Namespace

Kubernetes supports multiple virtual clusters backed by the same physical cluster. These virtual clusters are called namespaces.

Namespaces are intended for use in environments with many users spread across multiple teams, or projects. For clusters with a few to tens of users, you should not need to create or think about namespaces at all. Start using namespaces when you need the features they provide.

Namespaces are a way to divide cluster resources between multiple users (via resource quota).

Kubernetes already provides 4 namespaces by default, being:

• default
  • Holds the resources created by the user (if no other namespace is specified in the resource creation)
• kube-node-lease
  • Holds info about heartbeats of nodes
  • Each node has an associated lease object in the namespace
  • Determines the availability of a node
• kube-public
  • Contains public accessible date
  • Has a configMap with cluster information (kubectl cluster-info)
• kube-system
  • Should not be touched manually
  • Contains system processes

Cluster

A set of Nodes that run containerized applications managed by Kubernetes. For this example, and in most common Kubernetes deployments, nodes in the cluster are not part of the public internet.

Node

Server

Pod

A pod is the smallest execution unit in Kubernetes. A pod encapsulates one or more applications. Pods are ephemeral by nature, if a pod (or the node it executes on) fails, Kubernetes can automatically create a new replica of that pod to continue operations. Pods include one or more containers (such as Docker containers).

Usually we only use a container per each pod.

K8s manages an internal network and it assigns an address to each pod.

When a pod is recreated k8s assigns it a new IP address (that's why we should use Service and Ingress).

Example

apiVersion: v1
kind: Pod
metadata:
  name: static-web
spec:
  containers:
    - name: nginx
      image: nginx:1.7.9
      ports:
        - name: nginx
          containerPort: 80
          protocol: TCP

ReplicaSet

A ReplicaSet's purpose is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: guestbook-replicaset
  labels:
    app: guestbook
spec:
  # modify replicas according to your case
  replicas: 3
  selector:
    matchLabels:
      app: guestbook
  template: #this is the POD definition template
    metadata:
      labels:
        app: guestbook
    spec:
      containers:
      - name: php-redis
        image: gcr.io/google_samples/gb-frontend:v3