Deploy services on K8s

Architecture

Master node

Four processes run on every master node

• API Server: it’s a cluster gateway and gatekeeper for authentication (validate requests)
• Scheduler: schedule new pod, where to put the pod
• Controller manager: detect cluster states, restart pods if the pods crash
• etcd: cluster brain, all cluster changes stored in key-value format

Worker nodes

The worker nodes do the actual work meaning we will deploy services in these nodes, each node can have multiple pods (containers).

Thress processes must be installed on the node

• kubelet: communicate with master node
• kube poxy: internal communications
• pod: smallest unit of k8s

Components

Pod

Pods are the smallest computing units in Kubernetes. It can be created by using workload resources such as Deployment or Job. It has the following features:

• Usually only 1 application per pod
• Each pod has a virtual IP address (for internal interaction)
• Does not save any data (stateless)

Example for the Deployment file

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

Service

Service is an abstraction that defines a logical set of Pods and a policy by which to access them. It could be considered as an abstract way to expose an application running on a set of pods as a network service. The pods can not communicate by virtual IP because the IP changes every time when the pod restarts.

Example for the Service file

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: nginx
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376  # port exposed by the pod

Ingress

Ingress exposes HTTP and HTTPS routes from outside the cluster to services within the cluster. Traffic routing is controlled by rules defined on the Ingress resource.

Example for the Ingress file

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: minimal-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
spec:
  rules:
  - http:
      paths:
      - path: /yourpath
        pathType: Prefix
        backend:
          service:
            name: my-service
            port:
              number: 80