Kubernetes

• Open source container orchestration tool
• Developed by Google
• Helps manage containerised applications in different deployment environments.

Need for container orchestration tool

• Trend from Monolith to Microservices
• Increased usage of containers
• Demand for a proper way to manage containers

Features that orchestration tools offer

• High Availability or no downtime
• Scalability or high performance
• Disaster recovery - backup and restore

Kubernetes Architecture

A cluster is made of

• Control Plane - At least 1 Master Node
  • Important K8s processes run to manage
    • API Server - Entrypoint to K8s cluster
      • UI - Kubernetes Dashboard
      • Backend - Scripts to interact
    • Controller Manager
      • Keeps track of whats happening in the cluster
    • Scheduler
      • Ensures Pods placement
    • etcd
      • Kubernetes backing store - configuration and state data of every node
• Multiple Worker Nodes
  • Each node has a kubelet running on it
  • Higher workload, much bigger and more resources
• Virtual Network
  • Creates one unified network

Main Kubernetes Components

• Pod
• Service
• Ingress
• ConfigMap
• Secret
• Deployment
• StatefulSet
• DaemonSet

Pod

• Smallest unit in Kubernetes
• Abstraction over container - So that Kubernetes can interact consistently
  • Docker has container, Pod is an abstraction over it.
• Usually 1 application per Pod
  • Each Pod gets its own IP address
  • Pods are ephemeral, so new IP address on re-creation
• Example, a Node contains
  • App Pod - Pod to run Web App
  • DB Pod - Pod to run DB

Service

• Permanent IP address attached that will interact with Pod
• Lifecycle of Pod and Service - not connected
• App should be accessible through browser
  • External Service - So outside world could access
  • Internal Service - For example, DB connection

Ingress

We don’t want - IP address for user to access. We want something like “my-app.com”

Ingress helps in doing it.

ConfigMap

• Why ConfigMap?
  • Database URL usually in built application. If URL changes, re-build and push it to repo. Pull it in your Pod.
• External configuration of your application
• ConfigMap is for non-confidential data only - Not for Username and Passwords

Secret

• Used to store secret data
• Reference Secret in Deployment/Pod
  • Can be set as environment variable
• Default - base64. Anyone who has access to Kubernetes cluster can view password so 3rd party provider is encouraged to be used.

echo -n mongouser | base64

Volume

• Why Volume?
  • Data Storage - DB Pod that our app uses. If pod gets restarted, data will be gone
• Volume - Attaches physical storage to Pods
  • Could be local or remote outside of K8s container
• Kubernetes doesn’t manage data persistence

Deployment

• Blueprint for pods, for stateless apps
• You create Deployments
  • Specify how many replicas of Pods, scaling
• Abstraction of Pods
• DB can’t be replicated via Deployment

Stateful Set

• For stateful apps like - MySQL, Elastic, MongoDB
• It’s not easy

Example

(base) % kubectl apply -f mongo-config.yaml 
configmap/mongo-config created

(base) % kubectl get pod                   
No resources found in default namespace.

(base) % kubectl apply -f mongo-secret.yaml
secret/mongo-secret created

(base) % kubectl apply -f mongo.yaml       
service/mongo-service created

(base) % kubectl apply -f webapp.yaml
deployment.apps/webapp created
service/webapp-service created

(base) % kubectl get all
NAME                          READY   STATUS             RESTARTS   AGE
pod/mongo-85d45f7888-b9kd8    1/1     Running            0          102s
pod/webapp-6849d7476d-dwf9m   0/1     ImagePullBackOff   0          4m48s

NAME                     TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)          AGE
service/kubernetes       ClusterIP   10.96.0.1       <none>        443/TCP          40m
service/mongo-service    ClusterIP   10.96.190.173   <none>        27017/TCP        5m3s
service/webapp-service   NodePort    10.101.108.53   <none>        3000:30100/TCP   4m48s

NAME                     READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/mongo    1/1     1            1           102s
deployment.apps/webapp   0/1     1            0           4m48s

NAME                                DESIRED   CURRENT   READY   AGE
replicaset.apps/mongo-85d45f7888    1         1         1       102s
replicaset.apps/webapp-6849d7476d   1         1         0       4m48s

Kubernetes Architecture

• Master Node (Manages Cluster)
  • API Servier (kube-apiserver)
  • Distributed Database (etcd)
  • Scheduler (kube-scheduler)
  • Controller Manager (kube-controller-manager)
• Slave Node (Run Your Applications)
  • Node Agent (kubelet)
  • Networking Component (kube-proxy)
  • Container Runtime (CRI - docker, rkt, etc)
  • PODS (Multiple pods running containers)