A ReplicaSet is one of the Kubernetes controllers that makes sure we have a specified number of pod replicas running. 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.

 You can define Deployments to create new ReplicaSets, or to remove existing Deployments and adopt all their resources with new Deployments.

Example:

1. creating replicaset:

[centos@ip-172-31-22-14 harshit]$ kubectl create deployment harshit --image=scmgalaxy/nginx-devopsschoolv1 --replicas=3 -n=harshit
deployment.apps/harshit created
[centos@ip-172-31-22-14 harshit]$ kubectl get deployment -n=harshit
NAME      READY   UP-TO-DATE   AVAILABLE   AGE
harshit   3/3     3            3           15s
[centos@ip-172-31-22-14 harshit]$ kubectl get pod -n=harshit
NAME                       READY   STATUS    RESTARTS   AGE
harshit                    1/1     Running   0          109m
harshit-5gk8t              1/1     Running   0          8m58s
harshit-7c675b8d4f-dlv8l   1/1     Running   0          34s
harshit-7c675b8d4f-p9gmv   1/1     Running   0          34s
harshit-7c675b8d4f-xf7r7   1/1     Running   0          34s
harshit-xtftw              1/1     Running   0          8m58s

2.scaling replicaset:

[centos@ip-172-31-22-14 harshit]$ kubectl scale --replicas=1 deploy/harshit -n=harshit
deployment.apps/harshit scaled
[centos@ip-172-31-22-14 harshit]$ kubectl get deploy -n=harshit
NAME      READY   UP-TO-DATE   AVAILABLE   AGE
harshit   1/1     1            1           12m

Features of Kubernetes:

•  Automating many processes of applications and services deployment that before were carried out manually
•  Kubernetes checks constantly the health of nodes and containers
• It takes care of scaling and failover for the application
• Kubernetes is able to load balance and distribute the network traffic so that the deployment is stable.
• Scale resources and applications in real time
• Orchestrate containers on multiple hosts

2. A YAML file is used to configure all the application services

3. With a single command the services can be created and started using the defined configurations

4. Compose has command to start , stop and rebuild services.

5. docker-compose can be used to view the status of the running services.

6. It can stream the log output of running services.

7.docker-compose only recreates containers that have changed.

This is useful to temporarily store sensitive files that you don’t want to persist in either the host or the container writable layer

# example to use tmpfs mount 

docker run -itd --name harshit5 --mount type=tmpfs,destination=/opt/harshit ubuntu