- The Master is responsible for managing the cluster. The master coordinates all activities in your cluster, such as scheduling applications, maintaining applications' desired state, scaling applications, and rolling out new updates.
- master is a collection of three processes that run on a single node in your cluster.
- kube-apiserver.
- kube-controller-manager
- kube-scheduler.
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A node is a VM or a physical computer that serves as a worker machine in a Kubernetes cluster. Each node has a Kubelet, which is an agent for managing the node and communicating with the Kubernetes master.
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Each individual node runs two processes:
- kubelet : which communicates with the master.
- kube-proxy : a network proxy which reflects kubernetes networking service on each node.
- A Pod is the basic building block of Kubernetes–the smallest and simplest unit in the Kubernetes object model that you create or deploy.
- Like in Docker, container is the smallest unit, in k8s Pod is the smallest unit.
- Pod is a wrapper around container in k8s.
- You can run single container or multi-container inside a Pod
- Example: if you have static web-app running inside a container in docker environment, in k8s that single container will run inside Pod.
- Let's take a case where your application is made of multiple container ( client, server ) client in one container and server in different container than these two container can run inside your Pod and can communicate with each other.
kubectl create -f pod.yml
- Also knows as rc/rcs
- A ReplicationController ensures that a specified number of pod “replicas” are running at any one time.
- In the example here replicaController.yml, We tell rc what is the Pod label and how many Pods we want with that label.
- If there are sufficient Pods present that it will not create any Pod but if the desired number of Pods are not available than it will create more Pods. --If pod doesn't exist it will create as per the spec in yml or json file
kubectl create -f replicaController.yml
- A Deployment provides declarative updates for Pods and ReplicaSets (the next-generation ReplicationController).
- Replica Controller is old way to manage the desired state of the pod.
- With Deployment you can manage the desired state of the pod, deployment is not supported in apiVersion: v1 but use apiVersion: apps/v1beta1 for deployment.
- Deployment is more about rolling updates and rollback.
kubectl create -f deploy.yml --validate=false
kubectl apply -f deploy.yml --record
Service gets single IP,DNS,Port in kubernetes that never changes, supporting pods to a service keeps changing (goes-down and comes-up with new ip).
- A Kubernetes Service is an abstraction which defines a logical set of Pods and a policy by which to access them from outside world.
- Let say there is a pod running your web application, to expose this web application you need services.
- It gives you a way to access your Pods, load balancing among your Pods.
- It hides how it manages the Pods internally.
kubectl expose deployment/hello-world --name=hello-world --target-port=80 --type=NodePort
kubectl create -f Service.yml
kubectl describe service static-site-svc
- Pods are the basic unit which actually have your application up and running.
- Deployments are more about updating the app and managing the app (like : how many replica you want and what Pod with label to choose for running the application)
- Service is all about exposing your application to outside world.
- To Start with Kubernests follow the following instruction
- Install minikube
- Install kubectl
- Put these in your Path.
- start minikube
minikube start
- There are two ways for creating, updating - pods, deployment, replica-controller, service, either CLI or you can define in yaml/json file.
- For More info check
- Basic structure of yaml file
apiVersion: v1
Kind: Service, Pod, Deployment, ReplicationController
metadata:
name: <name-of-the-service>
spec:
kubectl.exe run static-site --image=docker.io/seqvence/static-site:latest --port=80
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This will create the deployment and one Pod for this deployment
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Check your deployment
kubectl.exe get deployments
O/p
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
static-site 1 1 1 1 24s
- Check Pod associated with this deployment
kubectl.exe get pods
Output:
NAME READY STATUS RESTARTS AGE
static-site-3755874646-1nvz2 1/1 Running 0 2m
- Try deleting the Pod associated with this deployment
kubectl.exe delete pod static-site-3755874646-1nvz2
Output:
pod "static-site-3755874646-1nvz2" deleted
- Now again check the Pods -- Your current pod must be deleted or in terminating mode and new pod will be created to support the deployment.
kubectl.exe get pods
output:
NAME READY STATUS RESTARTS AGE
static-site-3755874646-1nvz2 1/1 Terminating 0 2m
static-site-3755874646-x6mk1 0/1 ContainerCreating 0 4s
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To expose this application to outside world you need to create service.
-
Create service
kubectl expose deployment/static-site --type="NodePort" --port 80
output:
service "static-site" exposed
- Check your service
kubectl.exe get service
output:
NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
static-site 10.0.0.31 <nodes> 80:30634/TCP 5s
- Access your service from browser
From the get service you can see the expose port here is 30634, it can be anything, it is generated dynamically. We can also explicitly tell on which Port we want to expose. Now you should be able to acces the service from your ip:exposed-port Alternatively you can use following command to check the URL
minikube service static-site --url
- Update Image
- You can update the image make sure it should be a valid image and tag name should be correct
./kubectl set image deployments/static-site static-site=seqvence/static-site:latest
- Scale App
- Scale app to two replicas now application will be served from two Pods
./kubectl.exe scale deployment static-site --replicas 2