Assignment for Kubernetes ReplicasSets, DeamonSets & Statefulsets, & Services – by Vishal

Assignment – 1

What is ReplicasSets?

A: A ReplicaSet ensures that a specified number of pod replicas are running at any given time. 
How to work with ReplicasSets?

A: A ReplicaSet is linked to its Pods via the Pods’ metadata.ownerReferences field, which specifies what resource the current object is owned by. All Pods acquired by a ReplicaSet have their owning ReplicaSet’s identifying information within their ownerReferences field. It’s through this link that the ReplicaSet knows of the state of the Pods it is maintaining and plans accordingly.
Example of ReplicasSets?

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

Assignment – 2

What is DeamonSets?

A DaemonSet ensures that all (or some) Nodes run a copy of a Pod. As nodes are added to the cluster, Pods are added to them. As nodes are removed from the cluster, those Pods are garbage collected. Deleting a DaemonSet will clean up the Pods it created.

How to work with DeamonSets?

You can describe a DaemonSet in a YAML file. For example, the daemonset.yaml file below describes a DaemonSet that runs the fluentd-elasticsearch Docker image:

Example of DeamonSets?

apiVersion: apps/v1
kind: DaemonSet
metadata:
name: fluentd-elasticsearch
namespace: kube-system
labels:
k8s-app: fluentd-logging
spec:
selector:
matchLabels:
name: fluentd-elasticsearch
template:
metadata:
labels:
name: fluentd-elasticsearch
spec:
tolerations:
# this toleration is to have the daemonset runnable on master nodes
# remove it if your masters can’t run pods
– key: node-role.kubernetes.io/master
operator: Exists
effect: NoSchedule
containers:
– name: fluentd-elasticsearch
image: quay.io/fluentd_elasticsearch/fluentd:v2.5.2
resources:
limits:
memory: 200Mi
requests:
cpu: 100m
memory: 200Mi
volumeMounts:
– name: varlog
mountPath: /var/log
– name: varlibdockercontainers
mountPath: /var/lib/docker/containers
readOnly: true
terminationGracePeriodSeconds: 30
volumes:
– name: varlog
hostPath:
path: /var/log
– name: varlibdockercontainers
hostPath:
path: /var/lib/docker/containers

Assignment – 3

What is Statefulsets?

StatefulSet is the workload API object used to manage stateful applications.

Manages the deployment and scaling of a set of Pods, and provides guarantees about the ordering and uniqueness of these Pods.

How to work with Statefulsets?

Like a Deployment, a StatefulSet manages Pods that are based on an identical container spec. Unlike a Deployment, a StatefulSet maintains a sticky identity for each of their Pods. These pods are created from the same spec, but are not interchangeable: each has a persistent identifier that it maintains across any rescheduling.

Example of Statefulsets?

apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx # has to match .spec.template.metadata.labels
  serviceName: "nginx"
  replicas: 3 # by default is 1
  template:
    metadata:
      labels:
        app: nginx # has to match .spec.selector.matchLabels
    spec:
      terminationGracePeriodSeconds: 10
      containers:
      - name: nginx
        image: k8s.gcr.io/nginx-slim:0.8
        ports:
        - containerPort: 80
          name: web
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: "my-storage-class"
      resources:
        requests:
          storage: 1Gi

Assignment – 4
What is Service?

An abstract way to expose an application running on a set of Pods as a network service.

How Service Works?

Kubernetes Pods are created and destroyed to match the state of your cluster. Pods are nonpermanent resources. If you use a Deployment to run your app, it can create and destroy Pods dynamically.

Each Pod gets its own IP address, however in a Deployment, the set of Pods running in one moment in time could be different from the set of Pods running that application a moment later.

This leads to a problem: if some set of Pods (call them “backends”) provides functionality to other Pods (call them “frontends”) inside your cluster, how do the frontends find out and keep track of which IP address to connect to, so that the frontend can use the backend part of the workload?

What are the types of Service?

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376

Example Yaml of Service

Assigment5
What is Loadbalancer?

ItsA load balancer is a device / component that acts as a reverse proxy and distributes network or application traffic across a number of servers.

What are the types of Loadbalancer?

Network

Application loadbalancer

How Loadbalancer works?

The former default kube-proxy mode was userspace, which allocates the next available Kubernetes pod using round-robin load distribution on an IP list, and then rotates or otherwise permutes the list. Modern kube-proxy default mode, called iptables, enables sophisticated rule-based IP management. In iptables mode, random selection is the native method for load distribution. In this situation, an incoming request goes to one of a service’s pods that is randomly chosen

Assigment6
What is Discovery Services?

Service discovery is the automatic detection of devices and services offered by these devices on a computer network. A service discovery protocol (SDP) is a network protocol that helps accomplish service discovery. Service discovery aims to reduce the configuration efforts required by users and administrators.

How Discovery Services works?

Server-side service discovery allows clients applications to find services through a router or a load balancer. Client-side service discovery allows clients applications to find services by looking through or querying a service registry, in which service instances and endpoints are all within the service registry.

What is CoreDNS?

CoreDNS is a flexible, extensible DNS server that can serve as the Kubernetes cluster DNS. Like Kubernetes, the CoreDNS project is hosted by the CNCF.

Use of CoreDNS?

The CoreDNS service pods are deployed on the same master node. When the node is lost, the DNS service is down on the cluster until the pod eviction timeout expires.