1. What Kubernetes is

Kubernetes is a platform which allows to run and manage containerized applications.

2. Kubernetes concepts

Annotation

Allows to store addition information with a Kubernetes object; can have a big size.

ConfigMap

Stores environment variables.

CronJob

Used for background tasks run on time; under different conditions may run 0, 1 or more than 1 time.

DaemonSet

Runs a copy of a pod on each node; usually used for logging and metric collection.

Deployment

Updates a replica set or a pod; implements rolling updates and rolbacks.

Job

Creates one or more pods and ensures they are successfully terminated after execution; can restart if it fails.

HorizontalPodAutoscaler

Scales deployment or replica set basing on CPU usage or other metrics.

Ingress

Is a router that accepts incoming traffic and maps URLs to services; can implement TLS termination.

Label

Are key-value pairs that are used to identify Kubernetes objects.

Name

Every object in Kubernetes has a human-readable name and unique id (UID).

Namespace

Allow to organize a set of Kubernetes resources inside a Kubernetes cluster into a virtual cluster; nodes and persistent volumes doesn't belong to a namespace.

Pod

Runs one or more containers with the same IP Address.

ReplicaSet

Allows to run multiple copies of a pod for higher availability.

Secret

Used to store backend credentials.

Service

Allows to address a collection of pods for network communication.

Volume

Is a directory that is accessible to all containers in a pod.

Liveness probe

Liveness probe checks if a pod is working: if not then Kubernetes restarts the pod.

Readiness probe

Readiness probe checks if a pod is able to serve requests: if not then Kubernetes will not send requests to the service. Pod can accept traffic only if all containers are ready.

3. Kubernetes commands

Useful aliases

kp='kubectl -n my-namespace --kubeconfig ~/.kube/config-prod'

kp-port-forward='kp port-forward svc/elasticsearch-master 9200:9200'

Show pods in all namespaces

kp get pod -A

Show services

kp get service

Show deployments

kp get deployment

Show nodes

kp get node

Show persistence volume claims

kp get pvc

Run shell in a pod container

kp exec -ti pod-name -c container-name -- /bin/bash

Copy a file from a pod

kp cp -c container-name pod-name:/srv/app/file_name.log file_name.log

Double dash in a command

The double dash ( -- ) in kubectl command separates the arguments you want to pass to the command from the kubectl arguments.

4. Kubernetes service types

ClusterIP

Default service type; exposes service by IP inside Kubernetes cluster.

NodePort

Exposes service externally using each node IP address and a predefined port.

LoadBalancer

Exposes service externally with a load balancer per one service.

ExternalName

Works like a proxy for an external service by DNS name.

5. Kubernetes volume types

EmptyDir

Not persistent (when pod is restarted its data is lost), starts empty and allows to share folder between containers of the same pod (when explicitly mounted).

GitRepo

Not persistent, starts with data from a public Git repo and allows to share folder between containers of the same pod

Secret

In-memory directory to pass credentials to pods.

HostPath

Persistent on a node, usually is used with DaemonSet.

PersistentVolume

Persistent, created by admin with a given technology (like GCEPersistentDisk), capacity and access modes.

PersistentVolumeClaim

Abstracts away from a given technology by pointing to PersistentVolume.