Kubernetes Overview.

Before we start dealing with kubenetes we need a ready docker image. In the below image we've got some javascript files and a Dockerfile. We took all these files feed them to docker and created a image. We can not create any instances or containers of this image.

Enable kubenetes Docker Dashboard > Setting > kubenetes > Enable Kubenetes switch

Kubernetes Terminilogy.

Cluster

A kubenetes cluster is a collection of nodes and a master to manage all of them. This is a entire set of infrastructure that is going to run our code. Whenever we want to use kubernetes we always should have a set of images ready to go. We now deploy this images in the kubernetes cluster. In the below picture the blue box denotes kubernetes clustor.

Node

A node is a virtual machine that runs all the different containers that we throw at our kubenetes cluster. A single cluster might have multiple nodes assigned to it. It might only have one node. In the below picture there are three separate nodes being shown. A node is a virtual machine essentially a computer that is going to run some number of container for us.

Now if you are running kubernetes in your local computer. It is extremely likely you are running only one node by default. It is only once we start deploying to some cloud provider then we are going to have multiple nodes.

Pod

A pod is something that wraps up a container or it can wrap multiple differnet containers together. After it find the instructed image it will create 2 containers (because we asked for 2) and put them on the available node. Each container that is created is hosted withing something called Pod A Pod technically wraps up the container and it also can have multiple containers in it.

# look for the default v1 kubenetes list of objects
apiVersion: v1 
# type of object that we want to create.
kind: Pod 
# options that we provide object that we are going to create
metadata: 
    name: transit

spec: 
    containers: 
        - name: transit-container
          image: reponame/transit:0.0.1

 
kubectl get pods
 

Deployment

In order to manage the pods and containers kubenetes also creates something called a deployment. This deployment is going to be incharge of managing both these pods. If anything ever goes wrong with this pod for eg: crash or stops running. That pods will automatically get recreated by the deployment.

Depoloyment monitors a set of identical pods (Pods that are meant to run the same container inside them). If anything ever happens to that Pod for eg: if it crashes the deployment will automatically restart that pod for us.

Services

In the kubenetes file we also write instruction asking for network accesibility from other pods or container inside of kubenetes clustor. To do so kubenetes create something called service. The service gives us access to running pods or container inside of our cluster.

A kubernete service provide easy to remember URL so that other running pods or container can very easily access another each other. Below is the overall diagram of everything combined

Config File

The config is a YAML file that tells kubenetes about different deployments, pods and services that we want to create. This always resides in project's codebase and commit to gihub. Config files provide a precise definition of what your cluster is running.

In order to create a container out of the image we first need a config file. In this configuration file we are going to write directions for kubernetes eg: create 2 copies of a particular image, allow the copies of image accessible from network

Now with the given instruction kubenetes is going to search for the provided image referenece. First it is going to search image in the docker daemon running withing the local machine. If not available it will search on dockerhub.

Common Kubernetes commands

1. kubectl apply -f [config file name] to create a Pod
2. kubectl get pods List out all the available pods
3. kubectl describe pod [pod name] print out inforamtion about running pod
4. kubectl exec -it [pod_name] [cmd] Execute given comman in a running pod
5. kubectl logs [pod name] print out logs of the given pod
6. kubectl delete pod [pod name] delete a pod.