Kubernetes in 33 points — Bird’s eye view
This blog is just a very short personal summary of the book Kubernetes in Action. Feel free to skip it if you have read that book
First there were monoliths deployed as a big components and then world moved to microservices (ms). As the companies grew, the number of ms built and deployed were enormous and managing the life cycle of these ms needed a special role and skillset to manage infrastructure. Thats where devops role came in to manage the infra/hardware failure/migration of failure components to healthy ones and so forth. MS deployment was solved with dockerized containers but automation of container’s life cycle and ops task like resource, computation management was still unsolved. Thats where k8s came in to ease out dev and devops life.
What is Kubernetes -K8S?
Kubernetes enables developers to deploy their applications themselves and as often as they want, without requiring any assistance from the operations (ops) team. But Kubernetes doesn’t benefit only developers. It also helps the ops team by automatically monitoring and rescheduling those apps in the event of a hardware failure. The focus for system administrators (sysadmins) shifts from supervising individual apps to mostly supervising and managing Kubernetes and the rest of the infrastructure, while Kubernetes itself takes care of the apps.
Kubernetes abstracts away the hardware infrastructure and exposes your whole data-centre as a single enormous computational resource. It allows you to deploy and run your software components without having to know about the actual servers under-neath. When deploying a multi-component application through Kubernetes, it selects a server for each component, deploys it, and enables it to easily find and communicate with all the other components of your application.
K8S Architecture — Bird’s eye view
2 types of nodes:
Master Node [Control Plane]— consists of following components:
>>keyval store (etcd) to save cluster configs,
>>an API server to query configs etc,
>>Scheduler to schedule the deployments on different pods,
>>Controller Manager to manage failures, replication of components other cluster level functions
Worker Nodes — runs apps with following components:
>>Container runtime like Docker or rkt
>>Kubelet: a component to talk to API server
>>kube-proxy: a LB to balance traffic between app components
Node is a single physical hardware or a single machine. Multiple Pods are deployed on a node
Pods vs containers?
The basic building block in Kubernetes is the pod. Containerised apps run in PODs and when scaling we create more PODs inside which there are containerised apps. A POD is an isolated unit of capacity and resources. Each pod has its unique private IP and hostname. If multiple containers are deployed in a single POD they will share the IP, namespace and network interfaces.
Why need a ReplicationController/ReplicaSet?
Whenever you deploy an app, have it done via Replication controller or ReplicaSet. RC’s responsibility is to track health, node failure etc and scale the pods up and down based on rules defined
K8S Concepts, commands and more:
1. Run a docker image from docker rgistr
docker run <imagename>
2. Create Docker Image with following Dockerfile
docker build -t apurav .
Above command means build an image with name/tag apurav from the current directory (. dot means current directory). Docker will search for Dockerfile in current directory to build image. See below cur dir
ADD app.js /app.js
ENTRYPOINT [ “node”,”app.js” ]
FROM: tells the base image for this app
ADD: adds app code in docker container fie system
ENTRYPOINT: defines command arguments to run on startup of container
3. Check All running containers or inspect
docker inspect <container-name>
>brew install minikube
5. Install kubectl client to interact with k8s cluster
>> minikube kubectl -- get po -A
6. Run an image in k8s cluster using below
kubectl run <any-name> --image=<dockerhub-image-name> --generator=run/v1
!!The above internally creates pods and deploy container in it!!
6. Display all POD details OR a specific POD
kubectl get pods -o wide OR kubectl get pods <pod-name>-o wide
kubectl get pods -o yaml OR kubectl get pods <pod-name>-o yaml
kubectl get pods -o json OR kubectl get pods <pod-name>-o json
kubectl describe pod <pod-name>
7. See cluster dashboard
8. POD, service or any object in k8s can be created via json or YAML file as well. For Pod we created above, you can explore the object definition file in YAML using:
kubectl get po <pod-name> -o yaml
9. To create a POD using YAML file
kubectl create -f <pod-manual.yaml>
10. Check logs of a Pod OR specific container in a POD
kubectl logs <pod-name>
kubectl logs <pod-name> -c <container-name>
11. Sending Request/connect to a specific POD via port forward
kubectl port-forward <pod-name>8888:8080
12. You can add/edit multiple labels to PODS which later help in listing certain Pods only based on label-selectors
Create label>>kubectl label po <pod-name> env=staging
Label Selector>>kubectl get po -l env=staging
13. Node selectors can be used to selector machine where the POD is preferred to be deployed
kubectl get nodes (to fetch node name and other details)
kubectl label node <node-name> gpu=true
Now while creating a POD via YAML file you can define a node selector to find right set of nodes to create our POD
Sample YAML showing labels and node selector
- image: apuravchauhan/kubia
14. You can logically and horizontally segregate all resources by creating namespaces
15. Liveness probes can be configured that help k8s restart app in case of node failure or app crash. Readiness probes can be used to tell k8s when app is ready to serve traffic after startup
16. Creating Pods indirectly via Replication Controller/ReplicationSets yaml file. As see we can define replicas
kubectl create -f rc.yaml
Below is sample rc.yaml
- name: kubia
- containerPort: 8080
17. Scale up replicationcontroller
kubectl scale rc kubia --replicas=10
18. You can run exactly one pod on each node with DaemonSet
19. You can run pods that perform a single completable task using Job Resource
20. As a client you communicate with a group of a specific type of Pods via service resource. Below is an example YAML that can create a service using
kubectl create -f orderloadbalancer.yaml
We are defining a service called orderloadbalancer, which will accept connections on port 80 and route each connection to port 8080 of one of the pods matching the app=orderservice label selector.
- port: 80
21. When exposing multiple service group/ load balancers to external world, you can use something called Ingress Resource to define multiple rules and mappings based on paths
22. Pods can be decoupled from underlying storage technology by mounting different storage volumes in k8s like direct gitrepo, aws, gce, nfs, worker’s node fs etc
23. Special types of volumes exist to store configurations and secrets that can be mounted on worker nodes.
Example of volume mounting yaml
- image: nginx:alpine
- name: html
- name: html
24. To further decouple the Pods from underlying storage, we can use an abstraction called PersistentVolumeClaim
25. For managing production build updates and patch, use Deployment resource to handle rolling updates gracefully. Other deployment strategies are also available. Deployment resources maintains history of deployments and can even rollback the last deployment.
You can also pause, resume deployments and even block rollouts of bad versions using Deployments.
Example deployment yaml to be created:
- image: luksa/kubia:v1
after this you can roll out a new version like:
$ kubectl set image deployment kubia nodejs=luksa/kubia:v2
26. Use Statefulset resource to maintain the app state in case a Pod/container goes down. StatefulSet helps maintain the disk state and other identity details when a new POD is created.
27. Make sure your k8s is secured including API server using proper authentication/ role based access. Same for cluster network
28. Post-start and pre-stop hooks can be registered with Pods
29. LimitRange resources can be used to define computational limits for PODs
- type: Pod
- type: Container