Advanced Concepts — Stateful applications

Amulya Reddy Konda

Mar 21·3 min read

This article is part 2 of https://amulyareddyk97.medium.com/advanced-kubernetes-with-doks-d5fdd829eed4. Please go through the link.

1. Volumes and Claims
2. Life cycle of volumes and claims
3. Provisioning types
4. Binding and Mounting volume
5. Statefulsets

# Containers are not persistent
# PV is accessible at cluster level, independent of pods
# PVs life cycle is not tied to container lifecycle# Types = Static PV and Dynamic PV

Static PV

kubectl create -f static-pv.yml
kubectl get pv
kubectl get pv <name>

Life cycle of PV and PVC

# PV is at cluster level
# When developer claims PVC is created at container level

# Though the dev requested for 3GB volume, if the access mode matches, bigger volume is allocated. 10GB here# PVC
kubectl create -f static-pvc.yml
kubectl get pvc           # state goes from available to bound state
kubectl get pvc <name>

How to make PV visible inside a container

# Mount PVC to a mount path of a pod

kubectl create -f static-pv-pod.yml

Volume Binding — Phases

Available
Bound
Released
Fail

Reclaim Policy

Delete    # If pod is deleted PV gets deleted
Retain    # Even if PVC & pods are deleted, PV remains. We need to              #         delete PV manually
Recycle   
Reclaim

CSI (Container Storage Interface)

# Without CSI, different volume plugins has to be used to interact with different storages
# CSI is single interface for different external storage systems
# Supports secure 3rd party storages without being shipped with Kubernetes code
# They communicate over gRPC

In CSI, lets create Dynamic PV

kubectl create -f csi-pvc.yml
kubectl get pvc                      # gets created
kubectl get pv                       # gets created automatically
kubectl get pod
kubectl describe pod <name>          # can see the volume

Other topics

1. Protection
2. Expanding of Volume — change the size
3. Cloning of Volume

Deletion order

Static: Delete Pod (removes mounting) -> Delete PVC (removes binding)-> Delete PV

Dynamic: Does automatically. Deleting PVC deleted PV.

Volume Cloning

Create snapshot out of PVC

# Check if the PVC exists
kubectl get volumesnapshot

Creating new PVC out of snapshot

kubectl get pvc      # They have same state, same config

Clone PVC — not snapshot (pvc to pvc)

StatefulSets

# Stateful applications store data in StatefulSets (db data, kv pairs) for servers, clients to use# Creates in order -> 0,1,2...
# Once 0 is completely up, 1 starts and so on..

kubectl scale sts <pod_name> replica=5
kubectl get sts

ReplicaSet is the opposite of StatefulSet. It is good for stateless applications.

ReplicaSet, StatefulSet, DeamonSet and Job/CronJobs are workloads.

DeamonSet — defines Pods that provide node-local facilities.

Other Useful commands

kubectl get pod -w
kubectl describe pod <name>
kubectl get events