Core Concepts

[TOC]

Recap

Nodes

• node is a machine, physical or virtual.
• for availability we need more than one node.
• a set of nodes is a cluster
• every cluster needs a master node (responsible for orchestration) and worker nodes (responsible for actual work)

Components

Every kubernetes cluster has the following 6 components:

• API Server
• etcd
• Scheduler
• Controller
• Container Runtime
• kubelet

Brief explanation:

• API Server: acts as a frontend for kubernetes (when you use the kubectl command, you're interacting with the API Server)
• etcd: it's a key-value store used to store the data needed to manage the kubernetes cluster
• Scheduler: responsible to distribute work across multiple nodes.
• Controller: responsible to respond when containers/endpoints go down
• Container Runtime: underlying software responsible to run containers.
• Kubelet: an agent running on each node in the cluster, responsible to make sure the containers are running as expected.

kubectl

Used to interact with kubernetes

examples:

kubectl get-info

kubectl get nodes

Recap - Pods

A Pod is the smalles object that you can create in kubernetes.

A Pod can have one or more containers.

It's recommended to group in the same pod only containers that need to scale together.

Very minimum Pod yaml:

# example with nginx
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: container-nginx
    image: nginx:1.23

Creating a Pod from CLI:

# example with nginx
kubectl run nginx --image=nginx:1.23

Recap - YAML in Kubernetes

Always present 4 top-level keys in a k8s yaml file:

apiVersion: 
kind: 
metadata: 
spec: 

Pods labs

https://kodekloud.com/topic/pods-4/

Recap - ReplicaSets

Replication Controller

The Replication Controller is responsible to respawn a new pod when the current one fails.

It can also scale the numbers of pods based on the workload, and spawn new pods on different nodes in order to distribute the workload.

Replication Controller != ReplicaSet

Replication Controller is a technology that is being replaced by ReplicaSet

yaml of a ReplicationController:

apiVersion: v1

kind: ReplicationController

metadta:
  name: myapp-rc
  labels:
    app: myapp
    type: front-end
    
spec:
  template:
    metadata:
      name: myapp-pod
      labels:
        app: myapp
        type: front-end
    spec:
      containers:
      - name: nginx-containers
        image: nginx
        
  replicas: 3
  
  # apparently there's no 'selector'

yaml of a ReplicaSet

apiVersion: apps/v1

kind: ReplicaSet

metadta:
  name: myapp-replicaset
  labels:
    app: myapp
    type: front-end
    
spec:
  template:
    metadata:
      name: myapp-pod
      labels:
        app: myapp
        type: front-end
    spec:
      containers:
      - name: nginx-containers
        image: nginx
        
  replicas: 3
  
  # the difference is that a ReplicaSet definition is able
  # to manage replication of pods not defined in this
  # file's spec.template
  selector:
    matchLabels:
      type: front-end

Scale

Directly from command line:

# already running ReplicaSet
kubectl scale --replicas 6 replicaset myapp-replicaset

# overwriting the value in the definition file
kubectl scale --replicas 6 -f replicaset-definition.yaml

It's recommended to change the YAML file, in order to keep the change for the future.

Recap - Deployments

Features

1. deploy pods
2. easily upgrade
3. rolling updates
4. rollback
5. pause to make some changes (??)
6. resume after changes (??)

Definition

same as ReplicaSet, just replace kind: ReplicaSet with kind: Deployment.

Namespaces

https://www.udemy.com/course/certified-kubernetes-application-developer/learn/lecture/12350560#overview

In the pod's definition we can set the namespace in the metadata.namespace key.

namespace definition file

apiVersion: v1
kind: Namespace
metadata:
  name: dev

For getting info for a namespace different from the default one:

kubectl get pods --namespace=dev
kubectl get pods --namespace=prod

To change the default namespace:

kubectl config set-context \
  $(kubectl config current-context) \
  --namespace=dev

View pods on all namespaces:

kubectl get pods --all-namespaces

I'm not really sure why in the end of the lecture the tutor showed a ResourceQuota definition file:

apiVersion: v1
kind: ResourceQuota
metadata:
  name: compute-quota
  namespace: dev
spec:
  hard:
    pods: "10"
    requests.cpu: "4"
    requests.memory: 5Gi
    limits.cpu: "10"
    limits.memory: 10Gi