existence on different worker nodes. Even then these containers will require configuration for interacting with each other.
Containers inside a pod run under the same Network namespace, hence share the same IP address (hostname) and port space. Containers in a pod have the same loopback network interface, hence these containers can interact with each other through localhost.
Can containers running in the same pod share the file system also?
NO
Container’s file system comes from the container image, and the filesystem of each container is fully isolated from other containers. Still, in Kubernetes there is a feature to share file directories using Volume, which we will read later.
Should containers running inside Pod use the same Port binding?
NO
Since containers running inside Pod share the same IP address and ports space, this means that if both containers will try to bind to the same port number, they will have a port conflict.
Can Pod interact with each other inside the cluster?
YES
All pods in the Kubernetes cluster are given IP addresses under the same network address space, this means that no NAT (Network Address Translation) gateway is required.
It doesn’t matter if two pods are scheduled on the same or different worker nodes, Pods can interact with easily using each other IP address.
How to specify container port on Pod definition?
Here is an example of the same:
apiVersion: v1
kind: Pod
metadata:
name: my-pod
spec:
containers:
- image: <docker-image-to-pull>
name: my-docker-container
ports:
- containerPort: 8080
protocol: TCPNote: Providing port information in Pod definition is just for information, even if you will remove it, it will have no impact.
0 Comments