Pods
- Pods are the central, most important, concept in Kubernetes.
- Pod represents the basic building block in Kubernetes and a place where containers are run.
- When using Kubernetes you always deploy and operate on a pod of containers instead of deploying containers individually.
- Pods can run more than one container, but it is good practice running one container per pod.
- We need to remember that Kubernetes manages Pods not containers.
- So Kubernetes cannot scale individual containers, instead, it scales whole pods.
Points when running multiple container
- Pods with multiple containers, are always run on a single worker node — it never spans multiple worker nodes.
- A pod of containers allows you to run closely related processes together and provide them with (almost) the same environment as if they were all running in a single container, while keeping them somewhat isolated.
- One thing to stress here is that because containers in a pod run in the same Network namespace, they share the same IP address and port space.
- This means processes running in containers of the same pod need to take care not to bind to the same port numbers or they’ll run into port conflicts.
Pods Configuration
- I am using this kub-pod-initial branch for this chapter.
- We can create a Pods using command line but let's do it using
yamlfile.
apiVersion: v1 # 1
kind: Pod # 2
metadata: # 3
name: v1-web # 4
spec: # 5
containers: # 6
- image: amantuladhar/docker-kubernetes:v1-web # 7
name: v1-web # 8
ports: # 9
- containerPort: 8080
#1apiVersionconforms which version to Kubernetes API to use.#2kinddescribes what kind of Kubernetes Resources we want to define.#3metadatais a place where you put information about the pods#4nameSetting name for pod.#5speccontains a pods specification. Specification like container template, volumes etc.#6containerslist of container you want Pods to have#7imagedefines which image to use.#8namename of container.#9portsList of port container listens to.- Specifying ports in the pod definition is purely informational.
- Omitting them has no effect on whether clients can connect to the pod through the port.
- But it makes sense to define the ports explicitly so that everyone using your cluster can quickly see what ports each pod exposes.
Creating pods
- To create a pod we have a simple command.
kubectl create -f pods.xml
kubectl create -f pods.yaml
pod/v1-web created
List Pods
- To list the running pods, we can use
kubectl get podscommand.
➜ kubectl get pods
NAME READY STATUS RESTARTS AGE
v1-web 1/1 Running 0 21m
View logs
- Viewing logs is simple too
kubectl logs @pod_name kubectl logs v1-web- If your pod includes multiple containers, you have to explicitly specify the container name by including the
-c <container name>.
Access your app
- We ran our Pod, if you check the status it states it is running as well.
- But how do we access our app?
- Note: Pods are not meant to be accessed directly. We create some higher labels resources that abstracts these detail.
- For debugging purposes, Kubernetes lets us access our Pods directly using port-forwarding technique.
kubectl port-forwarding @pod_name @host_port:@container_pod
➜ kubectl port-forward v1-web 9090:8080
Forwarding from 127.0.0.1:9090 -> 8080
Forwarding from [::1]:9090 -> 8080
- You need to keep this command running if you want to access your pod.
- If you visit
localhost:9090/test, you will be able to access you app.
➜ http localhost:9090/test
HTTP/1.1 200
Content-Type: application/json;charset=UTF-8
Date: Sun, 17 Feb 2019 20:41:22 GMT
Transfer-Encoding: chunked
{
"app.version": "v1-web",
"host.address": "172.17.0.5",
"message": "Hello Fellas!!!"
}
Deleting the pods
- Deleting the pods is simple too.
kubectl delete pods @pod_name
➜ kubectl delete pods v1-web
pod "v1-web" deleted
Pod Labels
- Labels are simply a tag given to pods which may help us identify the specific pods.
- As our application grows number of pods increases, we need to categorize them so that we can manage them efficiently.
- Labels are a simple, yet incredibly powerful feature in Kubernetes.
- Labels can be added to pods, but also for all other resources Kubernetes has.
- A label is an arbitrary key-value pair you attach to a resource.
- It can be utilized when selecting resources using label selectors.
- Resources are filtered based on whether they include the label specified in the selector.
- A resource can have more than one label, as long as the keys of those labels are unique within that resource.
- You usually attach labels to resources when you create them, but you can also add additional labels or even modify the values of existing labels later without having to recreate the resource.
- Let's see how we can add them in
yaml - Labels are added in
metadatasection usinglabelsproperty.
apiVersion: v1
kind: Pod
metadata:
name: v1-web
labels:
version: v1
env: dev
spec:
containers:
- image: amantuladhar/docker-kubernetes:v1-web
name: v1-web
ports:
- containerPort: 8080
- You can create a resource using
kubectl create -f pods.yaml.
See Pods labels
- You can see pods labels in multiple ways
--show-labels
- If you want to see all labels use
--show-labelsoption when usingkubectl get pods. kubectl get pods --show-labels.
➜ kubectl get pods --show-labels
NAME READY STATUS RESTARTS AGE LABELS
v1-web 1/1 Running 0 2m38s env=dev,version=v1
using -L option
- While
--show-lablesoptions works fine. - What if we have lots of labels. Not that I am saying you should add many labels.
- We can use
-Loptions with the list of labels you want to see. -L label_key1,label_key2
➜ kubectl get pods -L version,env
NAME READY STATUS RESTARTS AGE VERSION ENV
v1-web 1/1 Running 0 6m2s v1 dev
Adding new label
- If you want to change/add the labels for pads you can do it by stopping the pod and creating again.
- But let's see how we can add new labels to running container
- We use
kubectl label @resource @resource_name @label_key=@label_value - For us
@resourceis pods.
➜ kubectl label pods v1-web value=new
pod/v1-web labeled
- List the pods with labels and see the new value.
➜ kubectl get pods --show-labels
NAME READY STATUS RESTARTS AGE LABELS
v1-web 1/1 Running 0 14m env=prod,value=new,version=v1
Updating labels
- If we want to override the existing label, we can use the same command like above but we need to append
--overwriteoption as well. - We use
kubectl label @resource @resource_name @label_key=@label_value --overwrite - Let's change
env to devagain.
➜ kubectl label pods v1-web env=dev --overwrite
pod/v1-web labeled
- List the labels
➜ kubectl get pods --show-labels
NAME READY STATUS RESTARTS AGE LABELS
v1-web 1/1 Running 0 16m env=dev,value=new,version=v1
List the pods using labels
- Listing pods using a label selector.
- We use
-lto add label criteria when listing the pods.
Selecting pods with specific label and specific value
➜ kubectl get pods -l env=dev
NAME READY STATUS RESTARTS AGE
v1-web 1/1 Running 0 51m
Selecting pods that have specific label
- We can also select pods that have specific label, ignoring value it has.
➜ kubectl get pods -l env
NAME READY STATUS RESTARTS AGE
v1-web 1/1 Running 0 53m
Selecting pods that don't have specific label
➜ kubectl get pods -l '!env'
No resources found.
Deleting pods using label selectors
- We already know how to delete a specific pod with name.
- If we want to mass delete pods, we can use label selector criteria
-loption.
➜ kubectl delete pods -l env
pod "v1-web" deleted
Deleting all pods at once
- We can delete all pods at once using
kubectl delete pods --all
Deleting all resource
- We haven't learned about other type of resources, but if we want to delete all resources at once we can use.
kubectl delete all --all.